OSIRIS - The Scientific Camera System Onboard Rosetta
H. U. Keller1, C. Barbieri2, P. Lamy3, H. Rickman4, R. Rodrigo5, K.-
P. Wenzel6, H. Sierks1, M. F. A�Hearn7, F. Angrilli2, M. Angulo8,
M. E. Bailey9, P. Barthol1, M. A. Barucci10, J.-L. Bertaux11,
G. Bianchini2, J.-L. Boit3, V. Brown5, J. A. Burns12, I. B�ttner1,
J. M. Castro5, G. Cremonese2,20, W. Curdt1, V. Da Deppo2,22,
S. Debei2, M. De Cecco2,23, K. Dohlen3, S. Fornasier2, M. Fulle13,
D. Germerott1, F. Gliem14, G. P. Guizzo2,21, S. F. Hviid1, W.-H. Ip15,
L. Jorda3, D. Koschny6, J. R. Kramm1, E. K�hrt16, M. K�ppers1,
L. M. Lara5, A. Llebaria3, A. L�pez8, A. L�pez-Jimenez5, J. L�pez-
Moreno5, R. Meller1, H. Michalik14, M. D. Michelena8, R. M�ller1,
G. Naletto2, A. Orign�3, G. Parzianello2, M. Pertile2, C. Quintana8,
R. Ragazzoni2,20, P. Ramous2, K.-U. Reiche14, M. Reina8,
J. Rodr�guez5, G. Rousset3, L. Sabau8, A. Sanz17, J.-P. Sivan18,
K. St�ckner14, J. Tabero8, U. Telljohann6, N. Thomas19, V. Timon8,
G. Tomasch1, T. Wittrock14, M. Zaccariotto2
1Max-Planck-Institut f�r Sonnensystemforschung, 37191 Katlenburg-
Lindau, Germany
2CISAS, University of Padova, Via Venezia 1, 35131 Padova, Italy
3Laboratoire d�Astrophysique de Marseille, 13376 Marseille, France
4Department of Astronomy and Space Physics, 75120 Uppsala, Sweden
5Instituto de Astrof�sica de Andaluc�a - CSIC, 18080 Granada, Spain
6Research and Scientific Support Department, ESTEC, 2200 AG Noordwijk,
The Netherlands
7Department of Astronomy, University of Maryland, MD, 20742-2421, USA
8Instituto Nacional de T�cnica Aeroespacial, 28850 Torrejon de Ardoz,
Spain
9Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
10Observatoire de Paris - Meudon, 92195 Meudon, France
11Service d'A�ronomie du CNRS, 91371 Verri�re-le-Buisson, France
12Cornell University, Ithaca, NY, 14853-6801, USA
13Osservatorio Astronomico de Trieste, 34014 Trieste, Italy
14Institut f�r Datentechnik und Kommunikationsnetze, 38106
Braunschweig, Germany
15Institute of Space Science, National Central University, Chung Li,
Taiwan
16Institut f�r Planetenforschung, DLR, 12489 Berlin-Adlershof, Germany
17Universidad Polit�cnica de Madrid, 28040 Madrid, Spain
18Observatoire de Haute-Provence, 04870 Saint Michel l'Observatoire,
France
19Physikalisches Institut der Universit�t Bern, Sidlerstra�e 5, 3012
Bern, Switzerland
20INAF, Osservatorio Astronomico, Vic. Osservatorio 5, 35122 Padova,
Italy
21Carlo Gavazzi Space, Via Gallarate 150, 20151 Milano, Italy
22CNR - INFM Luxor, Via Gradenigo 6/B, 35131 Padova, Italy
23DIMS, University of Trento, Via Mesiano 77, 38050 Trento, Italy
Abstract
The Optical, Spectroscopic, and Infrared Remote Imaging System OSIRIS
is the scientific camera system onboard the Rosetta spacecraft (Figure
1). The advanced high performance imaging system will be pivotal for
the success of the Rosetta mission. OSIRIS will detect 67P/Churyumov-
Gerasimenko from a distance of more than 106 km, characterise the
comet shape and volume, its rotational state and find a suitable
landing spot for Philae, the Rosetta lander. OSIRIS will observe the
nucleus, its activity and surroundings down to a scale of ~2 cm px-1.
The observations will begin well before the onset of activity and will
extend over months until the comet reaches perihelion. During the
rendezvous episode of the Rosetta mission, OSIRIS will provide key
information about the nature of cometary nuclei and reveal the physics
of cometary activity that leads to the gas and dust coma.
OSIRIS comprises a high resolution Narrow Angle Camera unit and a Wide
Angle Camera unit accompanied by three electronics boxes. The NAC is
designed to obtain high resolution images of the surface of comet
67P/Churyumov-Gerasimenko through 12 discrete filters over the
wavelength range 250 - 1000 nm at an angular resolution of
18.6 �rad px-1. The WAC is optimised to provide images of the near-
nucleus environment in 14 discrete filters at an angular resolution of
101 �rad px-1. The two units use identical shutter, filter wheel,
front door, and detector systems. They are operated by a common Data
Processing Unit. The OSIRIS instrument has a total mass of 35 kg and
is provided by six European countries.
1 Introduction
1.1 History of the instrument
On March 14th 1986 at 00:03 Universal Time, the European Space
Agency�s (ESA) spacecraft Giotto made its closest approach to comet
1P/Halley. The only remote sensing instrument onboard the spacecraft
was the Halley Multicolour Camera (HMC), which was designed to image
the nucleus and innermost coma of the comet from the spinning
spacecraft. The instrument development was led by the Max-Planck-
Institut f�r Aeronomie (now Max-Planck-Institut f�r
Sonnensystemforschung, MPS) with the participation of several other
major institutes in Europe (Keller et al., 1995).
HMC was by far the most complex instrument onboard Giotto and a
remarkable success (Figure 2). After the International Rosetta Mission
(hereafter �Rosetta�) was selected as the 3rd Cornerstone Mission of
ESA�s Horizon 2000 programme, it was natural for a significant part of
the HMC team to come together again to build the imaging system for
the main spacecraft. Groups from MPS, the Laboratoire d�Astronomie
Spatiale in Marseille (now Laboratoire d�Astrophysique de Marseille,
LAM), the Osservatorio Astronomico di Padova (UPD), the Belgian
Institute for Space Aeronomy (BISA), the Rutherford Appleton
Laboratory (RAL) and the Deutsches Zentrum f�r Luft- und Raumfahrt
(DLR) started working together in 1995 to study a modern imaging
system which would be powerful enough to maintain Europe�s lead in the
remote sensing of cometary nuclei. The resulting proposal for the
Optical, Spectroscopic, and Infrared Remote Imaging System OSIRIS was
the only experiment proposed to ESA as the main imaging system on the
Rosetta spacecraft in response to ESA�s Announcement of Opportunity
(AO). The proposal included two cameras (one narrow angle, one wide
angle) with an infrared imager incorporated into the narrow angle
system (Thomas et al., 1998). There was also the possibility to
include a UV spectrometer to cover the wavelength range from 200 to
400 nm. The instrument was extremely ambitious.
The mission definition study, or �Red Report�, which outlined the
goals and implementation of the Rosetta mission, included a dedicated
scientific imaging system as part of the strawman payload. However,
funding problems led to considerable uncertainty as to whether the ESA
Member States could fund such an ambitious imaging system. These
problems were resolved about one year after the selection of the rest
of the payload when a descoped version of OSIRIS was finally approved.
The descoped version eliminated the IR imaging element of the cameras
(the main interest of the Belgian and UK partners, BISA and RAL).
However, additional support was offered by a group of Spanish
laboratories led by the Instituto de Astrof�sica de Andaluc�a (IAA),
by ESA�s Space Science Department (now Research and Scientific Support
Department, RSSD) and by the Astronomical Observatory of Uppsala (now
Department of Astronomy and Space Physics, DASP) in Sweden. The
contributions from the different institutes involved in OSIRIS are
listed in TABLE 1.
OSIRIS was delivered to ESA and integrated on the Rosetta spacecraft
in 2002. The launch of Rosetta, originally foreseen for January 2003,
was deferred to early 2004, changing the target comet from
46P/Wirtanen to 67P/Churyumov-Gerasimenko. OSIRIS was successfully
commissioned in-flight during the months after the exciting launch on
March 2nd 2004 and in the meantime has been used for scientific
measurements of comet 9P/Tempel 1 in the course of the Deep Impact
mission (Keller et al., 2005, K�ppers et al., 2005).
1.2 The OSIRIS name and symbol
The name, OSIRIS, standing for Optical, Spectroscopic, and Infrared
Remote Imaging System, was selected at the time of the first
instrument proposal which included infrared imaging capability and the
possibility of an ultraviolet spectrometer. Although several aspects
of the original instrument were descoped, the name was retained.
Osiris was the Egyptian god of the underworld and of vegetation. He
was the brother and husband of Isis who gave birth to their son,
Horus, after his death. He was killed by the rival god, Seth. As
legendary ruler of predynastic Egypt and god of the underworld, he
symbolised the creative forces of nature and the indestructibility of
life. The name was selected for the imaging system because Osiris is
identified with the �all-seeing eye� that is depicted in the
hieroglyph of his name (Figure 3).
1.3 Forthcoming sections
In section 2, an overview of the key questions in cometary physics is
presented. This is followed by a short section which describes the
dual camera concept under which OSIRIS was developed. In section 4,
the detailed scientific rationale and objectives of the instrument are
described. The subsequent sections describe the hardware in detail. We
begin with the optical active elements (sections 5 to 7), followed by
the filter wheel mechanisms (section 8), the shutter systems (section
9) and the front door mechanism (section 10). In section 11 we deal
with the image acquisition system. In sections 12 to 16, we describe
the overall control electronics, the digital interfaces, the onboard
software, the EGSE and the telemetry. The calibration and operations
are described in sections 17 and 18. We then summarise with a short
conclusion.
2 The origin of comets and solar system formation
Cometary missions such as Rosetta derive their greatest intellectual
excitement from their potential to address questions about the origin
of the Solar System. In order to apply data acquired by spacecraft
missions to our understanding of these questions, it is necessary to
understand in detail the physical and chemical processes that might
occur in, on, and near the nucleus.
Some of the key problems of the cosmogony of comets and the Solar
System include the nature of the accretion process in the
protoplanetary disc, the physical and chemical conditions
(temperature, pressure, molecular composition) that prevailed there,
the relationship between the original interstellar composition (both
gaseous and solid) and the disk composition, and the variation of its
properties with both time and heliocentric distance. To derive the
maximum scientific return, the camera system on Rosetta must be
designed to address as many of these questions as possible.
The size distribution of planetesimals and the degree to which they
come from different parts of the protoplanetary disc can be studied
directly by images from which the heterogeneity of a cometary nucleus
at all scales can be determined. Images can show the chemical
heterogeneity both on the surface and in the material released from
the interior, the structural heterogeneity as seen in activity and in
topography and its changes with erosion, and porosity and its
variations as seen in the bulk density and moments of inertia.
Heterogeneity at the largest scales, from comet to comet, is then
studied by comparison of the results from Rosetta with results for
other comets (such as 1P/Halley). This will show whether or not
phenomena such as resonances and instabilities in the protoplanetary
disc are important in creating a characteristic size for planetesimals
rather than a broad distribution of sizes characteristic of
agglomeration and collisional phenomena. Our lack of knowledge of the
structure of cometary nuclei is illustrated by the competing models
shown in Figure 4. Note particularly the differences in the scales of
the inhomogeneities. These models are further distinguished by the way
in which the building blocks adhere to one another. This can be
studied by determining the relationship between outgassing and
structural inhomogeneities and by analysing the changes in topography
and structure as the comet goes from a nearly inert state to a very
active state. It can also be addressed both by measuring the degree of
mixing between refractories and solids on the surface of the nucleus
and by analysing the material released from the nucleus. Species could
be mixed at the microscopic level, at macroscopic levels that are
still small compared to the size of the nucleus, or at scales
comparable to the size of the nucleus. We need to know the scale of
mixing in a cometary nucleus as this can tell us, for example, whether
large sub-nuclei with different histories were brought together in the
nucleus.
The physical and chemical composition of the protoplanetary disc can
be studied with calibrated images that provide abundances of species
which are sensitive to those conditions (such as the OH/NH ratio and
various mineralogical ratios). Questions of the nature of physical and
chemical variations within the disc can be addressed by comparisons,
both among the components of comet 67P/Churyumov-Gerasimenko nucleus
and among comets formed in different parts of the disc (e.g. by
comparing the properties of a Jupiter-family comet from the Kuiper
belt, like 67P/Churyumov-Gerasimenko, with the properties of a Halley-
family comet, like 1P/Halley itself, originally from the Uranus-
Neptune region).
It is also necessary to understand the evolution of comets since the
changes that have occurred over a comet's active lifetime will have
affected the observable properties of the nucleus. Do comets disappear
by gradually shrinking in size as the ices sublime, do they
disintegrate because of the activity, or do they become inert by
choking off the sublimation? Are the intrinsic changes important
compared to the extrinsic changes (collisions, perturbations that
dramatically change the orbit, etc.)? How do comets contribute to the
population of interplanetary dust and how do they contribute to the
population of near-Earth objects? The Rosetta mission and OSIRIS, in
particular, are well suited to study the evolution over a large
fraction of an orbit and to determine the actual contribution per
orbital period to interplanetary dust. They are also well suited to
study the evolution of the surface or mantle of the comet in order to
address, for example, the question whether devolatilisation is more or
less important than simple loss of the surface layers. Data obtained
by Rosetta will be compared to those of missions to Near-Earth
Asteroids.
Our understanding of the nature and origin of comets, and our use of
them as probes of the early Solar System, is critically dependent upon
understanding the cometary sublimation processes, because this
knowledge is needed before we can relate results from Earth bound
remote sensing to the nature of cometary nuclei. Although many
processes in the outer coma, beyond about 100 km, are well understood
already, the processes at the surface of the nucleus and in the near-
nucleus portion of the coma, closer than a few cometary radii, are
poorly understood and in some cases simply unknown. We need to
understand the process by which material leaves the nucleus. Are
observed variations in the 'dust-to-gas' ratio caused by intrinsic
differences in the bulk ratio of refractories to ice, or are the
variations dominated by properties and processes near the surface such
as gas flow and structural strength? Does the size distribution of the
particles change in the near-nucleus region because of either
vaporisation or fragmentation or both? What fraction of the volatiles
is released directly from the nucleus and what fraction is released
subsequently from particles in the inner coma? Is the gas released
from vaporisation at the surface or at some depth below the surface?
Do periodic variations in the properties of the mantle occur and do
they lead to variations in the coma that are, in fact, unrelated to
the bulk properties of the nucleus?
OSIRIS will directly determine the outflow of gas and dust from
different regions of the nucleus and will compare those variations
with variations in surface mineralogy, in topography, and in local
insolation. This will provide the context in which to interpret the
results from the Rosetta lander (Philae). The unique strength of
OSIRIS is the coverage of the whole nucleus and its immediate
environment with excellent spatial and temporal resolution and
spectral sensitivity across the whole reflected solar continuum up to
the onset of thermal emission. In the next section, we briefly
describe the imaging concept of OSIRIS. In the subsequent sections, we
will address the many, detailed observational programmes to be carried
out by OSIRIS and how they bear on the fundamental questions outlined
above.
