PDS_VERSION_ID = 3 RECORD_TYPE = STREAM RELEASE_ID = 0001 REVISION_ID = 0000 OBJECT = TEXT PUBLICATION_DATE = 2004-09-29 NOTE = "Description of OMEGA calibration" END_OBJECT = TEXT END Calibration goals =============================================== The calibration goals can be summarised as follows: to validate all operation modes; to determine the instrument responses, in a variety of environmental conditions likely to match those it will experience during the flight. Operating as a spectral imager, it is required to acquire its absolute radiometric, spectral and geometrical responses, as a function of the various instrument temperatures (focal plane, spectrometer), in the entire field of view. During calibration, the instrument must be maintained in an environment (mechanical, optical, thermal) simulating the space, illuminated by a variety of reference sources, and controlled by a GSE (Ground Support Equipment) equipped with a spacecraft simulator so as to validate all interfaces, including the data transfer and acquisition. Radiometric calibration ----------------------------------------------- The goal of radiometric calibration consists in connecting the instrumental signal (expressed in digit.s-1) to the spectral luminance coming from the observed source (expressed in W.m-2.str-1.µm-1). For each detection element the radiometric calibration evaluates two kind of instrumental responses: - the absolute radiometric response that connect in a univocal way the instrumental signal recorded with the spectral luminance emitted by the source - the relative radiometric response between the various photosensitive elements of the focal plane arrays. Furthermore, we will be also concerned about the contribution of the parasitic light, which is equivalent estimating the quantity of flux light, measured by the instrument, coming from a source located out of its nominal field of view. The experimental procedure to carry out an absolute radiometric calibration consists in using a source whose spectral luminance is known and who can vary in intensity. Spectral calibration ----------------------------------------------- The function of a spectrometer is to measure the quantity of light resulting from a source by spectral interval given. In addition to the radiometric measuring accuracy for each spectel which was the subject of last sub-section, it is necessary to evaluate the disperse function of the unit consisted by the dispersive and the detection systems. For each spectel, the calibration will have to specify the following characteristics: - the central position and the width in wavelength (band-width); - the continuity between spectral channels: presence of gaps or overlaps between spectels. This will lead to characterise the actual spectral filter profile of each spectel and thus to the actual spectral resolution of the imaging spectrometer. The experimental procedure of the spectral calibration consists in using a source with a narrow emission spectral width and a variable emission wavelength, scanning it according to the direction of spectral dispersion associated with a spectral image pixel. The spectral displacement of the source makes it possible to record the response profile of each scanned spectel. The analysis of these profiles will give us the required characteristics of the spectel, like revealing the presence or not of spectral discontinuity between spectels. Spatial calibration ----------------------------------------------- In a imager, each picture element, identified by its coordinates and its dimensions , defined in space object of the instrument optical system a direction and a instantaneous field of view. There thus exists, in theory, a univocal law between the space and angular properties of a pixel. Nevertheless, many defaults affects the validity of such relations assign to the whole detection elements. The spatial calibration will have to recover the following characteristics: - the aiming direction and the elementary field of view for each pixel of the entire data cube; - the "registration" defects, which correspond to variations in the relative directions of observation of the spectels associated to a given pixel. This effect is perhaps due to misalignments of the detector compared to the dispersed spectrum or to the heterogeneity of the layer sensitive to the surface of the detector. It can lead to an erroneous spectral interpretation which is, in fact, resulting from a spatial albedo variation; - in the case where a scanning mirror is used to produce a spatial dimension of the spectral image, it is needed to be ensured of the scanning direction precision with respect to the spectral dispersion orientation; - finally, the relative optical axis direction of the different detection channels. VIRTIS and OMEGA use each one three different detectors within two parallel optical assemblies. Optical stimuli ----------------------------------------------- Hot blackbody A blackbody spectral brightness is entirely determined by the equilibrium thermal temperature of the emissive cavity. It follows that this source is particularly well adapted for the radiometric calibration. Ribbon lamp This source is one of the TPIII2850-3000 tungsten ribbon lamps made in Russia. Monochromator A monochromator is a source whose characteristic is to be able to emit a radiation in a spectral width and with a wavelength of emission variables. A monochromator was thus selected to ensure the spectral calibration of the studied experiments. Cold blackbody A body in thermal balance at room temperature (20°C), has a non null emission in the near infrared range (0.54 W.m-2.sr-1.µm-1 at 4 µm and 2.05 W.m-2.sr-1.µm-1 at 5 µm). For the absolute radiometric calibration, it is significant enough to precisely qualify this contribution brought by the experimental set-up itself. Grid pattern The pinholes source is made of a grid of 25 individual sources pinholes source distribute on a 56.5 mm square mechanical interface. This source is dedicated for static geometrical calibration purpose (distortion analysis...), but as a complement to this, the source is mounted on a the same vertical M-MFN25CC Newport linear stage as the ribbon lamp mask assembly in order to calibrate pixels IFOV. Gas cells Within the framework of the spectral calibration, it could be useful to have spectral standards independently of the monochromator. One approach consists in put on the course of the light beam an object which will superimpose fine absorption lines and whose positions are very well known. For the optical bench, these spectral standards will be obtained by using mixtures of gases contained in tight cells with a pressure adapted to the desired widths absorption lines. Representative An important calibration goal of OMEGA experiment is to be able to samples observe during the calibration sessions analogues samples of rocks and minerals aimed by these experiments over Mars, and this, under the same observation conditions, i.e. in reflectance.