The Colour and Stereo Surface Imaging System (CaSSIS) camera was launched on 14 March 2016 onboard the ExoMars Trace Gas Orbiter (TGO) and it is currently in cruise to Mars.
The CaSSIS high resolution optical system is based on a TMA telescope (Three Mirrors Anastigmatic configuration) with a 4th powered folding mirror compacting the CFRP (Carbon Fiber Reinforced Polymer) structure. The camera EPD (Entrance Pupil Diameter) is 135 mm and the focal length is 880 mm, giving an F# 6.5 system; the wavelength range covered by the instrument is 400-1100 nm. The optical system is designed to have distortion of less than 2%, and a worst case Modulation Transfer Function (MTF) of 0.3 at the detector Nyquist spatial frequency (i.e. 50 lp/mm).
The Focal Plane Assembly (FPA), including the detector, is a spare from the Simbio-Sys instrument of the Italian Space Agency (ASI). Simbio-Sys will fly on ESA’s BepiColombo mission to Mercury in 2018. The detector, developed by Raytheon Vision Systems, is a 2k×2k hybrid Si-PIN array with 10 μm-pixel pitch. The detector allows snap shot operation at a read-out rate of 5 Mpx/s with 14-bit resolution. CaSSIS will operate in a push-frame mode with a Filter Strip Assembly (FSA), placed directly above the detector sensitive area, selecting 4 colour bands. The scale at a slant angle of 4.6 m/px from the nominal orbit is foreseen to produce frames of 9.4 km × 6.3 km on the Martian surface, and covering a Field of View (FoV) of 1.33° cross track × 0.88° along track.
The University of Bern was in charge of the full instrument integration as well as the characterisation of the focal plane of CaSSIS. The paper will present an overview of CaSSIS and the optical performance of the telescope and the FPA. The preliminary results of the on-ground calibration campaign and the first light obtained during the commissioning and pointing campaign (April 2016) will be described in detail. The instrument is acquiring images with an average Point Spread Function at Full-Width-Half-Maximum (PSF FWHM) of < 1.5 px, as expected.
On board of the ExoMars Trace Gas Orbiter (TGO), the Colour and Stereo Surface Imaging System (CaSSIS) developed under the lead of University of Bern, has the mission to provide stereo images of the planet’s surface in colour at a resolution of better than 5 m (4.54m from a circular orbit of 400 km) for enhancing our knowledge of the surface of Mars [1].
L. Gambicorti, D. Piazza, M. Gerber, A. Pommerol, V. Roloff, R. Ziethe, C. Zimmermann, V. Da Deppo, G. Cremonese, I. Ficai Veltroni, M. Marinai, E. Di Carmine, T. Bauer, P. Moebius, N. Thomas
A new technique based on photolithographic processes of thin-film optical pass band coatings on a monolithic substrate has been applied to the filters of the Focal Plane Assembly (FPA) of the Colour and Stereo Surface Imaging System (CaSSIS) that will fly onboard of the ExoMars Trace Gas Orbiter to be launched in March 2016 by ESA.
The FPA including is one of the spare components of the Simbio-Sys instrument of the Italian Space Agency (ASI) that will fly on ESA’s Bepi Colombo mission to Mercury. The detector, developed by Raytheon Vision Systems, is a 2kx2k hybrid Si-PIN array with a 10 μm pixel. The detector is housed within a block and has filters deposited directly on the entrance window. The window is a 1 mm thick monolithic plate of fused silica. The Filter Strip Assembly (FSA) is produced by Optics Balzers Jena GmbH and integrated on the focal plane by Leonardo-Finmeccanica SpA (under TAS-I responsibility). It is based on dielectric multilayer interference coatings, 4 colour bands selected with average in-band transmission greater than 95 percent within wavelength range (400-1100 nm), giving multispectral images on the same detector and thus allows CaSSIS to operate in push-frame mode.
The Field of View (FOV) of each colour band on the detector is surrounded by a mask of low reflective chromium (LRC), which also provides with the straylight suppression required (an out-of-band transmission of less than 10-5/nm). The mask has been shown to deal effectively with cross-talk from multiple reflections between the detector surface and the filter.
This paper shows the manufacturing and optical properties of the FSA filters and the FPA preliminary on-ground calibration results.
