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Instrument Design

OMEGA: Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité

Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) is an imaging spectrometer covering the visible and near infrared wavelength ranges. The instrument comprises two grating spectrographs, one working in the visible and near infrared range (VNIR channel), the other in the short wavelength infrared range (SWIR channel).

The OMEGA instrument is made up of two main components:

  • A Camera unit (OMEGA-CAMERA or OMEC) comprising the VNIR and SWIR spectrographs and their associated signal electronics
  • A Main Electronics (OMEGA-ME or OMEM) module responsible for the control and management of the instrument

OMEGA performance summary
Imaging capability 128 contiguous, across track fields of view, each of 1.2 mrad, corresponding to < 500 m at periapsis
Spectral capability 352 contiguous spectral channels to acquire the entire spectrum from 0.36 to 5.2 µm for each resolved pixel
Photometric capability SNR > 100 over the full spectral range, allowing the identification of percentage absorptions and thermal variations

Visible and Near Infrared Range (VNIR) channel

The VNIR channel is based on a push-broom imaging concept with a two-dimensional silicon charge coupled device (CCD) detector and a telescope covering an 8.8° total field of view that is defined by a slit placed in the focal plane of the telescope. Target radiation leaving the slit is dispersed onto the detector by a concave holographic diffraction grating. The spectrum of a pixel area on the observed target is dispersed along a column of the detector array while spatial resolution in the direction of the spectrograph slit is obtained across the lines of the array. The spectrograph slit is oriented perpendicular to the spacecraft track so the motion of the spacecraft provides the second spatial dimension of the image. During one exposure interval, the full spectrum of every spatial pixel along the slit is obtained. Successive exposures are combined as the spacecraft moves, with the exposure times selected so that a contiguous image swath at the desired resolution is obtained.

VNIR channel characteristics
Entrance optics
Telescope Refractive,
double Gauss objective
Aperture (mm) 15.6
f number 3.7
Field of view (rad) 0.154
Slit width (µm) 50
Dispersive element Concave holographic mirror grating
Groove density (mm-1) 65
Dispersion (Åmm-1) 1071
Grating size (mm) 40 × 10
Type Silicon CCD,
Thomson TH 7863
Size (pixels) 384 × 288
Pixel size (µm) 23 × 23
Maximum spatial resolution per pixel (mrad) 0.4
Spectral range (µm) 0.36 - 1.05
Spectral resolution (λ/Δλ) 70 - 200
Dynamic range (bits) 12

Short Wavelength Infrared Range (SWIR) channel

The SWIR channel of the camera has two sub-channels fed by a common telescope, slit and collimator. The collimator output beam is split into two parts by a dichroic filter, covering the 0.93 - 2.77 µm and 2.65 - 5.2 µm wavelength bands. The filtered collimator outputs are dispersed by two blazed, flat diffraction gratings operating at the first order. The dispersed slit images are re-imaged onto the detectors by two sets of optics (one per sub-channel), comprising spherical collecting mirrors, spherical field mirrors and objectives composed of four zinc selenide (ZnSe) lenses. Each sub-channel employs an indium antimonide (InSb) photovoltaic linear array detector with 128 pixels, cooled down to approximately 77 K by a Stirling cycle micro cooler.

The use of linear arrays as detectors leads to the use of a whiskbroom-imaging concept. A scanning mirror gives the cross-track field of view and the spacecraft along-track motion provides the second spatial dimension for the images.

SWIR channel characteristics
  0.93 - 2.77 µm sub-channel 2.65 - 5.2 µm sub-channel
Type Cassegrain
Focal length (mm) 200
f number 4
Primary mirror Parabolic, 55 mm diameter
Secondary mirror Hyperbolic, 20.5 mm diameter
Dispersive element Blazed reflective grating Blazed reflective grating
Groove density (mm-1) 180 120
Incidence angle (deg) -6.5 -10.6
Blaze wavelength (µm) 1.7 3.8
Type Photovoltaic linear array
Material Indium antimonide (InSb)
Size (pixels) 128
Pixel size (µm) 90 × 120
Pixel pitch (µm) 120
Cross-track field of view (deg) 8.8
Spectral resolution (nm) 13 - 20
Dynamic range (bits) 12

Last Update: 04 October 2017

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