3 NAC and WAC - a complementary system
During the proposal phase, it was immediately obvious that the
scientific objectives of the camera system on Rosetta would best be
served by a combination of a Narrow Angle Camera (NAC) and a Wide
Angle Camera (WAC). The NAC would be a system with high spatial
resolution which would allow an initial detection of the nucleus,
study its structure and rotation from relatively great distances
(typically 104 km), investigate the mineralogy of the surface, and
study the dust ejection processes. The WAC would have much lower
spatial resolution but, accordingly, a much wider field of view. This
would allow observations of the 3-dimensional flow-field of dust and
gas near the nucleus and, in addition, would provide a synoptic view
of the whole nucleus. In summary, the WAC would provide long-term
monitoring of the entire nucleus from close distances, while the NAC
would study the details. The two camera systems have therefore been
designed as a complementary pair which, on the one hand, addresses the
study of the nucleus surface, and on the other, investigates the
dynamics of the sublimation process. The resulting cameras have the
basic parameters shown in TABLE 2.
Optical designs with central obscuration are notorious for their stray
light problems. Therefore, off-axis designs with no central
obscuration were selected for both systems. These provide maximum
contrast between the nucleus and the dust. The internal baffle of the
cameras was optimised for stray light suppression.
The NAC angular resolution was chosen as a compromise between requests
for a high resolution required for investigation of unknown scale
lengths on the nucleus surface, the need to maintain the nucleus in
the FOV of the WAC when only a few nucleus radii above the surface,
and the mass requirements for a longer focal length system. A spatial
resolution of ~2 cm px-1 was favoured, corresponding to an angular
resolution of ~20 �rad px-1 at a distance of 1 km. This value is also
well adjusted to the limited data volume that can be transmitted back
to Earth.
The NAC focal ratio was set at 8, which is a compromise between speed
(required at high heliocentric distance, rh) and mass. Extensive
calculations were performed to compute the motion of the image
footprint over the surface during the mapping phase taking into
account the orbit of the spacecraft and the rotation of the target,
which would produce image smear. The calculations indicate that
exposure times shorter than 50 ms are probably not required, given the
resolution of the NAC. The WAC observations of the dust and gas
environment require narrower filter bandwidths and a reduced focal
ratio. Therefore the WAC exposure times are significantly longer.
The major considerations for the CCDs were:
� �full well� signal-to-noise ratio (in order to optimise the
dynamic range of the instrument)
� UV response (to give good signal-to-noise ratio for gas species)
� high Quantum Efficiency (QE) in the range 800 to 1000 nm
(information on olivine and pyroxene bands).
A backside illuminated detector with a UV optimised anti-reflection
coating was selected. This type of device has high QE over an extended
wavelength range. Full well signal-to-noise ratio for these devices is
of the order of 2 10^4. Over-exposure control is needed to allow
saturation on the nucleus while acquiring high signal-to-noise
information on the dust and gas. Custom CCDs with an anti-blooming
design were developed for OSIRIS. For cost reasons, identical devices
are used in the two cameras.
While the two cameras have different scientific objectives, the
similar nature of the instruments naturally led to our seeking cost
reduction through development of identical subsystems. Hence, the
mechanical design was adjusted so that identical Focal Plane
Assemblies (FPA) could be used. The large format CCD necessitated the
use of a mechanical shuttering of the exposure. Here again, identical
subsystems were designed.
The requirement to determine the chemical and physical structure of
the nucleus and the inner coma suggested the use of an extensive
filter set. Identical filter wheels were used in the two cameras
although each camera had its own filter complement adapted
specifically for its own science goals.
Both cameras need protection from dust impacts when not operating.
Hence, they have doors which can be opened and closed on command.
Although the apertures (and therefore the doors themselves) are
different, the drive mechanism is the same in both cases. In addition,
the doors can be used to reflect light from calibration lamps mounted
inside the baffles. The lamps in the NAC and the WAC are identical.
The modular concept of OSIRIS functional blocks, mechanisms, and
electronics subsystems can be seen in Figure 5 (including
redundancies).
The selection of identical subsystems in both cameras reduced the
management effort, cost, and overall complexity considerably although
interface definition and specification to accommodate these subsystems
was more difficult throughout the project and required additional
spacecraft resources (mass).
4 Scientific objectives
4.1 The cometary nucleus
The imaging systems on the Giotto and Vega spacecraft were remarkably
successful in providing our first glimpses of a cometary nucleus and
its immediate environment. Reviews of the results of these
investigations can be found, e.g. in Keller et al. (1996). Despite
this success, the imaging results were limited and many questions were
left unanswered, and additional questions arose, many of which will be
addressed by OSIRIS. We describe here the goals of our nucleus
observations.
4.1.1 Position and size of the nucleus
The first goal of OSIRIS will be to localise the cometary nucleus and
to estimate its size and shape as quickly as possible for mission
planning purposes. These properties must be coarsely known well before
the mapping phase commences. This should be performed near the end of
the approach phase when the spacecraft is between 103 and 104 km from
the nucleus. Determination of the radius to an accuracy of 10 % from
104 km can be performed with the NAC and will immediately yield an
estimate of the nucleus volume (and mass for an assumed density)
accurate to about a factor of 2.
4.1.2 Rotational state
The goal of OSIRIS is to determine the rotational properties of the
comet including the periods of rotation about three principal axes,
the total angular momentum vector L, the changing total spin vector
and the characteristics of any precessional behaviour. Measurements of
these quantities will constrain the inhomogeneity of the nucleus and
will also permit the development of time-dependent templates over
which other data sets may be laid. The secondary, more ambitious goal
is to use OSIRIS to monitor the rotational properties throughout the
entire mission to search for secular evolution in response to the
torques acting on the nucleus caused by the onset of jet activity as
the comet approaches perihelion. Model calculations indicate that for
a small nucleus, such as 67P/Churyumov-Gerasimenko, torques could
force re-analysis of the rotational properties of the nucleus on
timescales of days (Guti�rrez et al., 2005). The measured precession
rate, along with an estimate of the average reaction force (from the
non-gravitational acceleration of the nucleus) and an estimate of the
torque caused by outgassing, may allow an estimate of the absolute
value of the nucleus moment of inertia. This, in turn, would give
clues to the internal density distribution, especially when combined
with the gravity field determination (see P�tzold et al., this
volume), allowing us to distinguish between a lumpy, a smoothly
varying, and a homogeneous nucleus (see also Kofman et al., this
volume). The structural inhomogeneity would provide an important clue
for the size distribution of the forming planetesimals.
4.1.3 Shape, volume, and density
The concept of comets as uniformly shrinking spherical ice balls was
shattered by the Giotto results. The nucleus is expected to be highly
irregular on all scales as a consequence of cratering, outgassing, and
non-uniform sublimation (Keller et al., 1988). However, it is not
clear whether these irregular-shaped bodies reflect the shape of the
nuclei at their formation, or are the result of splitting during their
evolution, or are caused by non-uniform sublimation.
To accurately model such a craggy shape, techniques developed at
Cornell University (Simonelli et al., 1993) can be used. Although
OSIRIS has no stereo capability per se, the motion of Rosetta relative
to the nucleus can be used to produce stereo pairs. The shape model
will be based upon these stereogrammetric measurements in addition to
limb and terminator observations. Once the shape model is available,
it can be used to determine the surface gravity field and moments of
inertia and will also be used to reproject and mosaic digital images,
as well as to develop surface maps. This technique has yielded
accurate shapes for the Martian moons and Galileo's asteroid targets,
951 Gaspra and 243 Ida (Thomas et al., 1995; 1994).
To look for internal inhomogeneities of say 30 % implies that
differences in the geometrical and dynamical moments of inertia need
to be known to better than 10 %. We therefore need to measure both to
better than 1 %. Thus, the topography must be characterised over the
entire nucleus to an accuracy of �20 m.
4.1.4 Nucleus formation and surface topography
On the smallest scales, the building blocks comprising the cometary
nucleus may be a heterogeneous mixture of interstellar and inter
planetary dusts and ices, with a structure and composition reflecting
the physical conditions and chemistry of the protoplanetary disc. The
different accretion processes leading to the production of first,
grains, then, building blocks and, finally, cometary nuclei, are all
expected to have left their mark on a nucleus which has remained
largely unaltered since its formation. OSIRIS will therefore perform a
detailed investigation of the entire cometary surface over a range of
spatial scales as wide as possible to identify the hierarchy of
cometary building blocks.
In addition to its implications for nucleus formation, the topography
of the surface determines the heat flow in the uppermost layers of the
nucleus (Gutierrez et al., 2000, Colwell et al., 1997). High
resolution imaging will determine the normal to the surface and hence
provide input to surface heat flow calculations.
The Vega 2 TVS observations of jets were interpreted as showing a fan
generated from a few, kilometre-long, quasi-linear cracks (Smith et
al., 1986; Sagdeev et al., 1987). If fresh cracks appear on the
surface during the aphelion passage, then OSIRIS will be able to probe
the inner layers of the nucleus where some stratification is expected
from the loss of volatiles near the surface.
4.1.5 Colour, mineralogy, and inhomogeneity
Inhomogeneity of mineral composition and colour could provide the most
obvious clues to the size of building blocks. The Vega and Giotto
cameras were able to determine only rough estimates of the broad-band
(�/�� = 5) colour of the nucleus of 1P/Halley. OSIRIS will allow a
much more sophisticated study of the mineralogy of the nucleus surface
by recording images that span the entire wavelength range from 250 nm
- 1000 nm.
OSIRIS also has the opportunity to search for specific absorption
bands associated with possible mineral constituents. The wavelengths
of pyroxene absorptions are highly dependent upon their exact
structure (Adams, 1974). Hence, filters giving complete coverage of
the 750 nm to 1 �m regions at 60 nm resolution were incorporated.
Vilas (1994) suggested that the 3.0 �m water of hydration absorption
feature of many low albedo (including C-class) asteroids strongly
correlates with the 700 nm Fe2+? Fe3+ oxidised iron absorption
feature. Given the spectral similarity between C-class asteroids and
1P/Halley and the high water ice content in comets, a search for the
water of hydration feature at 700 nm will be made.
4.1.6 Surface photometry
Due to the limited information from high-velocity fly-bys, little was
learned of the photometric properties of the surface of comet
1P/Halley. The correct determination of the phase function for comet
67P/Churyumov-Gerasimenko will provide information on the surface
roughness through application of, for example, Hapke's scattering
laws. The Philae observations will provide the parameters necessary to
validate the surface roughness models used to interpret global data
provided by OSIRIS.
4.1.7 Polarization measurements
The properties which can be addressed by polarization measurements can
be obtained more accurately by observations of the surface from the
Philae or by in situ analysis. Implementation of polarization
measurements in OSIRIS was thought costly in terms of resources and
calibration and they were therefore not included.
4.1.8 Active and inactive regions
Modelling (K�hrt and Keller, 1994) suggests that debris from active
regions will not choke the gas and dust production in view of the
highly variable terrain, the extremely low gravity, and the lack of
bonding between particles forming the debris. Inactive regions can
only arise if either the material comprising the regions formed in the
absence of volatiles or, alternatively, if the regions have become
depleted in volatiles without disrupting the surface.
To verify this picture, a comparison of active and inactive regions on
comet 67P/Churyumov-Gerasimenko must be of high priority. If inactive
regions are merely volatile-depleted with respect to active regions,
high signal-to-noise observations at several wavelengths may be
required to differentiate between the two. Imaging of the interface
between active and inactive regions may provide evidence of surface
structures and tensile strength present in one type of region, but not
in the other.
As the observations of �filaments� indicate (Thomas and Keller, 1987),
there is no reason to suppose that active regions are homogeneous.
Activity may be restricted within an active region (see for example
the theoretical calculations of Keller et al., 1994). For this reason,
OSIRIS will identify the active fraction within what we call an active
region. Achieving this goal may lead to understanding how cometary
activity ceases, leaving an inert comet. Do inactive spots within
active regions spread to reduce cometary activity or does the infall
of material from the edge of the sublimation crater choke emission?
Directions of jets and locations of active spots are influenced by
topography. The distribution and orientation of near-nucleus jets can
be used to infer topographic features (Thomas et al., 1988, Huebner et
al., 1988). OSIRIS will investigate these correlations.
4.1.9 Physics of the sublimation process
The physical processes characterising the sublimation and erosion
processes in or above active regions depend on the physical structure
of the surface and the distribution of refractory and volatile
material within the nucleus. Dust particles have usually been treated
as impurities in the ice (icy conglomerate). Starting with the
interpretation of the images of 1P/Halley (Keller, 1989), it has
become clear that the topography requires a matrix dominated by
refractory material (K�ppers et al., 2005). The other extreme is the
model of a friable sponge, where the refractory material is intimately
mixed with the ice and where the erosion process maintains a balance
between the ice and dust. How are dust particles lifted off the
surface? The excellent resolution of the OSIRIS NAC, which will be
smaller than the mean free path of the gas near the surface, will
allow the detection and study of the relevant macrophysical processes.
4.1.10 The diurnal cycle
OSIRIS will be able to monitor short-term changes in active regions
very easily. Changes are most likely when active regions cross the
terminators. Cooling will lead to decreased activity, but on what
timescale? On the other hand, as the insolation increases, will there
be changes in the surface structure?
4.1.11 Outbursts
Outbursts (or rapid increases in the brightness of cometary comae)
have frequently been observed from the ground and recently also during
the approach of the Deep Impact spacecraft to comet 9P/Tempel 1. This
implies some sudden increase or even explosion of activity ripping the
surface crust apart. OSIRIS, and in particular the WAC, can be used to
monitor autonomously the nucleus activity over many months at various
scales. The NAC can then be used to look in detail at the source to
determine how the site has altered topographically and spectrally.
4.1.12 Mass loss rate
The floor of the active regions will be lower by several metres on
average after the passage of comet
67P/Churyumov-Gerasimenko through its perihelion. It is clear that if
an active area can be monitored by OSIRIS at a resolution of � 30 cm
the mass loss will be evident. If the density of the surface layer can
be determined by Philae or through joint OSIRIS/Radio Science
investigations, this is potentially the most accurate means to
determine the total mass loss rate particularly if the mass loss is
dominated by infrequently emitted large particles.
4.1.13 Characterisation of the landing site
The NAC was designed to remain in focus down to 1 km above the nucleus
surface. Mapping at ~ 2 cm px-1 will reveal inhomogeneities of the
nucleus at scale lengths comparable to the size of Philae. Homogeneous
sites would provide no difficulties in interpretation but
heterogeneous sites may be scientifically more interesting. As a
result, OSIRIS needs to be able to characterise the landing site and
to identify on what types of terrain Philae has landed.
4.1.14 Observation of the Philae touchdown
There is no guarantee that the orbiter will be able to observe Philae
when it strikes the surface. However, OSIRIS will provide valuable
information on the impact velocity, the result of the initial impact,
and the final resting position and orientation.
Outgassing from the impacted site may also occur. If fresh ice is so
close to the surface that the lander can penetrate the crust, emission
of gas and dust may be fairly vigorous. If so, OSIRIS can quantify
this emission with highest possible spatial and temporal resolution.
4.2 Near-nucleus dust
The near-nucleus dust environment of a comet is remarkably complex and
remains poorly understood. Understanding the near-nucleus environment
is necessary to understanding the nucleus itself. OSIRIS can
investigate global dust dynamics.
