TO APPEAR IN THE ROSETTA MISSION:SPACE SCIENCE REVIEW
ALICE: THE ROSETTA ULTRAVIOLET IMAGING SPECTROGRAPH
S. A. Stern1, D. C. Slater2, J. Scherrer2, J. Stone2, M. Versteeg2,
M. F. A'Hearn3, J. L. Bertaux4, P. D. Feldman5, M. C. Festou*, Joel
Wm. Parker1, and O. H. W. Siegmund6
1Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder,
CO 80302-5143, USA
2Southwest Research Institute, 6220 Culebra Road, San Antonio, TX
78238-5166, USA
3University of Maryland, College Park, MD 20742, USA
4Service d'Aeronomie du CNRS, 91371 Verrieres le Buisson Cedex, France
5Johns Hopkins University, Department of Physics and Astronomy,
Baltimore, MD 21218-2695, USA
6Sensor Sciences, 3333 Vincent Road, Pleasant Hill, CA 94523, USA
ABSTRACT
We describe the design, performance and scientific objectives of the
NASA-funded ALICE instrument aboard the ESA Rosetta asteroid
flyby/comet rendezvous mission. ALICE is a lightweight, low-power, and
low-cost imaging spectrograph optimized for cometary far-ultraviolet
(FUV) spectroscopy. It will be the first UV spectrograph to study a
comet at close range. It is designed to obtain spatially-resolved
spectra of Rosetta mission targets in the 700-2050 A spectral band
with a spectral resolution between 8 A and 12 A for extended sources
that fill its ~0.05deg. x 6.0deg. field-of-view. ALICE employs an
off-axis telescope feeding a 0.15-m normal incidence Rowland circle
spectrograph with a concave holographic reflection grating. The
imaging microchannel plate detector utilizes dual solar-blind opaque
photocathodes (KBr and CsI) and employs a 2-D delay-line readout
array. The instrument is controlled by an internal
microprocessor. During the prime Rosetta mission, ALICE will
characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus, and
the nucleus/coma coupling; during cruise to the comet, ALICE will make
observations of the mission's two asteroid flyby targets and of Mars,
its moons, and of Earth's moon. ALICE has already successfully
completed the in-flight commissioning phase and is operating normally
in flight. It has been characterized in flight with stellar flux
calibrations, observations of the Moon during the first Earth fly-by,
and observations of comet Linear T7 in 2004 and comet 9P/Tempel 1
during the 2005 Deep Impact comet-collision observing campaign.
1.0 INTRODUCTION
Ultraviolet spectroscopy is a powerful tool for investigating the
physical and chemical environments of astrophysical objects and has
been applied with great success to the study of comets (e.g., Feldman
1982, Festou et al. 1993, Stern 1999, Feldman et al. 2004a;
Bockelee-Morvan et al. 2004). The ALICE UV spectrograph, designed to
perform spectroscopic investigations of planetary atmospheres and
surfaces at extreme (EUV) and far-ultraviolet (FUV) wavelengths
between 700 and 2050 A, has been optimized for Rosetta cometary
science with high sensitivity, large instantaneous field-of-view, and
broad wavelength coverage.
For the Rosetta Orbiter remote sensing investigation of comet
67P/Churyumov-Gerasimenko (67P/CG), ALICE will search for noble gases
such as Ne and Ar; measure the production rates, variability, and
structure of H2O, CO, and CO2 molecules that generate the bulk of
cometary activity; measure the abundances and variability of the basic
* Deceased May 11, 2005
elemental species C, H, O, and N in the comet's coma; and measure
atomic ion abundances in the comet's tail. In addition, ALICE will
undertake an investigation of the FUV photometric properties of both
the cometary nucleus itself, and solid grains entrained in the comet's
coma. During Rosetta's cruise to comet 67P/CG, ALICE will also obtain
flyby observations of the mission's two target asteroids, the Earth's
moon, and Mars. ALICE has already successfully completed a number of
in-flight commissioning activities including in-flight calibration
observations of several UV stars, and observations of comet C/2002 T7
(LINEAR) in April and May 2004 and comet 9P/Tempel 1 during the Deep
Impact encounter in July 2005. By virtue of its location at the comet,
the ALICE spectrograph will provide significant improvements in both
sensitivity and spatial resolution over previous cometary UV
observations.