The Colour and Stereo Surface Imaging System (CaSSIS) is the high-resolution scientific imager on board the European
Space Agency’s (ESA) ExoMars Trace Gas Orbiter (TGO) which was launched on 14th March 2016 to Mars.
CaSSIS will observe the Martian surface from an altitude of 400 km with an optical system based on a modified TMA
telescope (Three Mirrors Anastigmatic configuration) with a 4th powered folding mirror. The camera EPD (Entrance
Pupil Diameter) is 135 mm, and the expected focal length is 880 mm, giving an F# 6.5 in the wavelength range of 400-
1100 nm with a distortion designed to be less than 2%.
CaSSIS will operate in a “push-frame” mode with a monolithic Filter Strip Assembly (FSA) produced by Optics Balzers
Jena GmbH selecting 4 colour bands and integrated on the focal plane by Leonardo-Finmeccanica SpA (under TAS-I
responsibility). The detector is a spare of the Simbio-Sys detector of the Italian Space Agency (ASI), developed by
Raytheon Vision Systems. It is a 2kx2k hybrid Si-PIN array with a 10 μm pixel pitch.
A scale of 4.6 m/px from the nominal orbit is foreseen to produce frames of 9.4 km × 47 km on the Martian surface.
The University of Bern was in charge of the full instrument integration as well as the characterization of the focal plane
and calibration of the entire instrument. The paper will present an overview of the CaSSIS telescope and FPA optical
performance. The preliminary results of on-ground calibration and the first commissioning campaign (April 2016) will
be described.
The Colour and Stereo Surface Imaging System (CaSSIS) is a high-resolution camera for the ESA ExoMars Trace Gas
Orbiter mission launched in March 2016. CaSSIS is capable of acquiring color stereo images of features on the surface
of Mars to better understand the processes related to trace gas emission.
The optical configuration of CaSSIS is based on a three-mirror anastigmatic off-axis imager with a relay mirror; to attain
telecentric features and to maintain compact the design, the relay mirror has power.
The University of Bern had the task of detector integration and characterization of CaSSIS focal plane. An OGSE
(Optical Ground Support Equipment) characterization facility was set up for this purpose. A pinhole, imaged through an
off-axis paraboloidal mirror, is used to produce a collimated beam.
In this work, the procedures to align the OGSE and to link together the positions of each optical element will be
presented. A global Reference System (RS) has been defined using an optical cube placed on the optical bench (OB) and
linked to gravity through its X component; this global RS is used to correlate the alignment of the optical components.
The main steps to characterize the position of the object to that of the CaSSIS focal plane have been repeated to guide
and to verify the operations performed during the alignment procedures. A calculation system has been designed to work
on the optical setup and on the detector simultaneously, and to compute online the new position of the focus plane with
respect to the detector.
Final results will be shown and discussed.
To change the intensity distribution of the fundamental mode in a Nd:YAG laser resonator to a top-hat profile we developed and used a dielectric graded-phase mirror. A super-Gaussian mode of the sixth order was generated by means of a graded-phase mirror with a simple ring-shaped phase step on a spherical reflector. The depth of the ring was 90 nm. The graded-phase mirror was manufactured with an ordinary vapor deposition technique. An annular mask with a thickness of 10 μm was used to avoid the deposition of the LaF3 vapor at the position of the ring. Laser experiments with continuous-wave and repetitively pulsed dioed-laser pumping were performed and compared. The results are in excellent agreement with the theory.
To change the intensity distribution of the fundamental mode in a Nd:YAG laser resonator to a top-hat profile we developed and used a dielectric graded-phase mirror. A super-Gaussian mode of the sixth order was generated by means of a graded-phase mirror with a simple ring-shaped phase step on a spherical reflector. The depth of the ring was 90 nm. Measurements with continuous-wave and repetitively pulsed diode-laser pumping were performed and compared. The results are in excellent agreement with the theory. To allow for real-time modification of the modes, adaptive optics such as deformable mirrors can be used. Experiments with an adaptive mirror featuring eight actuators on a glass substrate were performed. It was not yet possible to generate super-Gaussian modes with a deformable mirror but the beam quality in the multi-mode regime was improved significantly without closed-loop electronics. A reduction of the M2 value by 39 % was achieved.
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