4.2.1 Detection of emission at rendezvous
OSIRIS will be used to place constraints on distant activity of the
nucleus. It is evident, however, that detection of dust in the
vicinity of the nucleus will be extremely difficult at high
heliocentric distances. The dust production may decrease as steeply as
rh-2.9 (Schleicher et al., 1998), with a corresponding decrease in
flux proportional to rh-4.9. At 3.25 AU, we estimate the ratio of the
signal received from the dust to that from the surface (Id/Is) � 4 10-
4 based on scaling of Giotto measurements. Therefore, to quantify the
total dust production rate, a dynamic range of > 2000 is required.
Both the WAC and the NAC were designed with this contrast requirement.
4.2.2 Temporal evolution
4.2.2.1 Variation with heliocentric distance
HMC observations showed that the dust production rate of comet
1P/Halley during the Giotto fly-by was remarkably stable over the
three hours of the encounter. Ground-based observations have shown,
however, that comets exhibit large and rapid changes in dust
production. A key goal of OSIRIS will be therefore to monitor the
variation in the production rate and to compare it to the rotational
characteristics of the nucleus and the change in rh.
4.2.2.2 Variations with rotation
The lack of significant variation in the dust production rate with the
rotation seen at comet 1P/Halley was not expected. How does the
production vary with the solar zenith angle? How long does an active
region take to switch on after sunrise? These phenomena are determined
by the physical properties (e.g. the thermal conductivity) of the
surface layer. If sublimation occurs below the surface then a period
of warming may be required before dust emission starts. The surface
layer could act as a buffer to stabilise the activity. These questions
can be addressed using OSIRIS to monitor the active region during the
first minutes after it comes into sunlight.
4.2.2.3 Night side activity and thermal inertia
The inferred absence of night side activity during the Giotto fly-by
and the thermal map created from near-infrared spectral scans of comet
9P/Tempel 1 during the recent Deep Impact mission (A�Hearn et al.,
2005) suggest that the thermal inertia must be low. Observations of
comet Hale-Bopp (C/1995 O1) also suggest that the thermal inertia of
comets is low (K�hrt, 2002). The high porosity of the surface and the
resulting low thermal conductivity suggest that the activity should
decrease rapidly and stop when the energy source is removed.
Monitoring the dust emission as an active region crosses the evening
terminator can confirm this hypothesis.
4.2.2.4 Short-term variability
The dust emission from the nucleus of comet 1P/Halley showed no
evidence for short-term (order of minutes) temporal variations.
Because of the nature of the active regions one might expect, however,
that the emission should occasionally show an enhanced or reduced rate
on a timescale of perhaps a few seconds. A sudden burst offers the
possibility of following the emitted dust and using it to derive
streamlines and velocities in the flow. This would provide strong
constraints on the hydrodynamics of the flow and lead to increased
understanding of the dust-gas interaction a few metres above active
regions. If large enough, outbursts could also modify the flow field
itself allowing us to use OSIRIS to monitor the reaction of the inner
coma to changes in the emission rate.
4.2.3 Large particles in bound orbits
It was shown that gravitationally-bound orbits around cometary nuclei
are possible, in theory, for relatively small particles even in the
presence of radiation pressure (Richter and Keller, 1995). In
addition, evidence from radar measurements suggests that large clouds
of centimetre-sized objects accompany comets in their orbits (Campbell
et al., 1989). The high resolving power of OSIRIS combined with our
proximity to the nucleus will allow us to place constraints on the
number density of objects with a particle radius of a > 5 mm. Since it
is now widely believed that most of the mass lost by comets is in the
form of large particles (McDonnell et al., 1991), observations of this
phenomenon could prove very important in determining the dust to gas
ratio. Clearly, it would be a major discovery to find an extremely
large chunk which might be termed �a satellite� of the nucleus. Active
chunks, as seen in comet Hyakutake (Rodionov et al., 1998), may also
be evident.
4.2.4 How inactive are �inactive� regions?
The observations by HMC and more recent fly-bys (A�Hearn et al., 2005)
were not good enough to place firm constraints on the activity of so-
called inactive regions. Dust emission from the illuminated but
apparently inactive regions could have been up to 10 % of the emission
from active regions and remained undetected. This clearly has
implications for the evolution of the nucleus and for the flow field
of gas and dust emission about the nucleus.
4.2.5 Optical properties of the dust
The orbit of Rosetta and the broad-band filters in OSIRIS will allow
observations of dust at many phase angles (0�-135�) over a wide
wavelength range. The phase curve and colour are sensitive to particle
size, composition, and roughness. Deduction of these properties and
their variation with rh will be important for ground-based
observations of other comets since it will provide the single
scattering albedo, the phase function, and the characteristic particle
size.
4.2.6 Eclipses
Eclipse measurements are extremely interesting for the innermost dust
coma as they would allow OSIRIS to determine the forward scattering
peak of the dust phase function, which provides the best information
on the size distribution, and nature of the dust particles. The strong
forward scattering peak also yields the most sensitive measurement of
the dust column density (e.g. Divine et al., 1986).
4.2.7 Acceleration and fragmentation
Complications with the determination of local dust production rates
arise if the observations cover the dust acceleration region, if
fragmentation is significant, or if optical depth effects become
important. Measurements of the acceleration will quantify the drag
coefficient of the gas-particle interaction and characterise the near-
surface Knudsen layer. The fluffiness of the cometary dust can be
derived from these observations.
A complementary approach is to measure the radius and velocity of
large escaping dust agglomerates in dependence of heliocentric
distance. By knowing the gravitational forces, this would also provide
information on the physics of the gas-dust interaction (drag
coefficient) at and near the surface (Knudsen layer), on the cohesive
forces, and on the density of the agglomerates.
4.3 Gas emissions
Our current understanding of the composition of the nucleus and
variations within the nucleus is severely limited by our lack of
knowledge about the processes in the innermost coma. We know little
about the variations of the composition of the outgassing on any
scale, although there are indications from Earth-based measurements of
large-scale heterogeneity (e.g. in 2P/Encke and in 1P/Halley). There
are distributed sources in the coma which produce some of the species
in the coma, including H2CO, CO, and CN. Because we cannot separate
completely the extended coma source from the nuclear source, we cannot
determine reliably the amount of ice in the nucleus. We therefore plan
to make observations of the gas in order to address some of the most
crucial questions in relating abundances in the coma to abundances in
the nucleus.
4.3.1 Selected species
In order to constrain the heterogeneity of other parent molecules, we
will map the release of certain daughter species in the vicinity of
the nucleus. Dissociation products having short lifetimes and
identifiable parents are ideal for this task. In particular, NH at
336.5 nm and NH2 at 570 nm will be measured to trace the heterogeneity
of NH3 (and thus the nitrogen chemistry in the nucleus), CS at 257 nm
to trace the heterogeneity of CS2 (and thus the sulphur chemistry),
and OH at 309 nm and OI at 630 nm to trace H2O.
The heterogeneity of other fragments, such as CN (388 nm), will also
be measured, even though we do not know the identity of the parent
molecules, because these species show evidence of an extended source.
The recent interest in the distribution of Na has led us to introduce
a sodium filter at 589 nm.
4.3.2 Sublimation process and inactive areas
The results from 1P/Halley showed us that the release of dust is
confined to discrete active areas, comprising only a small fraction of
the surface (15 %). We have no information, however, on whether the
gas is similarly confined. One of the key questions to be answered is
whether gas is also released from the apparently inactive areas. The
mapping capability of OSIRIS is ideally suited to answer this question
and thereby to assess the effects of an inert layer on the release of
gas and dust.
4.4 Serendipitous observations
4.4.1 Asteroid fly-bys
The fly-bys of 2867 Steins and 21 Lutetia will provide interesting
secondary targets on the way to the comet. The main scientific goals
of OSIRIS observations of the asteroids are:
� Determination of physical parameters (size, volume, shape, pole
orientation, rotation period)
� Determination of surface morphology (crater abundance, crater
size distribution, presence of features such as ridges, grooves,
faults, boulders, search for the presence of regolith)
� Determination of mineralogical composition (heterogeneity of the
surface, identification of local chemical zones, superficial texture)
� Search for possible gravitationally bound companions (detection
of binary systems).
4.4.2 Mars fly-by
High-resolution images of Mars (> 200 px across the planet) can be
taken within two days of closest approach (cf. recent HST images).
This will provide data on the global meteorological conditions on Mars
and allow us to follow weather patterns over a period of about two
days. Images around 12 hours before closest approach would be of
sufficient resolution to allow us to resolve vertical structures in
the atmosphere at the limb and to estimate the global atmospheric dust
content. The solar occultation during Mars fly-by would allow
detection of the putative Martian dust rings.
4.4.3 Earth-Moon system fly-bys
As with the space missions Galileo and Cassini/Huygens, the Rosetta
remote sensing instruments can perform testing and calibration during
the fly-bys of the Earth-Moon system. There are also several
interesting possibilities for new science. For example, the Moon is
now known to have a tenuous sodium atmosphere (�exosphere�). The Na
filter on the WAC can be used to acquire maps of Na near the Moon.
Similarly, OI emission from the Earth may be detectable at high
altitudes. Vertical profiles of OI in the atmosphere of the Earth can
be derived by stellar occultations.
5 The NAC telescope
The Narrow Angle Camera is designed to obtain high-resolution images
of the comet at distances from more than 500 000 km down to 1 km, and
of the asteroids 2867 Steins and 21 Lutetia during the interplanetary
cruise. The cometary nucleus is a low-albedo, low-contrast object;
hence, good optical transmission and contrast-transfer characteristics
are required. The camera also should be able to detect small ejected
particles close to the comet nucleus (brightness ratio = 1/1000),
placing strict tolerances upon stray light rejection.
The scientific requirements for the NAC translate into the following
optical requirements. A square field of view (FOV) of width 2.2� and
an instantaneous field of view (IFOV) of 18.6 �rad (3.8 arcsec) per
pixel, a spectral range from 250 nm to 1 �m, and a moderately fast
system (f/8) are needed. An unobstructed pupil is required to minimise
stray light. This is particularly important for the study of gas and
dust surrounding the bright nucleus. The requirements are fulfilled
with an all-reflecting system of 717 mm focal length and an off-axis
field, using a 2048 2048 px, UV-enhanced CCD array. The high
resolution over a large flat field requires a system of three optical
surfaces.
5.1 Optical concept and design: the Three-Mirror Anastigmat
A flat-field, three-mirror anastigmat system, TMA, is adopted for the
NAC. Anastigmatism (freedom from third-order spherical aberration,
coma, and astigmatism) is attained by appropriate aspheric shaping of
the three mirror surfaces, and a flat field (zero Petzval sum) is
achieved by appropriately constraining the system geometry. Our
solution (Dohlen et al., 1996) has an axial pupil physically placed at
the second mirror M2, an off-axis field of view, appropriate baffle
performance and a large back-focal clearance. The optics requires only
two aspheric mirrors, the tertiary remaining spherical. This
considerably reduces fabrication cost and alignment difficulty. The
three mirror surfaces are rotationally symmetric about a common
optical axis, but the field of view is sufficiently removed from the
axis to ensure that all rays pass through the system without
vignetting. The mirrors are made of silicon carbide; details of their
fabrication, polishing and alignment can be found in Calvel et al.
(1999).
The system is equipped with two filter wheels placed in front of the
CCD. In order to cope with the presence of ghost images, the filters
are tilted by 4� to the optical axis and wedged by 10'. In addition to
the bandpass filters, the filter wheels contain anti-reflection-coated
focusing plates of varying thickness which, when used with the filters
of the other wheel, allow two different focusing ranges: far focus
(infinity to 2 km, optimised at 4 km) and near focus (2 km to 1 km,
optimised at 1.3 km). Nominal operation is defined as far focus
imaging with an orange filter (centred at 645 nm with a bandwidth of
94 nm). This filter has similar characteristics to that of the orange
filter in the Halley Multicolour Camera.
A plane-parallel, anti-reflection coated plate, referred to as Anti-
Radiation Plate (ARP), was added to the front of the CCD for radiation
shielding. Its effect for monochromatic light is negligible, but the
shift of focus is considerable for the two UV filters, and the Far UV
and clear filters are affected by acceptable amounts of longitudinal
chromatic aberration. TABLE 3 lists the construction parameters for
the optimised camera design, including filter, refocusing plate and
ARP. The system includes an external baffle for stray light rejection
and a front door for protection.
5.2 Optical performance
Figure 6 shows a ray tracing diagram of the system, and Figure 7 shows
spot diagrams and root-mean-square wave front errors (WFE) at six
points in the FOV located at the centre, the edges and the corners.
Since the system is symmetrical about the y-z plane, the
characteristics are identical for positive and negative x co-
ordinates. The wave front error is calculated for the central
wavelength of the orange filter (� = 0.645 �m). As seen in Figure 7,
the WFE is in the order of 0.04 � over the entire FOV. The performance
is limited primarily by a triangular-type (trifle) aberration which is
present in varying degrees over the entire FOV. Astigmatism and coma
are close to zero at the centre but become significant towards the
edges.
5.3 Stray light rejection
The observation of faint cometary physical and chemical phenomena,
such as dust and gas jets from localised vents on the nucleus, require
good optical transmission and high contrast with strict tolerances on
stray light. There are two types of stray light sources. One
originates from the cometary nucleus itself (considered as an extended
object), the image of which is in the focal plane. The second source
is the sun, which is allowed to reach an elongation of 45� from the
optical axis of the instrument.
Rejection of stray light from the nucleus is insured by the TMA design
whose unobstructed pupil minimises diffraction phenomena and scattered
light. A low level of micro roughness of the optical surfaces
(< 2 nm rms) was specified to limit the stray light contribution. The
internal baffle of the instrument was optimised by adding vanes and
protective black tapes in critical areas. An internal black foil
envelops the whole instrument to prevent light leakage.
Rejection of stray light from the sun requires an external baffle
which can be closed with the front door whenever the solar elongation
is less than 45�. The fraction of the power incident onto the detector
surface to that entering the aperture of the telescope baffle was
required to be < 10-9 at angular distances from the centre of the FOV
exceeding 45�. The external baffle comprises a two stage cylinder with
four vanes which have a square aperture with rounded corners to fit
around the scientific beam. The baffle is made of aluminium alloy and
all internal surfaces are coated with a black paint. The vanes have a
thickness of 0.5 mm with sharpened edges in order to reduce the
reflecting area.
5.4 NAC structure
The main function of the structure of the NAC is to carry the three
mirrors of the TMA, the dual filter wheel mechanism, shutter
mechanism, focal plane assembly, the external baffle, the front door
mechanism (Figure 8), and to maintain them in proper position during
the long interplanetary cruise and the phases of nucleus observations.
The basic concept is an athermal design, achieved by using the same
material for the mirrors and the supporting structure. Therefore the
optical properties are maintained during temperature changes, as long
as thermal gradients are limited. Silicon carbide (SiC), a very rigid
ceramic material with good thermal properties (low coefficient of
thermal expansion, good thermal conductivity), is used.