2.0 SCIENTIFIC OBJECTIVES AND CAPABILITIES
The scientific objectives of the ALICE investigation can be summarized
as follows:
(1) Search for and determine the evolved rare gas content of the
nucleus to provide information on the temperature of formation and
thermal history of the comet since its formation. Some of the most
fundamental cosmogonic questions about comets concern their place and
mode of origin, and their thermal evolution since their formation. As
comets are remnants from the era of outer planet formation, one of the
most important things to be learned from them about planetary
formation and the early solar nebula is their thermal history (see
Bar-Nun et al. 1985; Mumma et al. 1993; Stern 1999). Owing to their
low polarizabilities, the frosts of the noble gases are both
chemically inert and extremely volatile. As a result, the trapping of
noble gases is temperature dependent, so noble gases serve as
sensitive "thermometers" of cometary thermal history. ALICE will
determine (or set stringent limits) on the abundances of the He, Ne,
Ar, Kr sequence from observations of their strongest resonance
transitions at 584 A (He I, to be observed in second order), 736/744 A
(Ne I), 1048/1067 A (Ar I), and 1236 A (Kr I). In addition to their
importance as thermal history probes, evolved rare gas abundances also
provide critical data for models requiring cometary inputs to the
noble gas inventories of the planets. ALICE will also determine the
abundances of another important low-temperature thermometer species,
N2 (electronic transitions in the 850-950 A c'4 and 1000- 1100 A
Birge-Hopfield systems). He emission at 584 A detected in comet C/1995
O1 (Hale-Bopp) by Krasnopolsky et al. (1997) from EUVE was accounted
for by charge exchange of solar wind He ions. These authors also set a
very stringent upper limit on Ne emission. Sensitive upper limits on
Ar and N2 have been obtained for four comets using FUSE (Weaver et
al. 2002; Feldman 2005); a weak detection of Ar was published for
Hale-Bopp (Stern et al. 2000). With long integration times ALICE
should be able to detect and monitor several noble gases, and possibly
also N2, in comet 67P/CG without the m/e ambiguities of mass
spectroscopy and at levels significantly below their cosmogonic
abundance levels.
(2) Determine the production rates of the parent molecule species,
H2O, CO and CO2, and their spatial distributions near the nucleus,
thereby allowing the nucleus/coma coupling to be directly observed and
measured on many timescales. Cometary activity and coma composition is
largely driven by the sublimation of three key species: H2O, CO, and
CO2. ALICE has been designed to directly detect each of these key
parent molecules. Sunlight scattered by the nucleus, and probably
background interplanetary H I Lyman-a, will be absorbed at a
detectable level by water, and possibly by other molecules such as
CO2. By moving the slit to various locations around the coma (either
by spacecraft pointing or simple changes in spacecraft location), it
will be possible to map, even tomographically, the H2O distribution
around the comet. By measuring the H2O column abundance in absorption
in the UV, rather than by fluorescence (as in the IR), ALICE's
measurements will provide a more direct, less model dependent
signature for interpretation. CO will be observed via fluorescence in
the well-known Fourth Positive band system from 1300 to 1700 A, which
will give total CO content; the CO Cameron bands between 1900 and 2050
A will measure the CO produced by CO2 photodissociation, and therefore
the total CO2 content. With most ALICE observations being directed
toward the nucleus, the H2O, CO, and CO2 gas abundances will directly
measure the production source(s) on the nucleus. The combination of
these various types of observations will allow ALICE to completely map
the H2O and CO distribution in the near environment of the nucleus and
address the question of the coupling between the gas and the nucleus
near the surface, and thus characterize the outgassing pattern of the
surface. The latter will be particularly important for determining how
much H2O and CO is derived from discrete, active regions, and how much
from the "background" subsurface flow on the nucleus as a whole.
Any volatile species released on the night side of the nucleus will
also be studied. When coupled with dust measurements in the coma,
these data will yield information on the temporal and spatial
variation of the dust/gas ratio in the cometary coma. When coupled
with IR mapping measurements by Rosetta-VIRTIS, these data will yield
information on the depths of the various icy reservoirs from which
H2O, CO, and CO2 can be derived.
(3) Study the atomic budget of C, H, O, N, and S in the coma as a
function of time. As a UV instrument, ALICE is unique among the remote
sensing investigations aboard Rosetta in its ability to detect atoms
in the cometary atmosphere. Among the most important atomic species in
comets are C, H, O, and N (Feldman et al. 2004a). ALICE's bandpass
includes the strongest resonance lines of all of these (C I 1561,
1657, 1931 A; H I 973, 1025, 1216 A; O I 989, 1304 A; and N I 1134 and
1200 A), as well as those of another important, cosmogonically
abundant species, S I (1425, 1474, 1813 A). When ALICE is viewing the
coma, we will obtain species abundance ratios. When the field-of-view
encompasses the entire coma (e.g., during approach), the total content
of the coma can be measured and its variation with time can be
monitored. These quantities can be measured independently of any
model. In addition, the most abundant coma ion, O+, has its strongest
resonance line at 834 A that will allow an investigation of the
ionization mechanism in the coma and the interaction of the comet
ionosphere with the solar wind. Another observable ion, C+ (1335 A),
will give complementary information on the competing ionization
processes (photoionization vs. electron impact) as the two species are
created from neutral atoms that are not similarly spatially
distributed and the ions are thus differently affected by solar wind
particles. And O I 1356 A emission, though weak, will be an excellent
tracer of electron impact processes in the coma. Together, these
various probes will provide measurements of the abundances and spatial
structures (e.g., distributed sources from CHON particles and as yet
undetected organic molecules) in the atomic coma of comet 67P/CG. As a
remote sensing instrument, ALICE enjoys the advantage that it can
obtain such measurements throughout Rosetta's long comet rendezvous,
independently of the orbital location of the spacecraft.
(4) Study the onset of nuclear activity in ways Rosetta otherwise
cannot. ALICE is particularly well-suited to the exploration of one of
the most fascinating cometary phenomena: the onset of nuclear
activity. This area of interest has important implications for
understanding cometary phenomenology, and for the general study of
cometary activity at large distances (e.g., in Halley, Hale-Bopp,
Skiff, Schwassmann-Wachmann 1, Chiron, etc.). ALICE will accomplish
this by searching for and then monitoring the "turn-ons" of
successively "harder volatiles" including N2, CO2, and H2O (in
absorption), and the noble gases, CO, and atomic sulfur (all in
fluorescence) as a tracer of H2S and CS2.
(5) Spectral mapping of the entire nucleus of 67P/CG at FUV
wavelengths in order to both characterize the distribution of UV
absorbers on the surface, and to map the FUV photometric properties of
the nucleus. With its inherent long-slit imaging capability, ALICE can
obtain either multispectral or monochromatic images of the comet at a
resolution of 500/R50 meters, where R50 is the comet-Orbiter range in
units of 50 km. The UV images can be used to: (i) search for regions
of clean ice; (ii) study the photometric properties of small
(10-9-10-11 g) surface grains (both at and away from active zones) as
a function of solar phase angle; and (iii) search for regions of
electrical or photoluminescent glows on the surface. Further, by
correlating regions that are dark below the ice absorption edge of H2O
(1600-1700 A), with visible albedo measurements made by the Rosetta
imager OSIRIS, ALICE will be able to search for nuclear regions rich
in this important volatile.
(6) Study the photometric and spectrophotometric properties of small
grains in the coma as an aid to understanding their size distribution
and how they vary in time. UV photometry can be carried out with ALICE
(a) using the solar continuum near 2000 A, and (b) at H I Lyman-a
(1216 A). In both of these passbands, the photometric phase function
of coma grains can be measured in order to map the distribution of
grains with 10 to 100 times less mass than can be well-observed with
the Rosetta imager, OSIRIS. We will separate the total optical depth
so derived into icy and non-refractory components using the depth of
the characteristic H2O absorption near 1650 A as a compositional
constraint.