The structure is U-shaped with two walls cemented on a connecting
tube, see Figure 9. The main wall carries the external baffle, the
secondary mirror and a magnesium interface plate (PPE) which receives
the mechanisms and the focal plane. The second wall carries the
primary and the tertiary mirrors, which are bolted directly on it. The
thickness of the two walls is reduced to the minimum feasible in order
to minimise mass. The NAC structure is kinematically mounted on the
main spacecraft structure via three titanium bipods, decoupling it
from panel distortion and reducing thermal flows.
5.5 Thermal design
The athermal concept requires that thermal gradients be minimised. A
thermal decoupling is necessary between the SiC parts constituting the
telescope and the subsystems which have to be maintained within their
temperature ranges. The PPE is connected to the main SiC wall via
three flexible titanium blades and plastic insulator washers. The
entrance baffle and the FDM are connected to the main SiC wall via
three insulator washers. In order to minimise thermal losses, the
instrument is wrapped in a thermal blanket made of Multi-Layer
Insulation (MLI). The strong limitation on available power during the
cruise phase required a careful optimization of the thermal design.
With a non-operational power of 7.5 W, the subsystems are maintained
in their allowed temperature ranges while the SiC telescope can float
to a minimum temperature of �70�C.
5.6 Internal calibration
The internal calibration of the NAC is achieved by illuminating the
rear side of the lid of the front door which acts as a diffusing
screen. The illumination system is composed of two redundant sets of
two small lamps placed inside the external baffle, between two vanes.
The four lamps form a rectangle but only two lamps on one side are lit
at a time. The tungsten lamps (colour temperature 2410 K) were
manufactured by Welch Allyn (USA); they have a quartz envelope and are
mechanically mounted with a glass diffuser. The in-flight calibration
system provides a reference illumination to the camera in cruise for
comparison to the ground calibration flat fields. Deviations allow the
determination of long-term degradation in flight.
6 The WAC telescope
The prime objective of the WAC is the study of the weak gas and dust
features near the bright nucleus of the comet. For this purpose, the
WAC has to satisfy a number of scientific requirements. The WAC needs
a rather large field of view, 12� 12�, to observe both the nucleus
and the features of emitted gas and dust. It has to cover a relatively
wide spectral range, from UV to visible, and it has to provide a high
contrast ratio, of the order of 10�4, to be able to observe the bright
nucleus and the weak coma simultaneously.
6.1 Optical concept and design
To obtain the required camera performance, an unobstructed all-
reflective, off-axis optical configuration using two aspherical
mirrors was adopted. With this system, best performance over the
entire field of view is obtained, providing a spatial resolution of
about 20 arcsec. The all-reflective solution, unlike a lens design,
allows observation in the ultraviolet spectral range. The unobstructed
solution provides the optimal contrast ratio. With the 20� off-axis
design, the whole field of view can be covered without significant
aberrations. Moreover, a fast f/5.6 ratio was adopted to allow
detection of the cometary nucleus and of the asteroids from a distance
of 106 km in 1 s exposure time. The characteristics of the optical
solution are summarised in TABLE 4.
6.2 Two-mirror off-axis system
A sketch of the WAC optical design is shown in Figure 10 and described
in more detailed in Naletto et al., 2002. The primary mirror (M1)
collects the light from the object at an angle of 20� with respect to
the camera axis and reflects it towards the secondary mirror (M2),
which focuses the light onto the focal plane assembly. In contrast to
the majority of all-reflective cameras, in which the instrument stop
is located on the primary mirror, the WAC stop coincides with M2. The
mirrors were produced and polished by Officine Galileo. Figure 11
shows the flight mirrors prior to their assembly. To assure a good
reflectivity over the whole spectral range, the mirrors were
aluminized and protected with MgF2.
The instrument design includes a set of 14 wedged filters, tilted at
an angle of 8.9� with respect to the central ray. The filters are
mounted on two wheels. The filter set comprises narrow and wide
bandpasses for observations of emission bands and lines, and of the
continuum. A 4 mm thick Suprasil anti-radiation plate was installed
directly above the CCD.
The sun angle is greatly variable when in orbit around the comet
nucleus. To protect the system from direct sun light, a movable front
door is positioned at the instrument entrance aperture. Moreover, to
reduce the stray light into the instrument, a rather complex baffle
system was realised (Debei et al., 2001; Brunello et al., 2000).
6.3 Optical performance
Figure 12 shows the spot diagrams at the centre, the edges, and the
corners of the FOV. The square boxes correspond to the pixel size. The
geometrical performance of the camera is optimised with residual
aberrations essentially lower than the pixel size. The diffraction
effect is negligible, with more than 90 % of the energy falling into a
single pixel. The optical performance is maintained essentially
unchanged from infinity down to almost 500 m, so that no refocusing
system is required.
The off-axis design produces slightly different scales in the image
plane: the scale in y is almost constant, 19.9 arcsec px-1, while the
scale in x direction varies from 20.9 arcsec px-1 to 21.8 arcsec px-1.
The acquired field width seen by the CCD is subsequently about 11.35�
in y and between 12.09� and 12.16� in x direction.
The nominal photometric aperture, that is the projection of the M2
stop onto the primary mirror, is circular with a radius of 12.5 mm.
However, the distortion causes the aperture to be slightly elliptical
and position dependent. The corresponding photometric distortion can
be removed during calibration of the images.
6.4 Structure of the camera
The lightweight, stiff structure is based on a closed box made of
aluminium alloy machined by electro-erosion. The optical bench ribs
are optimised to prevent noise induced through vibration and to
minimise vibration amplification at interfaces with mechanisms. For
thermo-structural stabilization, three kinematic mounting feet and an
external baffle are implemented. A truss structure was designed to
improve the thermal decoupling between the external baffle and the
optical bench, and to minimise the temperature gradient. The telescope
is covered by a thermal blanket, while the inner parts were painted
with electrical conductive black paint. The optic supports are made of
the same material as the optical bench to minimise distortion.
6.5 Thermal behaviour
The WAC thermal control system was designed for the operational
temperature range of the optical bench, 12 � 5�C. This requirement is
derived from the tolerances in the position of the telescope optical
elements.
The total electrical power dissipated into the camera is less than
2 W. The heat leak of the FPA is in the order of 1 W. With only 1 W to
dissipate, the camera should be thermally insulated from the
environment. Thus, the whole external surface was covered with MLI and
the WAC was mounted to the S/C with insulating feet made of a titanium
alloy (Figure 13). The strongest disturbances to the WAC thermal
control is due to the large optical aperture, an area of about
300 cm2, which can point towards many different thermal sources.
The external baffle is the most critical element of the WAC thermal
design. A trade-off analysis was performed. A glass reinforced epoxy
structure with absorber coating, thermally insulated from the camera,
was shown to be the best solution. To extend the operational
capability for sun incidence angles < 45�, a radiator was added in the
upper part of the baffle to reduce the heat flux to the camera. Below
45�, operation of the WAC is intermittent, with observational phases
until the allowed upper temperature limit is reached, followed by
phases of cooling with closed door.
The other important requirement for the camera thermal control system
is to provide a temperature greater than �40�C during the non-
operative phases. This is achieved using a heater dissipating 5 W.
6.6 Baffle system and stray light performance
The primary scientific requirement for the WAC is to be able to image
dust and gas in the proximity of the comet nucleus at heliocentric
distances from 3.25 AU to perihelion. The WAC baffle system has
therefore to perform two different functions. The first is to
attenuate at least 4 10�9 times (at the detector) the light coming
from any source, in particular the sun, at angles larger than 45� off-
axis, which is the nominal operational mode. The second is related to
the characteristic of the WAC optical design with the system stop
located at the second mirror M2. Light entering the system aperture
and reaching M1, not being collected by M2, acts as internal source of
stray light. The baffle has to attenuate this stray light contribution
by a factor of at least 10�3.
The baffle system is made of two main parts: the first with
rectangular cross section is localised in front of the M1 mirror and
has 17 vanes, the second is accommodated between M2 and the detector,
and has 4 deep vanes (see Figure 14).
6.7 Calibration lamps
The calibration lamps used in the WAC are manufactured by Welch Allyn
and are identical to the NAC lamps. They are flame-formed bulb lamps
with a colour temperature of 2410 K. Four lamps (two main and two
redundant) are mounted at the fifth vane of the external baffle. The
lamps illuminate the inside of the front door, which diffuse the light
into the optical path. Obviously, this illumination cannot provide a
flat field, but a reference illumination pattern monitoring the system
transmission.
7 Interference filters
Sets of 14 filters for the WAC, and 12 for the NAC, were selected. For
the WAC, the principal aim is to study the intensity of gas emissions
and dust-scattered sunlight as functions of position and viewing angle
in the vicinity of the nucleus. This is accomplished by centring
narrow bandpass filters on a set of emission lines with slightly
broader bandpass filters to measure the continuum.
The NAC filters will be used to characterise the reflectivity spectrum
of the nucleus surface over as wide a spectral range as possible, and
to focus in particular on some possible or likely absorption bands.
With no need for isolating narrow spectral features, the bandpasses
are generally wider than for the WAC, i.e. typically from 24 to
100 nm.
7.1 Selected filters
7.1.1 NAC bandpass filters
The selected filters for the NAC are shown in TABLE 5. The NAC filters
were optimised to provide a low resolution spectrum from 250 nm to
1 �m. The orange filter at 645 nm will allow a close comparison of the
results from comet 67P/Churyumov-Gerasimenko with those obtained from
comet 1P/Halley with HMC. A cluster of filters was placed in the
wavelength range between 800 nm and 1 �m to investigate possible
pyroxene and olivine absorptions. A neutral density filter was added
to reduce the photon flux from the comet in the event that very
bright, pure ice structures are revealed by activity near perihelion.
The neutral density filter can also be used if the shutter fails and
will be used for resolved observations of the Earth and Mars.
Clear filters of selected thicknesses were included to modify the
focus position so that the NAC remains in focus down to a distance of
just 1 km from the target. The thickness of each filter was chosen
individually (dependent upon wavelength and substrate refractive
index) to maintain the system in focus.
7.1.2 WAC bandpass filters
The selected filters for the WAC are shown in TABLE 6. Most of the
filters are narrow band filters to study gas and radical emissions.
The minimum filter bandwidth allowed by the f/5.6 optical design is
4 nm, because narrower bandpass filters would produce variations in
the transmitted wavelength over the field. Continuum filters were
incorporated to allow straightforward subtraction of the dust
continuum from images acquired in gas emission filters. Calculations
indicate that high signal-to-noise ratios in CN, OH, OI, and CS will
be easily achieved. Na, NH, and NH2 should be detectable in binned
data within 1.2 AU from the Sun. A broad-band R filter was included
for nucleus detection and mapping, in the event of failure of the NAC.
A green filter, identical to that in the NAC, was included for simple
cross-correlation of the data between NAC and WAC. No refocusing
capability is required for the WAC.
7.2 Orientation and properties
7.2.1 Materials and radiation tolerance
Radiation tests were performed to ensure that the performance of the
filters is not seriously degraded by cosmic ray damage during the 9
years in cruise. Many of the substrates are made of Suprasil, which is
known to be radiation hard, but some filters are Schott coloured
glasses to achieve a proper out-of-band blocking. Since too little was
known about the radiation hardness of such glasses, and unacceptable
damage levels could not be excluded, laboratory experiments with the
Uppsala tandem Van de Graaff accelerator were performed. A 2 MeV
proton beam was shot onto OG590, KG3, and Suprasil blanks to simulate
the solar proton exposure during the Rosetta cruise. The resulting
change in spectral transmission was measured (Possnert et al., 1999;
also Naletto et al. 2003).
Figure 15 illustrates the obtained results. It has to be noted that a
proton fluence of 1013 cm-2 exceeds the expected fluence for Rosetta
by almost two orders of magnitude. Other experiments using smaller
fluences or lower dose rates yielded much smaller effects, and the
general conclusion is that the expected damage levels are indeed
acceptable. Moreover, the figure shows that annealing at room
temperature causes a rapid recovery towards the initial transmission.
The experiments on Suprasil verified that no visible damage occurred
in this case.
7.2.2 Physical parameters
The filters are placed relatively close to the detector in the optical
path. This minimises the size of the filters but, because of the large
CCDs used by OSIRIS, the required aperture is still fairly large. The
required clear aperture is 37.5 37.5 mm2. The physical size of the
applied filters is therefore 40.0 40.0 mm2 with rounded corners. They
are wedged to reduce ghosts and are optimised for operation at +10�C.
7.2.3 NAC ghosts
The NAC suffers from a complex combination of ghost images due to
three transmission elements in front of the CCD: the filters, the
refocusing plate and the anti-radiation plate. Two types of ghosts may
be distinguished. The �narcissic� ghosts are caused by light reflected
from the CCD surface and back reflected from transmissive elements.
Filter ghosts are caused by two successive reflections from
transmissive elements.
The ghost images are out-of-focus replicas of the scientific image,
and the amount of defocus is different for each ghost image according
to the extra optical path travelled. For a point source, the diameter
of the ghost image increases with increasing optical path and so the
ghost intensity decreases. For extended objects however, such as the
comet nucleus, the integrated ghost intensity is independent of
defocus distance and equals the product of the two reflections
encountered.
In order to take advantage of cases where one ghost type is weaker
than the other, the two types are physically separated. This is
achieved by introducing a 4� tilt of the filter wheel, hence of
filters and refocusing plates, with respect to the optical axis,
sending filter ghosts to one side of the scientific beam and narcissic
ghosts to the other side. The slight dispersion effect introduced by
this tilt is compensated by the 10' wedge of the filters. Also, ghost
reflections from the refocusing plates are reduced by using
specialised plates for the UV, visible and IR ranges.
The most problematic ghost components are produced by the ARP and the
CCD. While efforts were made to reduce their reflectance over the
entire wavelength range, ghost performance is optimised in the
orange/red region, where other performance criteria (stray light
rejection, efficiency, etc.) are also optimal. In this region, ghost
intensity of less than 10-3 is required.
7.2.4 WAC narcissus ghosts
The peculiar orientation of the WAC filters with respect to the light
beam has to be emphasised. The beam incidence is not normal. The non-
wedged surface of the filter is parallel to the CCD plane which is
orthogonal to the camera optical axis. The angle between the optical
axis and the chief ray of the light beam is 8.7�. The thickest filter
side is towards the filter wheel rotation axis.
The thicknesses of the WAC filters were calculated to have the same
focus shift for all filters taking into account the focus shift
introduced by the Suprasil window (ARP).
Ghost minimization with suitable AR coatings for the WAC is even more
stringent than for the NAC because of the initial contrast
requirements. Analysis of the ghost images has shown that the
Secondary Narcissus ghost (SNgh) is the most intense one. This ghost
is produced by back-reflection of the beam from the CCD surface and
the outermost filter surface. The ratio of total SNgh over image
intensity depends on the actual filter and is between 0.16 (for the
worst case, NH filter) and 10�3 (for the best case, Green and R
filters).
8 Filter wheel assembly
The Filter Wheel Mechanism, FWM, positions the optical filters in
front of the CCD detectors with high accuracy. The assembly is
composed of
� a support structure
� a common shaft with two parallel filter wheels
� two stepper motors with gears (crown and pinion)
� position encoders and mechanical locking devices.
Figure 16 shows the fully assembled FWM. The mechanism provides the
space for 16 optical elements (6 filters and 2 refocusing elements per
wheel in the NAC, 7 filters and a hole per wheel in the WAC). The
selected filters for both cameras are described in section 7.1.