(7) Map the spatial and temporal variability of O+, N+, S+ and C+
emissions in the coma and ion tail in order to connect nuclear
activity to changes in tail morphology and structure near
perihelion. These ions have resonance transitions at 1036 A and 1335 A
(C+), 1085 A (N+), 910 A and 1256 A (S+), and 834 A (O+), with which
ALICE will be able to probe the ion formation and tail region behavior
of the comet at any time when the comet is active.
3.0 TECHNICAL DESCRIPTION
3.1 Instrument Overview
An opto-mechanical layout of ALICE is shown in Figure 1. Light enters
the telescope section through a 40 x 40 mm2 entrance aperture and is
collected and focused by an f/3 off-axis paraboloidal (OAP) mirror
onto the entrance slit and then onto a toroidal holographic grating,
where it is dispersed onto a microchannel plate (MCP) detector that
uses a double-delay line (DDL) readout scheme. The 2- D (1024 x
32)-pixel format, MCP detector uses dual, side- by-side, solar-blind
photocathodes: potassium bromide (KBr) and cesium iodide (CsI). The
measured spectral resolving power (lambda/d lambda) of ALICE is in the
range of 70- 170 for an extended source that fills the instantaneous
field-of- view (IFOV) defined by the size of the entrance slit. ALICE
is controlled by an SA 3865 microprocessor, and utilizes lightweight,
compact, surface mount electronics to support the science detector, as
well as the instrument support and interface electronics. Figure 2
shows both a 3D external view of ALICE, and a photograph of the flight
unit.
3.2 Optical Design
The OAP mirror has a clear aperture of 41 x 65 mm2, and is housed in
the telescope section of the instrument (see Figures 1 and 2). The
reflected light from the OAP enters the spectrograph section, which
contains a holographic grating and MCP detector. The slit, grating,
and detector are all arranged on a 0.15-m diameter normal incidence
Rowland circle.
The spectrograph utilizes the first diffraction order throughout the
700-2050 A spectral passband. The lower half of the first order
wavelength coverage (700-1025 A) also shows up in second order between
the first order wavelengths of 1400 and 2050 A.
Both the OAP and grating, and their mounting fixtures, are constructed
from monolithic pieces of Al, coated with electroless Ni and polished
using low-scatter polishing techniques. The OAP and grating optical
surfaces are
Text Box: Fig. 1. (a) The opto-mechanical layout of ALICE. (b) A
photograph of the ALICE flight unit.
overcoated with sputtered SiC. Control of internal stray light is
achieved with a well- baffled optical cavity, and a holographic
diffraction grating that has low scatter and near-zero line ghost
problems.
For contamination control, heaters are mounted to the back surfaces of
the OAP mirror and grating to prevent cold trapping of contaminants
during flight. To protect the sensitive photocathodes and MCP surfaces
from exposure to moisture and other harmful contaminants during ground
operations, instrument integration, and the early stages of the
mission, the detector tube body assembly is enclosed in a vacuum
chamber with a front door that was successfully (and permanently)
opened during the early commissioning phases of the flight. For
additional protection of the optics and detector from particulate
contamination during the flight, a front entrance aperture door is
included that can close when the dust and gas levels are too high for
safe operation and exposure (i.e., when the Rosetta Orbiter is close
to the comet nucleus). The telescope baffle vanes also help to shield
the OAP mirror from bombardment of small particles that can enter the
telescope entrance aperture.
3.3 Entrance Slit Design
The spectrograph entrance slit assembly design is shown in Figure
3. The slit is composed of three sections plus a pinhole mask. The
center section of the slit provides high spectral resolution of ~8-12
A FWHM with an IFOV of 0.05deg. x 2.0deg.. Surrounding the center slit
section are the two outer sections with IFOVs of 0.10deg. x
2.0deg. and 0.10deg. x 1.53deg.. A pinhole mask, located at the edge
of the IFOV of the second outer section, provides limited light
throughput to the spectrograph for bright point source targets (such
as hot UV stars) that will be used during stellar occultation studies
of CG's coma.