Each filter wheel is turned by a stepper motor to position a filter in
front of the CCD in less than 1 s (half wheel turn). All filters are
positioned with an accuracy of �135 �m (10 CCD pixels) relative to the
optical axis with �30 �m of repeatability (two pixels). This is
achieved by V-shaped Vespel cams, one on top of each filter, which are
locked by stationary V-shaped stainless steel springs attached to the
mechanism support.
8.1 Filter accommodation
Each wheel has eight square openings to accommodate the filters. The
filters are mounted in cover frames of aluminium alloy and further
positioned by elastic joints, which preclude damage to the filter�s
surface upon thermal expansion. In order to minimise light reflection,
all mechanism surfaces (except the gear teeth, the pinion and the
motor fixation) are finished in black.
8.2 Wheel drive mechanism
Due to the tight mass, power and timing allocations, titanium alloy
was selected for the central shaft while aluminium alloy is used for
the filter wheels and for the assembly support. The filter wheel
support has three mechanical interface points to the camera that allow
adjustment by shimming to obtain the required alignment to the optical
path.
The wheels are mounted to the central shaft by double ball bearings,
coupled back-to-back with 50 N pre-load. These space qualified
bearings are dry-lubricated by lead ion sputtering of the stainless
steel races. The wheels support the Vespel crown gear at one side. The
pinion on the motor shaft is made of stainless steel. The gap between
the pinion and the crown is adjusted to 50 �m, which is equivalent to
0.07o backlash in the wheel. SAGEM 11PP92 type stepper motors were
fabricated with redundant windings and were further modified to
provide a high holding torque of 3 N cm at a power of 10.5 W. Perfect
operation is obtained by a ramped step rate provided by the Mechanism
Controller Board.
8.3 Positioning accuracy and filter encoder
Motor movement is achieved by sequential activation of the 4 motor
phases where always two adjacent phases are simultaneously powered.
Each activation step moves the motors by one rotation step. A change
to the next filter position requires 27 motor steps in either
direction. Filter changes are completed in less than 1 s. As the
motors do not have permanent magnets (variable reluctance type), they
consequently do not have a holding force when not powered. A
mechanical locking device is required to keep the filter wheels in
place when a filter change is completed.
The filter selection is monitored by a binary system where the code is
given by 1 to 4 SmCo magnets beside each filter and a stationary set
of four reed switches. The field distribution of the magnets is
focussed towards the reed switches thus creating a well-defined
activation area.
9 Shutter mechanism
In each camera an electromechanical shutter in front of the CCD
controls the exposure. The shutter is designed to support exposure
times between 10 ms and > 100 s with a maximum repetition rate of 1 s-
1. Typical imaging might use exposure times of 100 ms and repetition
rates of one image every 7 s. The shutter is able to expose the 28 28
mm2 active area of the detector with uniformity of better than 1/500.
A total of 50 000 shutter operations is anticipated throughout the
mission.
The shutter comprises two blades travelling across the CCD parallel to
the CCD plane. They are each driven by four-bar mechanisms from
brushless dc motors (Figure 17). To determine exposure with high
accuracy, a customised encoder for each blade is mounted to the motor
shaft.
A position sensor at the final position verifies that the first blade
has completed its travel. A mechanical locking device locks the first
blade in open position until it is released by the second blade at the
end of travel, when the exposure is completed. The back-travel of both
blades is provided by springs.
The exposure time is precisely defined by the relative distance (e.g.
by the delay) between the moving blades. The exposure time can be any
multiple of 0.5 ms, 10 ms minimum.
9.1 Blade movement
The blades are moved in the direction of CCD columns with a constant
velocity of 1.3 m s-1. The blades are accelerated and decelerated by a
current waveform controlling the motors in 512 steps each at 8 bit
resolution. Figure 18 shows a typical waveform for the actuation of
the first blade, which is completed in 53 ms. The blade movement
across the CCD lasts 21.3 ms (or 96 px ms-1).
The blade velocity is measured by an optical encoder mounted on the
shaft of the motor. The encoding accuracy leads to a blade position
resolution of about 0.08 mm.
9.2 Performance verification
Uniform exposure across the CCD is achieved by constant blade velocity
passing the detector. In order to satisfy the long-term stability
requirements, a calibration scheme for the shutter blade movement was
established. The shutter movement is optimised by adapting the current
waveform for the motors by analysis of the encoder data. The Data
Processing Unit evaluates the encoder data onboard and generates an
optimised waveform in order to achieve uniform exposure of the CCD. A
shutter calibration cycle lasts approx. 15 min per camera and is
executed routinely once after each system initialization.
9.3 Shutter electronics
The shutter electronics controls the operation of the shutter
mechanism. As shown in Figure 19, it is split into a digital and an
analogue module. The boards are accommodated in the NAC and WAC CRB
box.
The digital module stores the current waveform data for both blades in
FIFOs. These FIFOs are loaded from the Data Processing Unit with the
actual waveform data. Updated waveforms can be calculated onboard or
received by telecommand. The waveforms for both blades can be
different. The digital module checks continuously the status of the
memory and the functionality of the mechanism. The electronics is
prepared to identify 11 different types of errors. If an error is
detected, the actual status is immediately reported to the Data
Processing Unit.
The analogue module is composed of a capacitor bank with associated
current switches and the circuitry to select the charge mode for the
capacitors. The capacitor bank is needed to feed the motors with a
peak power of 20 W during the acceleration and the deceleration phases
(10 ms each). Three different charge modes, e.g. fast, nominal and
slow mode, are implemented according to the desired shutter repetition
time.
9.4 Fail-safe mechanism
The fail-safe mechanism configures the shutter into a pseudo frame
transfer CCD mode in case an unrecoverable mechanism failure occurs.
It forces the first blade to cover one half of the CCD while the
second blade is blocked in the starting position. The open section is
then used for imaging. The acquired charge is rapidly shifted into the
covered section for intermediate storage and subsequent readout.
10 Front Door Mechanism
The Front Door Mechanism, FDM, is primarily designed to protect the
optical components inside the NAC and the WAC by a reclosable door.
The inner side of the door can be used for in-flight calibration in
combination with the calibration lamps. The mechanisms for the NAC and
WAC telescopes are identical with the exception of the shape of the
doors, as these are different in order to fit the entrance baffles of
the two cameras. As the front doors cover the field of view of the
cameras, the reliability of the entire subsystem during the mission�s
lifetime requires highest attention.
10.1 Requirements and design
The main functional and environmental constraints to the mechanism can
be identified and summarised as follows:
� the door has to prevent contamination of the internal surfaces of
the telescope
� single-point failure tolerance requires redundancy and the
ability to open the door permanently in the case if an irreversible
system failure occurs (fail-safe device)
� requirement to validate open and closed positions
� dynamic load during launch
� non-operational temperature range (-50 to +70�C) implies a design
for high differential thermal loads within the mechanisms.
The door mechanism is designed to maintain the moving door always
parallel to its closed position plane, thus avoiding direct exposure
of the inner surface to open space, to the sun, or to cometary dust
particles, because collected contaminants could be re-emitted into the
telescope once the door is returned to its closed position. The
parallel motion is achieved with two coupled cams that initially lift
the door followed by a rototranslation which completes the lift and
rotates the door. The shape of the two cams was designed in such a way
that both final positions (open and closed) are self-locking states,
so that no electrical power is required to maintain these positions,
even if the system is exposed to vibrations.
Figure 20 shows the main components of the mechanism generating the
movement of the door. The internal cam is activated by a stepper motor
with a step angle of 0.3� and a gearhead with a reduction ratio of
100:1. The combined motion is transferred to the door by an internal
shaft rigidly fixed to the coupling peg and to the supporting arm.
The actual position of the door can be determined from the number of
applied motor steps. Nevertheless, two ABB micro switches are employed
to identify the open and the closed positions for the housekeeping
monitoring. These switches are located on the external cam and are
activated by a disk that is fixed on the peg.
A preload of the door against the external baffle of the camera
improves the stiffness of the system composed of the sustaining arm,
the door and the external baffle. Potential damage to the baffle due
to vibrations of the door, especially during launch, is avoided by a
damping seal. Figure 21 shows the completed Front Door Mechanism.
10.2 Reliability
High reliability of the Front Door Mechanism for the extended lifetime
of the instrument is of utmost importance. The concept comprises not
only redundant drivers and motor windings, but also extensive safety
margins in the mechanical design. The latter includes particularly the
mechanical load during launch, the specific implementation of sliding
parts and, finally, decreased sensitivity to the long-term mission
environment.
Differential thermal expansion was taken into account by a number of
elastic elements which absorb thermally induced loads. The FDM is
covered with a thermal blanket that efficiently isolates the
structural parts of the mechanism from thermal paths to the
environment.
All moving parts must be coupled tightly together to make the arm
stiff enough to sustain the mechanical load during launch. Increased
bearing friction by adhesion or cold-welding phenomena must be
avoided. Therefore, an innovative lubricant coating has been applied,
which relies on the low friction properties of the MoS2, but is not
affected by its sensitivity to humidity. This so-called MoST coating
is a vacuum deposition of MoS2 in a matrix of titanium that preserves
the lubricant properties of the coating.
10.3 Fail-safe device
A fail-safe device is required beyond the general redundancy concept
to make the front door single-point failure tolerant should an
irreversible system failure ever occur. This fail-safe device would
open the door once and forever.
The device is located within the arm holding the door and, to make it
fully independent, is operated on an axis parallel to the cam axis. It
provides for a lifting of the door and a subsequent rotation of 90� by
preloaded springs.
The arm supporting the door has been divided into two parts which are
kept together by a locking slider. The lock can be released by a Shape
Memory Alloy actuator. Once the lock is released and the slider is
pulled away by a spring, the arm supporting the door is lifted-up by a
coaxial spring. A torsion spring finally rotates the door and keeps
the door in the open position. Figure 22 shows the released state of
the fail-safe mechanism.
A high preload of 70 N at the main spring was applied to overcome
adhesion or cold-welding phenomena which could appear between the
moving parts in the course of a long-term mission. Friction
coefficients for the moving parts were minimised also by a sputtered
MoST coating on the relevant surfaces of the arm and by a chromium
coating deposit on the slider.
11 Image acquisition system
Both cameras use identical image acquisition systems, consisting of
two separate subsystems: (1) the Focal Plane Assembly (FPA),
accommodating the CCD detector, the Sensor Head Board (SHB) with the
front end electronics, heaters, temperature sensors and radiation
shielding, and (2) the CCD Readout Box (CRB box), with the CCD Readout
Board (CRB), the Housekeeping Board (HKB), the CRB Power Converter
Module (PCM) and the Shutter Electronics (SHE). The subsystems are
about 50 cm apart and are interconnected by a cable of 62 lines.
11.1 Detector selection criteria
The detector is a key element of the OSIRIS cameras. Its format and
performance has major influence on the parameters of the optical
system. The pixel size determines the focal length for a defined
angular resolution, and the QE relates to the f-ratio. These
parameters strongly influence the dimensions of the optical systems. A
constraint on the detector selection was the requirement to select a
CCD device that needed only little further development for space
application saving cost, development time and risk.
The requirement of highest possible QE over the wavelength range from
250 nm to 1000 nm leads to the choice of backside illuminated CCDs. A
minimum pixel capacity of 10^5 electrons was considered as acceptable.
Low readout noise in the order of a few electrons per pixel was
required to achieve sufficient dynamic range in the image data.
Large CCDs of 2k 2k pixels have some drawbacks compared to smaller
devices. Foremost, they are more sensitive to Charge Transfer
Efficiency (CTE) degradations, which occur under high energy
irradiation in space. Therefore, tight shielding and the capability to
anneal defects at elevated temperatures up to +130�C were implemented
for OSIRIS. The storage temperature of the detectors during cruise is
kept near room temperature. A further drawback of the large CCD format
is the increased readout time. Two readout amplifiers cut this
interval in half and also provide required redundancy.
11.2 OSIRIS CCDs
The OSIRIS CCD design is based on the commercially available, backside
illuminated non-MPP E2V CCD42-40 devices with 2 output channels. These
CCDs feature the desired pixel size of 13.5 �m sq. and excellent wide-
band QE. High dynamic range and low power consumption make them well
suited for space applications. The CCD performance characteristics are
summarised in TABLE 7.
The non-MPP clocking register technique yields high full well capacity
but also high dark charge generation. Since the dark charge is almost
negligible at the in-flight operational temperature range of 160 to
180 K, the OSIRIS CCD takes advantage primarily of the enhanced charge
capacity.
An innovation for the OSIRIS devices was the introduction of lateral
(shielded) anti-blooming overflow protection, so that weak cometary
features can be imaged near bright regions in long duration exposures.
The lateral anti-blooming keeps the entire pixel area light-sensitive
so that the QE is not affected. Nevertheless, the full well charge
capacity is reduced by the anti-blooming from 140,000 e- to about
100,000 e- per pixel.
Dark current becomes a significant component at temperatures above
230 K. Therefore, during the device evaluations at room temperature,
full pixel wide clock dithering at a fixed rate of 80 �s/cycle was
applied, yielding typical dark charge reduction rates by a factor of
up to 15. With the help of such dithering, useful images could be
obtained up to exposure times of 40 s. Below 230 K, clock dithering is
no longer useful, because spurious charge becomes dominant (Kramm et
al., 2000; 2004).
11.3 Detector packaging concept
With high thermal insulation between the CCD substrate and the focal
plane housing, the CCD can be operated either at low temperatures
(down to 160 K) or can be heated up to 400 K to anneal radiation
defects. A special detector packaging was required therefore to
provide sufficient thermal insulation.
The CCD substrate die is glued to a 10 mm Invar carrier plate that is
attached to the housing structure with perfect thermal insulation by
two stages of three glass spheres to mount the device. Cooling (by a
thermal radiator) and (electrical) heating is applied from the back
side of the Invar plate. The Invar plate keeps the detector flat to
< 10 �m and also provides shielding against irradiation.
The electrical interface is obtained via a small ceramic interface
board mounted to the Invar plate. On its top side, the board provides
32 gold plated bond pads which are aligned with the substrate bond
pads on the CCD. A flex circuit connects the CCD to the SHB through
gold plated contacts on the ceramics.
11.4 Readout concept and image formats
In stand-by mode, the CCD is continuously clocked at a moderate rate
of about 3 ms/line. Prior to an exposure, the CCD is entirely cleared
by a fast vertical dump of 25 �s/line. The exposure is started by the
shutter opening and completed by the movement of second shutter blade.
Full frame or sub-frame (window) images can be read with or without
binning and via either one or both channels. If both channels are used
to read a sub-frame of the CCD, the centre of the sub-frame must be
aligned to the centre axis of the CCD. Binning formats of 2 2, 4 4
and 8 8 pixels are supported.
11.5 Sensor Head Boards
The sensor head provides only limited space for electronics. As a
consequence, the front end electronics accommodates just the
preamplifiers and protection circuitry. This concept minimises the
power dissipation in the FPA section and thus protects the CCD
detector from heating up. The total power loss in the FPA section is
made up of approximately 90 mW within the CCD substrate and about
270 mW on the sensor head board.
11.6 CCD Readout Boards
Each CCD readout board contains two complete signal chains, including
line receiver, the correlated double-sampling (by clamping), the
buffer amplifier and analogue-to-digital converter (ADC). All clock
signals are provided by an ACTEL 1280 Field Programmable Gate Arrays
(FPGA). A second FPGA handles the high speed serial (LVDS) data link
to the DPU.