3.4 Detector & Detector Electronics
The 2-D imaging photon-counting detector located in the spectrograph
section utilizes an MCP Z-stack that feeds the DDL readout array
(Siegmund et al. 1992). The input surface of the Z-stack is coated
with opaque photocathodes of KBr (700-1200 A) and CsI (1230-2050 A)
(Siegmund et al. 1987). The detector tube body is a custom design made
of a lightweight brazed alumina-Kovar structure that is welded to a
housing that supports the DDL anode array (see Figures 4 and 5).
To capture the entire 700-2050 A bandpass and 6deg. spatial FOV, the
size of the detector's active area is 35 mm (in the dispersion
direction) x 20 mm (in the spatial dimension), with a pixel format of
(1024 x 32)-pixels. The 6deg. slit- height is imaged onto the central
20 of the detector's 32 spatial channels; the remaining spectral
channels are used for dark count monitoring. Our pixel format allows
Nyquist sampling with a spectral resolution of ~3.4 A, and a spatial
pixel resolution of 0.3deg..
Text Box: (a) (b) Fig. 2. (a) 3D external view of ALICE. (b)
Photograph of the ALICE flight unit.
The MCP Z-Stack is composed of three 80:1 length-to-diameter (L/D)
MCPs that are all cylindrically curved with a radius-of- curvature of
75 mm to match the Rowland-circle for optimum focus across the full
spectral passband. The total Z-Stack resistance at room temperature is
~500 M ohm. The MCPs are rectangular in format (46 x 30 mm2), with
12-um diameter pores on 15-um centers. Above the MCP Z-Stack is a
repeller grid that is biased ~1000 volts more negative than the top of
the MCP Z-Stack. This repeller grid reflects electrons liberated in
the interstitial regions of the MCP back down to the MCP input surface
to enhance the detective quantum efficiency of the detector.
The expected H I Lyman-a (1216 A) emission brightness from comet
67P/CG is ~4 kR at a heliocentric distance of 1.3 AU (based on IUE
observations of this comet in 1982; Feldman et al. 2004b). To prevent
saturation of the detector electronics, it is necessary to attenuate
the Lyman-a emission brightness to an acceptable count rate level well
below the maximum count rate capability of the electronics (i.e.,
below 104 c s-1). An attenuation factor of at least an order of
magnitude is required to achieve this lower count rate. This was
easily achieved by physically masking the MCP active area where the H
I Lyman-a emission comes to a focus during the photocathode deposition
process. The bare MCP glass has a quantum efficiency about 10 times
less than that of KBr at 1216 A.
Surrounding the detector tube body is the vacuum chamber housing made
of aluminum and stainless steel (see Figures 4 and 5). As mentioned
above, this vacuum chamber protected the KBr and CsI photocathodes
against damage from moisture exposure during ground handling and from
outgassing constituents during the early stages of the flight. It also
allowed the detector to remain under vacuum (< 10-5 Torr) during
ground operations, testing and handling, and transportation. Light
enters the detector vacuum chamber through an openable door, which
contains a built-in MgF2 window port that transmits UV light at
wavelengths > 1200 A. This window allowed testing of the detector with
the door closed, and provided redundancy during flight if the door
mechanism had failed to open.
The detector vacuum chamber door was designed to be opened once during
flight, using a torsion spring released by a dual-redundant
pyrotechnic actuator (dimple motor). During instrument integration and
test (I&T), the door was successfully opened numerous times and
manually reset. In flight, the
Text Box: Fig. 3. ALICE entrance slit design.
Text Box:
Fig. 4. Schematic of the ALICE DDL detector vacuum chamber housing.
detector was successfully opened; however, the primary side of the
actuator did not open the door--the redundant side was required to
successfully open the door.
The detector electronics includes preamplifier circuitry, time to
digital converter circuitry (TDC), and pulse-pair analyzer (charge
analysis) circuitry (PPA). All of these electronics are packaged into
three 64 x 76 mm2 boards. These three boards are mounted inside a
separate enclosed magnesium housing that mounts to the rear of the
spectrograph section (just behind the detector vacuum chamber). The
detector electronics require +/- 5 VDC, and draw ~1.1 W.