The implementation of the ADC section was complex because the required
data resolution of better than 14 bit could not be achieved with a
single ADC in each channel. A sub-ranging technique was applied using
two simultaneously operated 14-bit ADCs in each channel, one for the
range up to 50,000 e- and the second covering the range up to
200,000 e-. The criterion for the selection of the conversion result
is based on the occurrence of an overflow at the low range converter.
We selected the LTC1419 type ADC because it was found to be radiation
resistant to at least 50 krad (Tomasch et al., 2000).
The entire readout electronics could be accommodated on one board with
the help of four different types of thick film hybrid circuits
especially designed for OSIRIS to save both, space and power (Figure
23).
About 1.6 W are sufficient for stand-by operation, and 3.2 W are
required during readout.
11.7 Housekeeping Boards
The operating conditions of the CCD, the FPA and the CCD Readout Board
are monitored by the Housekeeping Board. It measures the voltage and
current on all six power lines, and determines the temperatures of the
CCD, the Shutter actuators, the ADCs and the related PCBs.
Furthermore, the HKB reads the dosimeter-FET that was implemented to
track the camera total ionizing dose. All housekeeping data are
incorporated in the image header. The HKB requires less than 190 mW.
11.8 FPA mechanical design and thermal control
As shown in Figure 24, the Invar plate is thermally insulated by two
concentric groups of 3 glass spheres. A titanium ring in-between holds
the glass spheres in place and serves as an additional insulator. The
thermal resistance between Invar plate and FPA housing is in the order
of 500 K/W.
A cold finger connects the Invar plate to a radiator for passive
cooling. The thermal conductivity of the cold finger is a trade-off
between two contradictory requirements. Operating the CCD requires low
gradients between the radiator and the CCD. On the other hand,
annealing the CCD with limited heater power requires a considerably
higher gradient on this path. Developing a �thermal switch� between
the radiator and the CCD would have been an elegant solution but was
judged to be a technological risk and thus was rejected. The
compromise is a cold finger consisting of 45 single aluminium sheets
of 0.1 mm thickness. The temperature gradient between the radiator and
CCD during operational conditions is in the order of only 1 K. The
lightweight radiator is mechanically mounted to the FPA housing by a
supporting structure of 7 GFRP (glass fibre reinforced plastic) tubes
(Figure 25).
The total conductive and radiative heat input onto the cold parts from
the warm FPA housing, from the telescope in front of the CCD and from
the SHB electronics sums up to nearly 1 W. To reach the nominal
operating temperature of 160 K, a radiator surface of 430 cm2 is
needed. The radiator has a view factor of nearly 2p sr to deep space.
Its outer surface is painted with a conductive white paint having a
low solar absorption coefficient a and a high infrared emissivity e.
The low a helps to reduce the heat input and temperature changes of
the CCD during asteroid flyby phases, when sun incidence on the
radiator cannot be avoided.
The CCD temperature is passively controlled by the efficiency of the
radiator, but it can be adjusted electrically by 4 heaters mounted to
the cold inner parts. A 0.5 W heater is used to adjust the operating
temperature to levels higher than defined by the passive cooling. Two
3 W heaters are powered by the spacecraft during non-operational
phases to keep the CCD at elevated temperatures and minimise radiation
damage effects. A 25 W heater is installed to decontaminate and anneal
the CCD.
11.9 Radiation protection
Solar protons are the population dominating the radiation effects to
the CCD in the inner heliosphere. They generate lattice defects at a
sensitivity threshold of a proton fluence of 108 cm-2 or a dose
equivalent of 100 rad (Holland et al., 1990). Passive shielding of the
CCD was implemented in both cameras to ensure high performance at
maximum solar activity after 9 years cruise.
Dense absorbers and a dedicated quartz window plus the optical filters
in front of the CCD reduce significantly the number of incident
protons. However, radiation shielding to 20 g cm-2 or 150 MeV kinetic
energy is required for appropriate reduction of the proton fluence for
the Rosetta mission.
Mass and volume restrictions do not allow full implementation of
shielding. Additional measures were taken to ensure a CTI (Charge
Transfer Inefficiency) at levels better than 2 10-5. Raising the CCD
temperature to near room temperature during cruise reduces
significantly the damages by long-term annealing (Hopkinson, 1989).
Additional annealing at elevated temperature up to +130�C will remove
accumulated degeneration up to 85 % (Abbey et al., 1991) but is
limited due to high mechanical risks and stress at the CCD.
Thus, the OSIRIS CCD radiation protection is designed to an
accumulated proton fluence of 109 cm-2 with passive shielding to
minimum of 5 g cm-2 or 70 MeV equivalent. Heaters ensure long-term and
high-temperature annealing during mission and stacking of low and high-
z absorbers along the particle track reduces x-ray and neutron
generation inside the shielding (Dale, 1993). The nominal CCD
operational temperature (160 K) will freeze out traps caused by
residual lattice defects and enlarges the emission time constant. CTI
verification in flight will allow real-time determination of defects.
Radiation effects were studied on two OSIRIS CCD samples that were
exposed to 10 MeV and 60 MeV proton irradiation up to a fluence of
2 1010 cm-2. We found that long-term room temperature annealing
significantly reduces the increased dark charge leakage, while high
temperature annealing is particularly suitable to cure degraded CTE
(Kramm et al., 2002).
12 Data Processing Unit
12.1 Architectural design
The main driver for the design of the OSIRIS Data Processing Unit
(DPU) is the need to control camera operations as well as to acquire
and process image data from the two rather large CCD arrays. This made
a Digital Signal Processor desirable. The DPU is based on a
development by ESA/TOS using the radiation-hard version of the Analog
Devices ADSP21020.
Because of power limitations, the speed of the processor had to be
substantially reduced. A large local memory was implemented so that
extensive processing tasks could be executed by batch processing
rather than in real-time. This scheme does not impose a significant
limitation to the operations, because the data transmission rate is
the most restricting requirement.
The DPU architecture is driven by the following requirements:
� high data rate from both cameras operated simultaneously (up to
40 Mbit s-1)
� output data rate (to S/C mass memory) limited to 10 Mbit s-1
� support of �movie� operation (up to 64 images with ~ 1 s image
repetition time)
� single-point failure tolerance
� limited resources of mass and power
� decision to base the architecture on an existing TSC21020
processor board.
The discrepancy between the input and output data rate necessitated a
DPU internal storage of minimum one WAC and one NAC image. The data
transfer rate to ground is limited to 100 Mbit/day. Image transfer
rates can be increased by image compression at the expense of image
fidelity. State-of-the-art (lossy) wavelet compression with a
compression ratio between 4 and 8 provides the best balance between
image degradation and a substantially increased image count. For the
observation of dynamic events, the optimum balance is expected to
require even higher compression factors.
Compression can be performed either in real-time (in step with the
incoming camera data) or by intermediate storage and subsequent
processing of the stored raw data. The latter was implemented because
the limited bandwidth of the telemetry restricts the time for image
acquisition but leaves time for S/W compression.
12.2 Implementation
The DPU block diagram is shown in Figure 26. The DPU consists of four
elements each mounted in one mechanical frame of the E-Box:
� Main Processing Element (PE), consisting of the processor board
with the DSP, local memory, spacecraft interface, and internal
IEEE 1355 interfaces, and a memory extension board
� Redundant Processing Element in cold redundancy
� Mass Memory Board (MMB), containing 4 Gbit of image memory,
control logic, and IEEE 1355 interfaces
� DPU Interface Board (DIB), with the power-up logic, the
interfaces to the NAC CRB, WAC CRB, MMB, MCB, PCM, and IEEE 1355
interfaces.
An overview of the DPU performance characteristics is given in TABLE
8. The unfolded DPU flight unit in a test configuration with a
prototype of the Mass Memory Board is shown in Figure 27.
12.3 Processing Element
The two Processing Elements of the DPU are identical and cold
redundant. The activation of the selected PE is done during power-up
of the DPU by the telemetry sample signal.
Each PE is implemented on two printed circuit boards and provides:
� TSC21020F 32-bit floating point digital signal processor clocked
at 20 MHz
� 1.5 Mbyte of zero wait state program memory
� 512 kbyte of zero wait state data memory
� 16 Mbyte image memory on the extension board
� 1.5 Mbyte E2PROM for non-volatile program and data storage
� 128 kbyte E2PROM for non-volatile parameter storage
� 32 kbyte dual port communication memory
� 3 IEEE 1355 high-speed communication links
� SMCS332 communication controller
� TM/TC interface controller.
The processing performance of the DPU allows wavelet image compression
of a 2k 2k image in less than 50 s at a compression ratio of c = 12
and in less than 30 s of a 1024 1024 image at c = 4 by using an
optimised assembler code (Christen et al., 2000).
12.4 Mass Memory Board
The MMB provides a user capacity of 4 Gbit, which is implemented in
4M4 DRAMs, 4-high stacks of 64 Mbit capacity each. The memory is
protected by an extended (80, 64) Reed-Solomon code against single 4-
bit wide symbol errors. The array is split into 2 partitions. A
partition is composed of 8 word groups, each of them containing
4 Mword of 80 bit, which are accommodated in twenty DRAM chips.
The word length of 80 bits is structured into 16 data symbols and 4
parity symbols. The code is capable of correcting errors in one symbol
of 4 adjacent bits and of detecting errors in 2 of such symbols
(Fichna et al., 1998). The address management provides logical
addressing of the configured memory space in terms of active word
groups, start address and block length.
Data integrity is provided by background scrubbing for SEU (Single
Event Upset) error removal. Overcurrent sensors and circuit breakers
in the supply lines protect each partition individually from Single
Event Latch-up (SEL) events.
The MMB has dual redundant communication interfaces and two
independent partitions. In case of a failure in one partition, this
partition can be switched off (graceful degradation). The processor
workspace can store and process a complete raw image in absence of
both partitions of the MMB. The MMB communicates via two differential
IEEE 1355 serial interface links, each providing simultaneous data
transfer of 38 Mbit s-1 in both directions. All peripheral functions
of the module are accommodated in Actel 1280 FPGAs. Each FPGA includes
functions for enhanced SEU tolerance.
12.5 Redundancy Concept
The DIB is split in two identical parts interfacing the cameras, DIB A
for the NAC and DIB B for the WAC. In consequence, the two camera
chains (CCD detector - readout electronics - DIB) are redundant, as
the cameras are. The Processing Elements containing the processor,
local memory, and glue logic are cold redundant. The two PEs and the
two detector chains are cross-strapped using the capability of the
IEEE 1355 ASICs to accept three interfaces.
13 Mechanism Controller Board
The Mechanism Controller Board, MCB, drives the motors at the front
doors and at the two filter wheels in both cameras. The four-phase
dual-step, variable reluctance stepper motors were built with
redundant windings which can be powered alternatively or in parallel.
The MCB also acquires housekeeping data from the position encoders on
the front doors and filter wheels and from the temperature sensors of
the two cameras.
13.1 Description of the Mechanism Controller Board
The MCB consists of two boards, the Control Board and the Drivers
Board, both mounted into the E-Box MCB frame (Figure 28). They are
interconnected via two flexible, low profile boards from Nicolitch
(Sferflex Technology). Connections to the cameras are provided via 62-
pin connectors. Communication with the DPU is established via RS-422
type line drivers and receivers.
The Control Board hosts the digital circuitry for communication and
mechanism control and the analogue housekeeping acquisition. The
digital functions are concentrated in two FPGAs performing the command
decoding, the data collection, the packaging and the data
transmission. They also translate the DPU mechanism commands into
motor phase pulses that are transferred to the Drivers Board. Full
dual redundancy was established for the line drivers and for the
digital and analogue conversion circuitry. The main and the redundant
analogue data acquisition modules can read the temperature sensors
regardless of which stage is in use. Similarly, both main and
redundant modules can read the position encoders of all mechanisms.
The Drivers Board accommodates the motor drivers. As each camera unit
contains three motors, 48 line drivers are required to feed the main
and the redundant phases of all motors. Driving the motors directly
from the +28 V S/C power rail requires electrical isolation of the
motor switches from the remaining electronics by optocouplers.
13.2 Controller FPGA description
The Controller FPGA establishes the communication, the command
decoding and execution, the housekeeping data acquisition and the
reset functions.
The communication link to the DPU uses standard RS-422 interfaces, one
in each direction. Each transferred byte is 8 bit wide with 1 start
bit, parity even and 1 stop bit. A communication packet consists of
one command or packet identification byte and of an appropriate number
of parameter bytes (0 to 5 bytes on receiving, 3 to 87 bytes when
transmitting). Each packet is finally terminated by a checksum byte.
The command decoder interprets the received commands. Commands are
related either to the data acquisition, to the stepper controllers or
to the parameters of the 6 motors, i.e. phases and step pattern. Each
command is approved for coherence and possible transmission errors in
parity, frame or checksum. Motor control commands are forwarded to the
Controller FPGA. Invalid commands are rejected, and an error flag is
returned in the status word.
The MCB recognises two types of reset signals, the (internal) power-on
reset and the system reset command from the DPU. Both resets re-
initiate the MCB to the default parameter set.
13.3 Stepper FPGA description
The Stepper FPGA consists of three blocks,
� controller block containing two independent controllers to allow
simultaneous operation of two motors with independent parameters
� motors block providing the generation of phases to the motors,
with the option to define the initial phase of movement
� communication block for the interface to the controller FPGA.
The program repertory includes parameters for the minimum (first and
last step) velocity, for the maximum velocity, for the acceleration
and the deceleration ramps and for the total number of required steps.
An example for a nominal ramping profile for a single filter change is
provided in Figure 29.
The stepper motor controller keeps the final phase powered for a
holding time of 463 ms to obtain high position accuracy in
deceleration. Another important functionality of the MCB is the warm-
up of the motors and its mechanisms by powering individual or dual
windings. The power command in heating mode must be repeated once a
second until the envisaged heating effect is achieved.
14 Power Converter Module
The Power Converter Module, PCM, provides power for the OSIRIS
instrument. High conversion efficiency is achieved with regulated
switching DC/DC converter technique. Unused instrument subsystems can
be switched-off by solid state relays. Distribution of noise is
substantially reduced by filtering and isolation.
The initial idea was to use a central power converter unit for the
entire instrument. During the development the power conversion tasks
were split into a main PCM and small dedicated converters close to the
CCD Readout Boards to reduce the risk of noise pick-up. Thus, the PCM
is comprised of a module in the lower compartment of the E-Box, and
the NAC CRB PCM and the WAC CRB PCM, which are located in the
respective CRB boxes. E-Box PCM to DPU communication is provided via
standard bi-directional RS-422 serial interface links.
14.1 E-Box PCM
The E-Box PCM is accommodated on two boards of 190 190 mm2, namely
the Power Control Board and the Power Distribution Board (Figure 30).
The Power Control Board hosts the digital circuitry that performs
primarily the following tasks:
� execution of commands from the DPU, e.g. the distribution scheme
for primary and secondary power or requesting housekeeping data,
supported by redundant microcontrollers
� generation of control signals to the switches and latching relays
on the Power Distribution Board
� collection and transmission of housekeeping data from the
Distribution Board
� determination of the primary current limitation threshold. The
threshold can be modified according to actual operational modes.
The Power Distribution Board contains the power units and analogue
circuitry:
� main and redundant DC/DC power converters to feed the DPU, the
MCB, the PCM, and other consumers such as lamps, heaters etc.
� power distribution as requested from the Control Board
� acquisition of associated housekeeping data.
The Power Distribution Board receives the main and redundant primary
power lines through separate connectors. The entire power conversion
stage, consisting of the EMI filter, the inrush control circuitry and
the DC/DC converters, is established with full redundancy. The
following power distribution stage (including the inrush current
control as well as the slope and delay control of the secondary
voltages) distributes the secondary power to the shutter electronics,
to the MCB and other consumers. Solid state relays and optocouplers
are used where necessary.
14.2 PCM software
The onboard software for the Power Converter Module is stored in an
8 kbyte wide PROM. The PCM software has the following autonomous
functions:
� supervision of the state of the non-active (redundant)
microcontroller
� detection of a primary current limit violation
� detection of a housekeeping limit violation
� detection of Single-Event Upset.
Limit overruns are reported to the DPU and, if necessary, endangered
subsystems are disconnected autonomously by the PCM onboard software.
14.3 CRB power converters
The two CRB Power Converter Modules (Figure 31) are located in the CRB
boxes. They provide six supply voltages for the detector electronics,
the housekeeping board and operational heater at the CCD.
The CRB power converters provide electrical isolation from the S/C
power bus. They are based on a regulated switching DC/DC converter
with high efficiency. Minimisation of conducted EMC, emission and
susceptibility is achieved by EMI filters and snubbers at the
switching stage of the converter.
Due to the sensitivity of the analogue signal chain to external noise,
converter switching was synchronized with the pixel readout. The CCD
Readout Board provides the synchronization signal to the converter.
The phase of this so-called �Sync Pos� signal can be shifted by one of
32 possible incremental steps relative to the pixel readout period to
achieve operation with lowest noise pick-up. If, however, the
synchronization signal is not available, the CRB power converter
operates in free-running mode. In nominal operation, this condition
occurs only during power-on when the circuitry for synchronization
signal is not yet settled.
15 Onboard software
The OSIRIS flight software is composed of three sections:
� Kernel software
� OSIRIS UDP library
� OSIRIS scientific library.
15.1 Kernel software
The kernel software provides the low level functionality of OSIRIS,
e.g. the TM/TC interface and the hardware drivers. A novel concept of
onboard operational procedures was implemented for instrument control,
image acquisition and process sequencing. The OSIRIS Command Language
(OCL) is a systematic approach to generate or to adapt on-board
application software for instruments with varying operational profiles
during mission. It continues and extends previous approaches, as e.g.
applied in SOHO/SUMER by Kayser-Threde (Birk, 1992). OCL comprises of
a middleware-system for (1) application layer functions with
operational sequences in a high-level language (so-called User Defined
Programs, UDPs, and Persistent Operational Programs, POPs), (2) upload
of UDPs, and (3) on-board script execution, supported by virtual
machines which interpret the precompiled scripts. Software integrity
is supported by on-board checks as well as by language inherent
features.
The architecture of the OSIRIS OCL system is shown in Figure 32. It
consists of a space segment with the UDP manager and virtual machines,
and a ground segment with a compiler to generate UDP token code
including a translator to convert the code to telecommands (TC). The
UDP manager handles the UDP code onboard. The token code can be stored
in memory or transferred as POPs to non-volatile RAM. UDPs are invoked
either by the UDP manager directly or via a timeline.
The structure of the OSIRIS flight software is outlined in Figure 33.
The UDPs represent the upper layers (application layers). OCL is
located at the mid-layer (4) in the SW architecture.
The major functionalities of the five software layers are:
� Level 4: the token interpreter executes POPs and User Defined
Programs. UDPs can call functions in the low level kernel software
(the run time library) or call other UDPs. The UDPs are executed from
ground using an execute-by-name scheme (with parameters)
� Level 3 contains library functions such as image acquisition and
basic image processing. It contains a library of image evaluation
functions, like histogram, sub-framing, binning, bright point
determination, and the SPIHT (Set Partitioning In Hierarchical Trees)
wavelet based image compressor supporting lossy and lossless
compression
� Level 2 interfaces to the Run Time Operation System, RTOS, and
the driver software. It consists of service functions to the serial
devices of NAC, WAC, MCB and PCM and to the MMB. Telemetry,
telecommand and the IEEE 1355 interfaces to the SSMM are served. All
software interfaces above this level are hardware independent
� Levels 0 and 1 interface the H/W of the DPU with the next higher
S/W level. Level 0 consists of hardware descriptions, like address,
port, and data definitions. Level 1 is shared by the RTOS Virtuoso
(Eonic Systems, 2000) and driver software. RTOS interacts with
processor devices. The drivers serve dedicated hardware. The boot
loader program can load program data from the internal E2PROM or from
the S/C mass memory via the IEEE 1355 interface.
The kernel modules and the data flow are shown in Figure 34.
Interconnections between the modules are made in different ways,
depending on data and command flow. Data driven program parts use
Virtuoso mailboxes (Eonic Systems, 2000), Virtuoso FIFOs (on a single
word base), as well as Virtuoso events (one-bit information used by
driver software) to communicate with other modules.
Maintainability of the OSIRIS DPU software is guaranteed by a typical
set of programming conventions (naming conventions, header format and
commenting rules, etc.), revision control and back-up strategies.
15.2 OSIRIS UDP library
The OSIRIS UDP library is a collection of routines written in OCL
language. The library provides an interface between the users of
OSIRIS and the hardware specific details required by the kernel
software. The following modules exist:
� Management of the parameter table. This module handles the
modification and dump of the parameter table via the TM/TC interface
and persistent storage of the parameters in the OSIRIS non-volatile
memory.
� The resource control module protects resources to be accessed by
parallel running UDPs.
� OSIRIS image and data memory is accessed via the memory
management module.
� A telemetry handling module implements the high-level protocol
for message events, generic data dumps and image data transfer.
� Downlink is managed by a module providing downlink prioritization
via a number of queues
� The hardware abstraction layer allows the user of OSIRIS to
command the hardware modules using logical parameters, e.g. move
filter wheel to a specified position instead of turn filter wheel a
number of steps in a given direction, and provides calibration of all
analogue HK channels.
� Health monitoring is provided by real-time monitoring of all
components of OSIRIS and will safeguard the instrument in case of
anomalies. All currents, voltages, and temperatures are monitored as
well as the radiation environment via data from the SREM radiation
monitor.
� A module providing self test and performance tuning functions.
15.3 OSIRIS scientific library
The OSIRIS scientific library is a collection of UDPs that implement
complex scientific observations as single commands. Various multi-
spectral sequences are implemented where full spectral cubes can be
acquired. The point of the scientific UDP library is that the library
can easily be expanded to serve future needs of both simple serial
activities and highly complex activities requiring onboard
intelligence and data processing. An example of a proposed UDP is an
onboard cometary outburst detector that periodically acquires images
and only downlinks the data if brightening of the comet is detected.
16 EGSE and telemetry
16.1 EGSE and associated software
The OSIRIS EGSE consists of a standard PC equipped with subsystem
simulators providing all interfaces for ground testing as well as for
flight operations (Figure 35). Hence, the EGSE supports the entire
instrument development and maintenance and also provides quick-look
presentations and health monitoring.
The operating software is based on the software package GSEOS V
running on Windows XP. GSEOS V supports the tests of the instrument
under near-real-time conditions. A data-driven concept is used instead
of less efficient polling. GSEOS V is configured for OSIRIS using the
built-in G-compiler, which is based on the C language and is enhanced
in some properties to support the data-driven concept.
GSEOS V provides numerous functional modules:
� Commands can be sent to the instrument directly or via network.
Manual, time-tagged or event-driven commands are accepted.
� Instrument data are grouped into blocks (e.g. science or
housekeeping data). Blocks are user-defined structures on bit level.
Data processing is data driven; if a data block is received, the
decoder module calls the related, user defined function for
processing.
� Instrument data can be displayed in various formats (hex,
decimal, text, bitmap, histogram, plot).
� Data can be checked and evaluated. If a limit is exceeded, a user-
defined reaction can be activated (e.g. power off, print of an error
log).
� Command, status and data protocol logging.
� Incoming and outgoing data can be saved on disk and can be
replayed.
� The EGSE can be connected to the network on IP level. All program
functions are accessible via network. Special communication protocols
(e.g. CCSDS, SFDU) are implemented.
16.2 Telemetry conversion
The DPU acquires the image and associated housekeeping data, performs
pre-processing (compression) if required and provides the packaging
for the telemetry. The information about the processing steps is
attached to the telemetry so that the data can be reconstructed on
ground.
When the telemetry data are received on ground, the images and their
headers are extracted and recovered from the data stream. The binary
telemetry header is converted to ASCII format and finally stored with
the image data in individual files.
The data archiving is organised by the OSIRIS software written in IDL.
A software library retains routines for reading, writing, and
processing the OSIRIS image data. The files are stored in eXternal
Data Representation (XDR) format. PDS (Planetary Data System)
formatted data are generated as well.
17 Calibration
17.1 Ground calibration
The OSIRIS system was calibrated in several stages. Prior to final
system integration, the NAC underwent a series of unit tests at LAM in
Marseille to verify focus, thermal stability, stray light performance,
geometric and absolute calibration. At UPD, the WAC underwent a series
of focus tests. After mating with the flight electronics at MPS, a
further series of calibration tests was performed at system level
(Figure 36): focus, flat-fielding, spectral response (including
temperature dependence), geometric distortion, as well as electrical
offset and system gain of the CCD readout chain were successfully
tested. More than 300 Gbyte of data were acquired with the OSIRIS
system at MPS.
17.2 In-flight calibration concept
The OSIRIS instrument is required to operate over a period of 11 years
in a harsh radiation environment. The geometric distortion of the
cameras needs to be re-measured in flight because it might have
changed during launch. The relative alignment of the cameras can be
affected by the change from the 1 g ground environment to zero gravity
in space. It is certain that the detectors will be affected by
energetic particles during cruise leading to changes in the detector
dark current and charge transfer efficiency. The optics might be
sputtered by energetic particles and the primary mirrors might
eventually further deteriorate because of dust particle impacts.
Pinholes might be created in the filters. Accurate calibration of the
image data can therefore only be obtained if additional in-flight
calibration is performed. The approach to in-flight calibration is
summarised by TABLE 9.
Dark current images are potentially costly in terms of data volume,
but must be acquired to reveal detector radiation damage. Ground-based
distortion maps are adopted as a baseline for optical correction, but
require verification by observation of star fields. This is fairly
straightforward using, e.g., fields studied by Landolt (1992). The co-
alignment of the cameras can be determined by observation of star
fields and of specific stars. Those are the same photometric standards
which provide point spread function (PSF) and spectral response
calibration. A list of photometric standards for OSIRIS is given in
TABLE 10. Relevant data can be found in Hamuy et al. (1992; 1994),
Bessell (1999), and the HST CalSpec data base (HST CalSpec, 2005).
The most complicated task is flat-fielding. To facilitate this, both
cameras carry main and redundant calibration lamps which can be turned
on to illuminate the front doors when they are closed. The
disadvantage of this approach is that the illumination of the
telescope aperture is not uniform, neither in the NAC nor in the WAC.
Therefore, only changes in the flat field between ground testing and
in-flight can be determined. A complicating factor is that the
calibration lamps have a limited lifetime. Hence, the main lamps will
be cross-calibrated against the redundant lamps on an occasional basis
(with the redundant lamps otherwise left off) to compensate for
deterioration over time. Attempts will also be made to check the flat-
fields by taking long exposures of the cometary nucleus during
spacecraft slews.
The first images taken with the OSIRIS cameras are shown in Figure 37
and Figure 38. These are �random� star fields (i.e. without requesting
specific spacecraft pointing) which showed that the performance of
OSIRIS lives up to expectations. A more detailed discussion of
calibration issues is the topic of a dedicated paper.
18 Operations
OSIRIS is operated via the Rosetta spacecraft instrument timeline
(ITL). The timeline allows commands to be executed at a specified time
or relative to timeline events (for example: time of closest approach
to Earth). ITL command sequences are transferred to the instrument
using Orbiter Operational Requests.
OSIRIS supports the use of high-level UDPs (see onboard software
section). UDP commands can initiate a single image acquisition as well
as complex observational scenarios. Hence, UDPs represent a toolbox
that can be used for scientific observations. Approved UDPs have a
higher reliability than dispatching a bulk of single commands from the
mission timeline. It has been suggested therefore to implement
substantial observation tasks, for example for the asteroid fly-bys,
by single UDPs. Commanding via UDPs definitely will be the preferred
way to operate the OSIRIS cameras.
It is envisioned that the science team will develop and implement UDPs
for it�s specific scientific investigations. These new UDPs can be
easily merged into the DPU after validation on the ground reference
model of OSIRIS.
19 Conclusions
There were considerable difficulties during the selection phase of the
scientific cameras of the Rosetta mission. However, the OSIRIS design
concept which was finally approved promised an instrument which would
provide an outstanding scientific return. Many strict requirements
were placed on the system during the design phase (e.g. operational
lifetime, stray light, shutter accuracy, filter wheel speed, CCD
readout rate). Most of these were achieved with only modest reduction
of requirements in one or two areas where technical constraints
demanded. The data to be returned by OSIRIS will provide a
comprehensive survey of the nucleus of comet 67P/Churyumov-Gerasimenko
and the surrounding dust and gas coma. OSIRIS will also make a major
contribution to asteroid science through its multi-spectral
capability. Its design is such that even in 2014 a superior system
would be difficult to build.
Two examples of the quality of OSIRIS images are presented in Figure
39, the Orion nebula M42 obtained with the NAC in commissioning, and
Figure 40, a glimpse back on Earth and Moon, acquired during the
Rosetta pointing campaign from a distance of nearly 0.5 AU.
20 Acknowledgements
The support of the national funding agencies of Germany (DLR), France
(CNES), Italy (ASI), Sweden (SNSB), and Spain (MEC) is gratefully
acknowledged. Substantial support for the development of the Data
Processing Unit was provided by the ESA Technical Directorate through
the Technical Research Programme.
In addition to the formal co-authors of this paper (comprising lead
scientists, Co-Is, project managers, and lead engineers), the project
was supported by an enormous number of scientists, engineers, and
technicians involved in the day-to-day development of the hardware.
These include J. C. Blanc, D. Pouliquen, M. Saisse (France),
A. �lvarez, A. L. Arteaga, A. Carretero, M. Fern�ndez, H. Guerrero,
P. Gutierrez, J. L. Lizondo, V. Luengo, J. A. Mart�n, M. A. Mart�n,
J. Meseguer, J. M. Mi, L. Moreno, J. Navarro, A. N��ez, E. Ragel,
D. Rodr�guez, G. Rosa, A. S�nchez, J. C. Sanmartin, G. Tonellotto
(Spain), W. Boogaerts, W. Engelhardt, K. Eulig, B. Fiethe, A. Fischer,
M. G�rtner, K. Gr�big, K. Kellner, A. K�hn, W. K�hn, J. Knollenberg,
W. Neumann, J. Nitsch, P. R�ffer, H. Sch�ddekopf, U. Sch�hle,
I. Sebastian, S. Stelzer, U. Strohmeyer, T. Tzscheetzsch,
M. Wassermeyer (Germany), B. Johlander (ESTEC), M. Baessato,
P. F. Brunello, S. Casotto, F. Don�, M. Lazzarin, E. Marchetti,
F. Marzari, P. G. Nicolosi, F. Peron, F. Rampazzi, B. Saggin,
G. Tondello, S. Verani, P. Zambolin� (Italy), J. Lagerros, and
B. Davidsson (Sweden).
21 References
Abbey, A., Holland, A., Lumb, D. and McCarthy, K.: 1991, Further
proton damage effects in EEV CCDs, in proceedings ESA Electronic Comp.