The detector electronics amplify and convert the detected output
pulses from the MCP Z-Stack to pixel address locations. Only those
analog pulses output from the MCP that have amplitudes above a set
threshold level are processed and converted to pixel address
locations. For each detected and processed event, a 10-bit x address
and a 5-bit y address are generated by the detector electronics and
sent to the ALICE command-and-data handling (C&DH) electronics for
data storage and manipulation. In addition to the pixel address words,
the detector electronics also digitizes the analog amplitude of each
detected event output by the preamplifiers and sends this data to the
C&DH electronics. Histogramming this "pulse-height" data creates a
pulse-height distribution function that is used to monitor the health
and status of the detector during operation. A built-in "stim-pulser"
is also included in the electronics that simulates photon events in
two pixel locations on the array. This pulser can be turned on and off
by command and allows testing of the entire ALICE detector and C&DH
electronic signal path without having to power on the detector
high-voltage power supply. In addition, the position of the stim
pixels provides a wavelength fiducial that can shift with operational
temperature.
3.5 Electrical Design
The instrument support electronics (see Figure 6) on ALICE include the
power controller electronics (PCE), the C&DH electronics, the
telemetry/command interface electronics, the decontamination heater
system, and the detector high-voltage power supply (HVPS). All of
these systems are controlled by a rad-hardened SA 3865 microprocessor,
supplied by Sandia Associates, with 32 KB of local program RAM and 64
KB of acquisition RAM along with 32 KB of program ROM and 128 KB of
EEPROM. All of the instrument support electronics are contained on 5
boards mounted just behind the detector electronics (see Figures 1 and
2).
Power Controller Electronics. The PCE are composed of DC/DC converters
designed to convert the spacecraft power to +/- 5 VDC required by the
detector electronics, the C&DH and TM interface electronics, and the
detector HVPS. Also located in the PCE is the switching circuit for
the heaters and the limited angle torque (LAT) motor controller that
operates the front aperture door.
Command-and-Data Handling Electronics. The C&DH electronics handles
the following instrument functions: (i) the interpretation and
execution of commands to the instrument, (ii) detector acquisition
control including the histogramming of raw detector event data, (iii)
telemetry formatting of both science and housekeeping data, (iv)
control of the detector HVPS, (v) the detector vacuum cover release
mechanism, (vi) the front aperture door control, (vii) the control of
the housekeeping ADC's which are used to convert analog housekeeping
data to digital data for inclusion into the TM data stream, and (viii)
on-board data handling.
Telemetry/Command Interface Electronics. The C&DH utilizes radiation
tolerant buffers and FIFO memory elements in the construction of the
spacecraft telemetry and command interfaces. A finite state machine
Text Box: Fig. 5. A photograph of the ALICE DDL flight detector with
the MgFB2B detector door in the closed position.
programmed into a radiation hardened Actel 1280 FPGA controls the
receipt and transmission of data. A bit-serial interface is used.
Decontamination Heater System. A single decontamination heater each
(~1 W resistive heater) is bonded to the backside surface of both the
OAP mirror substrate, and the grating substrate. Along with each
heater, two redundant thermistors are also mounted to the back of each
substrate to monitor and provide control feedback to the heaters. The
C&DH electronics can separately control each heater. Successful heater
activations have already taken place during the commissioning phase of
the flight. Additional activations are planned periodically during the
long cruise phase to comet 67P/CG.
High Voltage Power Supply. The HVPS is located in a separate enclosed
bay behind the OAP mirror (see Figure 1). It provides the -4.0 kV
required to operate the detector. The voltage to the Z-stack is fully
programmable by command in ~25 V steps between -1.7 and -6.1 kV. The
mass of the supply is ~120 g, and consumes a maximum of 0.65 W during
detector operation.
3.6 Data Collection Modes
ALICE can be commanded to operate in one of three data collection
modes: i) image histogram, ii) pixel list, and iii) count rate
modes. Each of these modes uses the same 32k word (16 bit) acquisition
memory. The first two acquisition modes use the same event data
received form the detector electronics but the data is processed in a
different way. Also, in these two modes, events occurring in up to
eight specific areas (each area is composed of 128 spectral pixels by
4 spatial pixels) on the array can be excluded to isolate high count
rate areas that would otherwise fill up the array. The third
acquisition mode only uses the number of events received in a given
period of time; no spectral or spatial information is used.
Image Histogram Mode. In this mode, acquisition memory is used as a
two dimensional array with a size corresponding to the spectral and
spatial dimensions of the detector array. The image histogram mode is
the prime ALICE data collection mode (and the one most often used
during flight). During an acquisition, event data from the detector
electronics representing (x,y)-pixel coordinates are passed to the
histogram memory in parallel form. The parallel data stream of x and y
values is used as an address for a 16 bit cell in the 1024 x 32
element histogram memory, and a read-increment-write operation on the
cell contents is performed for each event. During a given integration
time, events are accumulated one at a time into their respective
histogram array locations creating a 2-D image. The
read-increment-write operation saturates at the maximum count of
65,535 so no wrap around can occur in the acquired data. The time
information of the individual events within the acquisition is lost in
this process, but using appropriate acquisition durations high
signal-to-noise ratio data may be acquired even from dim objects. At
the conclusion of the integration period the acquired data can be down
linked in telemetry. In order to limit the required telemetry
bandwidth, the histogram memory can be manipulated to extract only
data from up to eight separate, 2-D windows in the array for downlink,
and within these windows rows and columns may be co-added to further
reduce the number of samples.
Pixel List Mode. In this mode the acquisition memory is used as a
one-dimensional linear array of 32,768 entries. The pixel list mode
allows for the sequential collection of each (x,y)-event address into
the linear pixel list memory
Text Box:
Fig. 6. ALICE's electronic block diagram.
array. Periodically, at programmable rates not exceeding 256 Hz, a
time marker is inserted into the array to allow for "time-binning" of
events. This mode can be used to either (a) lower the downlink
bandwidth for data collection integrations with very low counting
rates, or (b) for fast time-resolved acquisitions using relatively
bright targets in the ALICE FOV. At the conclusion of this acquisition
period the total amount of generated data can be further reduced by
selecting only events that have occurred within up to eight separate
windows for downlink.
Count Rate Mode. In this mode the acquisition memory is again used as
a one dimensional linear array of 32,768 entries. The count rate mode
is designed to periodically (configurable between 3 ms and 12 s)
collect the total detector array count rate sequentially in the linear
memory array, as if the entire instrument were an FUV photometer. This
mode allows for high count rates from the detector (up to 10 kHz),
without rapid fill up of the array. It does not, however, retain any
spatial or spectral information for broadband photometric
studies. Depending on the required periodic acquisition rate, total
acquisition durations of up to 98 seconds to 100 hours are possible.
4.0 MEASURED PERFORMANCE CHARACTERISTICS
An overview of the instrument characteristics, spacecraft resource
requirements, and a comparison of the ground and in-flight performance
of ALICE are summarized in Table I. A brief summary of the ground test
and in-flight radiometric performance is presented below.
4.1 Radiometric Ground Test
ALICE's radiometric performance was first measured prior to delivery
to the spacecraft. Both vacuum and bench level radiometric tests
allowed characterization of the detector dark count rate, the
instrument's wavelength passband, spatial and spectral resolution,
scattered light rejection, and the effective area as a function of
wavelength (see Slater et al. 2001 for details of these test
results). The vacuum tests were performed using the UV radiometric
test facility at Southwest Research Institute (SwRI) following the
successful completion of instrument environmental tests that included
vibration, EMI, and thermal-vacuum tests. Table I includes a summary
of the ALICE radiometric performance measured during these ground
tests, with a comparison to the in-flight performance measured during
the commissioning phase of the flight. Note that the ground and
in-flight performance tracks closely except for i) the detector
background rates, which are higher in flight as expected due to the
ambient spacecraft environment; and ii) the effective area, which was
found to be lower in flight by a factor of two from that measured
during ground test. The lower in-flight effective area performance is
not well understood; speculation includes possible degradation of the
detector photocathodes during the extended storage of the spacecraft
caused by the one-year launch delay.