Conf., ESA SEE-N91, 307-318.
Acton, P., Agnew, G., Cotton, R., Hedges, S., McKemey, A. K.,
Robbins, M., Roy, T., Watts, S. J., Damerell, C. J. S.,
English, R. L., Gillman, A. R., Lintern, A. L., Su, D. and
Wickens, F. J.: 1991, Future potential of charge coupled devices as
detectors of ionising radiation, Nucl. Instr. & Methods A305, 504-511.
Adams, J. B.: 1974, Visible and near-infrared diffuse reflectance
spectra of pyroxenes as applied to remote sensing of solid objects in
the solar system, J. Geophys. Res. 79, 4829-4836.
A�Hearn, M. F., Belton, M. J. S., Delamere, W. A., Kissel, J.,
Klaasen, K. P., McFadden, L. A., Meech, K. J., Melosh, H. J.,
Schultz, P. H., Sunshine, J. M., Thomas, P. C., Veverka, J.,
Yeomans, D. K., Baca, M. W., Busko, I., Crockett, C. J.,
Collins, S. M., Desnoyer, M., Eberhardy, C. A., Ernst, C. M.,
Farnham, T. L., Feaga, L., Groussin, O., Hampton, D., Ipatov, S. I.,
Li, J.-Y., Lindler, D., Lisse, C. M., Mastrodemos, N., Owen, W. M.
Jr., Richardson, J. E., Wellnitz, D. D. and White, R. L.: 2005, Deep
Impact: Excavating comet Tempel 1, Science 310, 258-264.
Bessell, M. S.: 1999. Spectrophotometry: Revised standards and
techniques, Publ. Astron. Soc. Pacific 111, 1426-1433.
Birk, M.: 1992, SUMER DPU Command Language (SCL), DPU-IF-E-0200-01-KT,
Kayser-Threde GmbH, Munich.
Breger, M.: 1976, Catalog of spectrophotometric scans of stars,
Astrophys. J., Suppl. Ser. 32, 7-87.
Brunello, P., Peron, F., Barbieri, C., Fornasier, S.: 2000, Baffling
system for the Wide Angle Camera (WAC) of ROSETTA mission, in Current
Development in Lens Design and Optical Systems Engineering,
Fischer, R. E., Johnson, R. B., Smith and W. J., Swatner W. H. (ed.),
SPIE 4093, 79-88.
Calvel, B., Castel, D., Standarovski, E., Rousset, G. and Bougon, M.:
1999, The monolithic SiC telescope of the OSIRIS Narrow-Angle Camera
for the cometary mission ROSETTA, SPIE 3785, 56-65.
Campbell, D. B., Harmon, J. K. and Shapiro, I. I.: 1989, Radar
observations of comet Halley, Astrophys. J. 338, 1094-1105.
Christen, A., Wiest, R., Rastetter, P., Weih, E., Fichna, T.,
Br�ggemann, M., Cassel, M., Fiethe, B., Reiche, K.-U., St�ckner, K.,
Doutreleau, J., Armbruster, P., Telljohann, U. and Wenzel, P.: 2000,
Data processing unit for the Rosetta OSIRIS instrument, in proceedings
DASIA 2000, ESA SP-457, 199-210.
Colwell, J. E.: 1997, Comet Lightcurves: Effects of archive regions
and topography, Icarus 125(2), 406-415.
Dale, C.: 1993, Displacement damage effects in mixed particle
environments for shielded spacecraft CCDs, IEEE Trans. Nucl. Sci.
40(6), 1628-1637.
Debei, S., Fornasier, S., Ramous, P., Barbieri, C., Da Deppo, V.,
Brunello, P. and Peron, F.: 2001, The Wide Angle Camera of the Rosetta
mission: Design and manufacturing of an innovative baffling system for
an aspherical optics telescope, in UV/EUV and Visible Space
Instrumentation for Astronomy and Solar Physics, SPIE 4498, 324-334.
Divine, N., Fechtig, H., Gombosi, T. I., Hanner, M. S., Keller, H. U.,
Larson, S. M., Mendis, D. A., Newburn, R. L., Reinhard, R.,
Sekanina, Z. and Yeomans, D. K.: 1986, The Comet Halley dust and gas
environment, Space Sci. Rev. 43, 1-104.
Dohlen, K., Saisse, M., Claeysen, G., Lamy, P. and Boit, J.-L.: 1996,
Optical designs for the Rosetta narrow-angle camera, Opt. Eng. 35,
1150-1157.
Donn, B.: 1991, The accumulation and structure of comets, in Comets in
the Post-Halley Era I, Newburn, R. L., Rahe, J. (ed.), Kluwer Academic
Publishers, 335-359.
Eonic Systems: 2000, Web page http://www.eonic.com.
Fichna, T., G�rtner, F., Gliem, F. and Rombeck, F.: 1998, Fault-
tolerance of spaceborne semiconductor mass memories, in proceedings
FTCS-28, Munich, 408-413.
Guti�rrez, P. J., Ortiz, J. L, Rodrigo, R. and L�pez-Moreno, J. J.:
2000, A study of water production and temperatures of rotating
irregularly shaped cometary nuclei, Astron. Astrophys. 355, 809-817.
Guti�rrez, P. J., Jorda, L., Samarasinha, N. H. and Lamy, P.: 2005,
Outgassing-induced effects in the rotational state of comet
67P/Churyumov-Gerasimenko during the Rosetta mission, Planet. Space
Sci. 53, 1135-1145.
Hamuy, M., Walker, A. R., Suntzeff, . B., Gigoux, P.,
Heathcote, S. R. and Phillips, M. M.: 1992, Southern
spectrophotometric standards. I, PASP, 104, 533-552.
Hamuy, M., Walker, A. R., Suntzeff, N. B., Gigoux, P.,
Heathcote, S. R. and Phillips, M. M.: 1994 Southern spectrophotometric
standards. II, PASP, 106, 566-589.
Hardy, T., Murowinsky, R. and Deen, M. J.: 1998, Charge transfer
efficiency in proton damaged CCDs, IEEE Trans. Nucl. Sci. 45(2), 154-
163.
Hayes, D. S.: 1985, Stellar Absolute Fluxes and Energy Distributions
from 0.32 to 4.0 micron, in Calibration of fundamental stellar
quantities, Hayes, D. S., Pasinetti, L. E. and Philip, A. D. G. (ed.),
IAU 111, Reidel, Dordrecht, 225-252.
Holland, A., Abbey, D., Lumb, D. and McCarthy, K.: 1990, Proton damage
effects in EEV charge coupled devices, SPIE 1344, 378-395.
Hopkinson, G. R.: 1989, Proton damage effects in an EEV CCD imager,
IEEE Trans. Nucl. Sci. 36(6), 1865-1871.
HST CalSpec: 2005, Web page
http://www.stsci.edu/hst/observatory/cdbs/calspec.html.
Huebner, W. F., Boice, D. C., Reitsema, H. J., Delamere, W. A.,
Whipple, F. L.: 1988, A model for intensity profiles of dust jets near
the nucleus of comet Halley, Icarus 76, 78-88.
Keller, H. U., Kramm, R. and Thomas, N.: 1988, Surface features on the
nucleus of comet Halley, Nature 331, 227-231.
Keller, H. U.: 1989, Comets: Dirty snowballs or icy dirtballs, in ESA
Physics and Mechanics of Cometary Materials, 39-45.
Keller, H. U., Knollenberg, J. and Markiewicz, W. J.: 1994,
Collimation of cometary dust jets and filaments, Planet. Space Sci.
42(5), 367-382.
Keller, H. U., Curdt, W., Kramm, J. R., Thomas, N.: 1995, Images of
the nucleus of comet Halley, Reinhard, R. and Battrick, B. (ed.), ESA
SP-1127.
Keller, H. U., Jorda, L., K�ppers, M., Guti�rrez, P. J., Hviid, S. F.,
Knollenberg, J., Lara, L. M., Sierks, H., Barbieri, C., Lamy, P.,
Rickman, H. and Rodrigo, R.: 2005, Deep Impact observations by OSIRIS
onboard the Rosetta spacecraft, Science 310, 281-283.
Kramm, J. R. and Keller, H. U.: 2000, CCDs for the Rosetta science
imaging system OSIRIS, in Optical Detectors for Astronomy II,
Amico, P. and Beletic, J. W. (ed.), Kluwer Academic Publishers, 55�62.
Kramm, J. R, Sierks, H., Barthol, P., M�ller, R., Tomasch, G. and
Germerott, D.: 2003, Benefit from annealing proton irradiation defects
on the OSIRIS CCDs, MPAE Report MPAE-W-472-03-02, Max-Planck-Institut
f�r Aeronomie, Katlenburg-Lindau, Germany.
Kramm, J. R., Keller, H. U., M�ller, R., Germerott, D. and
Tomasch, G.: 2004, A Marconi CCD42-40 with anti-blooming - Experiences
with the OSIRIS CCDs for the ROSETTA mission, in Scientific detectors
for astronomy - The beginning of a new era, Amico, P., Beletic, J. W.
and Beletic, J. E. (ed.), Kluwer Academic Publishers, 131�135.
K�hrt, E. and Keller, H. U.: 1994, The formation of cometary surface
crusts, Icarus 109, 121-132.
K�hrt, E.: 2002, From Hale Bopp�s activity to properties of its
nucleus, Earth, Moon and Planets 90(1), 61-65.
K�ppers, M., Bertini, I., Fornasier, S., Guti�rrez, P. J.,
Hviid, S. F., Jorda, L., Keller, H. U., Knollenberg, J., Koschny, D.,
Kramm, R., Lara, L. M., Sierks, H., Thomas, N., Barbieri, C.,
Lamy, P., Rickman, H. and Rodrigo, R. and the OSIRIS team: 2005, A
large dust/ice ratio in the nucleus of comet 9P/Tempel 1, Nature 437,
987-990.
Landolt, A. U.: 1992, UBVRI photometric standard stars in the
magnitude range 11.5<V<16.0 around the celestial equator, Astron. J.
104, 340-371.
McDonnell, J. A. M., Lamy, P. L. and Pankiewicz, G. S.: 1991, Physical
properties of cometary dust, in Comets in the post-Halley era,
Newburn, R. L., Neugebauer, M. and Rahe, J. H. (eds.), Kluwer Academic
Publishers, 1043-1073.
Naletto, G., Da Deppo, V., Pelizzo, M. G., Ragazzoni, R. and
Marchetti, E.: 2002, Optical design of the Wide Angle Camera for the
Rosetta mission, Appl. Opt. 41(7), 1446-1453.
Naletto, G., Boscolo, A., Wyss, J. and Quaranta, A.: 2003, Effects of
proton irradiation on glass filter substrates for the Rosetta mission,
Appl. Opt. 42(19), 3970-3980.
Possnert, G., Lagerros, J. and Rickman, H.: 1999, Radiation damage in
OSIRIS filter substrates, in Advances in Optical Interference
Coatings, SPIE 3738, 428-435.
Richter, K. and Keller, H. U.: 1995, On the stability of dust particle
orbits around cometary nuclei, Icarus 114, 355-371.
Rodionov, A. V., Jorda, L., Jones, G. H., Crifo, J. F., Colas, F. and
Lecacheux, J.: 1998, Comet Hyakutake gas arcs: First observational
evidence of standing shock waves in a cometary coma, Icarus 136, 232-
267.
Sagdeez, R. Z., Smith, B., Szego, K., Larson, S., Toth, I.,
Merenyi, E., Avanesov, G. A., Krasikov, V. A., Shamis, V. A. and
Tarnapolski, V. I.: 1987, The spatial distribution of dust jets seen
during the VEGA-2 Flyby, Astron. Astrophys. 187, 835-838.
Schleicher, D. G., Millis, R. L. and Birch, P. V.: 1998, Narrowband
photometry of comet P/Halley: Variation with heliocentric distance,
season, and solar phase angle, Icarus 132(2), 397-417.
Simonelli, D. P., Thomas, P. C., Carcich, B. T. and Veverka, J.: 1993,
The generation and use of numerical shape models for irregular solar
system objects, Icarus 103, 49-61.
Smith, B., Szego, K., Larson, S., Merenyi, E., Toth, I.,
Sagdeev, R. Z., Avanesov, G. A., Krasikov, V. A., Shamis, V. A. and
Tarnapolski, V. I.: 1986, The spatial distribution of dust jets seen
at Vega-2 fly-by, in Proceedings of the 20th ESLAB Symposium on the
Exploration of Halley's Comet, Battrick, B., Rolfe, E. J. and
Richard, R. (eds.), ESA SP-250, 327-331.
Thomas, N. and Keller, H. U.: 1987, Fine dust structures in the
emission of comet P/Halley observed by the Halley Multicolour Camera
on board Giotto, Astron. Astrophys. 187, 843-846.
Thomas, N., Boice, D. C., Huebner, W. F. and Keller, H. U.: 1988,
Intensity Profiles of Dust Near Extended Sources on Comet Halley,
Nature 332, 51-52.
Thomas, N., Keller, H. U., Arijs, E., Barbieri, C., Grande, M.,
Lamy, P., Rickman, H., Rodrigo, R., Wenzel, K.-P., A�Hearn, M. F.,
Angrilli, F., Bailey, M., Barucci, M. A., Bertaux, J.-L., Brie�, K.,
Burns, J. A., Cremonese, G., Curdt, W., Deceuninck, H., Emery, R.,
Festou, M., Fulle, M., Ip, W.-H., Jorda, L., Korth, A., Koschny, D.,
Kramm, J. R., K�hrt, E., Llebaria, A., Marzari, F., Moreau, D.,
Muller, C., Murray, C., Naletto, G., Nevejans, D., Sivan, J.-P. and
Tondello, G.: 1998, OSIRIS - The Optical, Spectroscopic, and Infrared
Remote Imaging System for the Rosetta orbiter, Adv. Space Res. 21(11),
1505-1515.
Thomas, P. C., Veverka, J., Simonelli, D., Helfenstein, P.,
Carcich, B., Belton, M. J. S., Davies, M. E. and Chapman, C.: 1994,
The shape of Gaspra, Icarus 107, 23-26.
Thomas, P. C., Black, G. J. and Nicholson, P. D.: 1995, Hyperion:
Rotation, shape, and geology from Voyager images, Icarus 117, 128-148.
Tomasch, G., Harboe-S�rensen, R., M�ller, R. and Tzscheetzsch, T.:
2000, Co-60 total dose test of 14- and 16-bit ADCs, in IEEE Nuclear
and Space Radiation Effects Conference, IEEE 00TH8527, 26�31.
Vilas, F.: 1994, A cheaper, faster, better way to detect water of
hydration on solar system bodies, Icarus 111, 456-467.