4.2 In-Flight Performance
During the commissioning phase of flight, ALICE performed a number of
in-flight radiometric characterization and calibration observations
using UV stars (focus, PSF, effective area, pointing, wavelength
calibration), the Sun (stray light characterization), and the Moon
during the first Earth-Moon flyby (effective area). Table I summarizes
the in- flight instrument performance characteristics. Figure 7 shows
the measured in-flight effective area based on a number of UV stars
observed with ALICE that have been calibrated with IUE. Effective area
values using ALICE acquired lunar spectral data also fits well with
the data acquired using UV-bright stars shown in Figure 7.
5.0 CONCLUSION
ALICE is a highly-capable, low-cost UV imaging spectrograph that will
significantly enhance Rosetta's scientific characterization of the
nature and origins of the cometary nucleus, its coma, and nucleus/coma
coupling. ALICE will do this by its study of noble gases, atomic
abundances in the coma, major ion abundances in the tail, and
powerful, unambiguous probes of the production rates, variability, and
structure of H2O and CO/CO2 molecules that generate cometary activity,
and the far-UV properties of the nucleus and solid grains. ALICE will
also deepen the Rosetta Orbiter's in situ observations by giving them
the global view that only a remote-sensing adjunct can provide.
The in-flight characterization and calibration performance of ALICE
has successfully been completed and the instrument is healthy and
ready to accomplish scientific tasks aboard the Rosetta Orbiter.
TABLE I
Rosetta-ALICE Characteristics, Spacecraft Resource Requirements, &
Measured Performance Summary (Ground cal & in-flight results).
Parameter
Description
Total Spectral Passband:
680 - 2060 A
Spectral Resolution:
(Point Source) Grd Cal: 4-8 A; Flight: 4-9 A
(Extended Source) Grd Cal and Flight: 8-12 A
Spatial Resolution:
0.05deg. x 0.6deg. (35 x 420 m2 at 40 km from nucleus)
Active FOV
0.05deg. x 2.0deg. + 0.1deg. x 2.0deg. + 0.1deg. x 1.5deg.
Pointing
Boresight with OSIRIS WAC and VIRTIS
Effective Area:
Flight: 0.02 (1575 A)-0.05 cm2 (1125 A)
Stray Light Attenuation
Grd Cal: < 10-4 at aoff > 4deg.; Flight: < 10-9 at aoff > 60deg.
Detector Dark Rate:
Grd Cal: 2.4 c/s; Flight: 18 c/s (total array)
Telescope/Spectrograph
Off-axis telescope, Rowland circle spectrograph
Detector Type
2-D Microchannel Plate w/ double-delay line readout
External Dimensions
204 x 413 x 122 mm3
Mass/Power
3.0 kg/4.0 W
Observation Types
Nucleus imaging and spectroscopy; coma gas
spectroscopy; jet and grain spectrophotometry; stellar
occultations (optional observations)
0.00010.0010.010.11000120014001600180020002200R-ALICE IN-FLIGHT
EFFECTIVE AREAHD 218045 (IUE) HD 218045 (Kurucz) HD 209952 (Kurucz) HD
207971 (IUE) HD 203245 (Kurucz) EFFECTIVE AREA (cm2) WAVELENGTH (A)
Fig. 7. The measured in-flight effective area of ALICE based on
observations of various UV stars during the Rosetta instrument
commissioning phase of the mission.
Acknowledgements
We would like to thank our engineering staff at SwRI, including Greg
Dirks, Susan Pope, and Peter De Los Santos, for their contributions to
the design of ALICE. We also want to thank Dr. John Vallerga and Rick
Raffanti of Sensor Sciences for their technical advice and assistance
with the detector and its associated electronics design and test, and
Joe Kroesche for the initial design of the ALICE flight software.
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