JANUS is a multi-filter optical camera part of the JUICE ESA Mission, that has been launched last April from the French Guiana towards the Giovian system, where it will arrive in 2031. During the design phase of the instrument an extensive Straylight Analysys has been carried on, but after AIV the need to update the analysis on the base of the 'as built' system has become desiderable, to better interpretate the calibration data and prepare for science phase. We here report about this update, covering the rationale of the update, the used methodology and the obtained results.
The STereoscopic imaging Channel (STC) is one of the three channels of SIMBIO-SYS instrument, whose goal is to study the Mercury surface in visible wavelength range. The SIMBIO-SYS instrument is on-board of ESA Bepicolombo spacecraft. STC is a double wide angle camera designed to map in 3D the whole Mercury surface. The detector of STC has been equipped with six filters: two panchromatic and four broad band. The panchromatic filters are centred at 700 nm with 200 nm of bandwidth, while the broad band ones have bandwidth of 20 nm and are centred at 420, 550, 750 and 920 nm, respectively. In order to verify the relative spectral response of each STC sub-channel, a spectral calibration has to be performed during the on-ground calibration campaign. The result consists in the transmissivity curve of each filter of STC as function of wavelength. The camera has been illuminated with a monochromator coupled with a diffuser and a collimator. The images have been acquired by changing the wavelength of the monochromator in the range correspondent to the filter bandwidth. The background images have been obtained by covering the light source and have been used to calculate and subtract the dark signal, fixed pattern noise (FPN) and ambient effects.
On December 2018, the Near Earth Commissioning Phase (NECP) has been place forSIMBIO-SYS (Spectrometers and Imagers for MPO BepiColombo Integrated Observatory – SYStem), the suite part of the scientific payload of the BepiColombo ESA-JAXA mission. SIMBIO-SYS is composed of three channels: the high resolution camera (HRIC), the stereo camera (STC) and the Vis/NIR spectrometer (VIHI) . During the NECP the three channels have been operated properly. For the three channels were checked the operativity and the performance. The commanded operations allowed to verify all the instrument functionalities demonstrating that all SIMBIO-SYS channels and subsystems work nominally. During this phase we also validated the Ground Segment Equipment (GSE) and the data analysis tools developed by the team.
BepiColombo is the first ambitious, multi-spacecraft mission of ESA/JAXA to Mercury. It will be launched in October 2018 from Kourou, French Guiana, starting a 7-year journey, which will bring its modules to the innermost planet of the solar system.
The Stereo Camera (STC) is part of the SIMBIO-SYS instrument, the Italian suite for imaging in visible and near infrared which is mounted on the BepiColombo European module, i.e. the Mercury Planetary Orbiter (MPO). STC represents the first push-frame stereo camera on board of an ESA satellite and its main objective is the global three-dimensional reconstruction of the Mercury surface.
The harsh environment around Mercury and the new stereo acquisition concept adopted for STC pushed our team to conceive a new design for the camera and to carry out specific calibration activities to validate its photogrammetric performance. Two divergent optical channels converging the collected light onto a unique optical head, consisting in an off-axis telescope, will provide images of the surface with an on-ground resolution at periherm of 58 m and a vertical precision of 80 m.
The observation strategies and operation procedures have been designed to optimize the data-volume and guarantee the global mapping considering the MPO orbit.
Multiple calibrations have been performed on-ground and they will be repeated during the mission to improve the instrument performance: the dark side of the planet will be exploited for dark calibrations while stellar fields will be acquired to perform geometrical and radiometric calibrations.
The ESA-JAXA mission BepiColombo toward Mercury will be launched in October 2018. On board of the European module, MPO (Mercury Planetary Orbiter), the remote sensing suite SIMBIOSYS will cover the imaging demand of the mission. The suite consists of three channels dedicated to imaging and spectroscopy in the spectral range between 420 nm and 2 μm. STC (STereo Imaging Channel) will provide the global three-dimensional reconstruction of the Mercury surface with a vertical accuracy better than 80 m and, as a secondary scientific objective, it will operate in target oriented mode for the acquisition of multi spectral images with a spatial scale of 65 m along-track at the periherm for the first orbit at Mercury. STC consists in 2 sub-channels looking at the Mercury surface with an angle of ±20° with respect to the nadir direction. Most of the optical elements and the detector are shared by the two STC sub-channels and to satisfy the scientific objectives six filters strips are mounted directly in front of the sensor. An off-axis and unobstructed optical configuration has been chosen to enhance the imaging contrast capabilities of the instrument and to allow to reduce the impact of the ghosts and stray light. The scope of this work is to present the on-ground geometric calibration pipeline adopted for the STC instrument. For instruments dedicated to 3D reconstruction, a careful geometric calibration is important, since distortion removal has a direct impact on the registration and the mosaicking of the images. The definition of the distortion for off-axis optical configuration is not trivial, this fact forced the development of a distortion map model based on the RFM (rational function model). In contrast to other existing models, which are based on linear estimates, the RFM is not specialized to any particular lens geometry, and is sufficiently general to model different distortion types, as it will be demonstrated.
BepiColombo is one of the cornerstone missions of the European Space Agency dedicated to the exploration of the planet Mercury and it is expected to be launched in July 2016.
One of the BepiColombo instruments is the STereoscopic imaging Channel (STC), which is a channel of the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIOSYS) suite: an integrated system for imaging and spectroscopic investigation of the Mercury surface. STC main aim is the 3D global mapping of the entire surface of the planet Mercury during the BepiColombo one year nominal mission.
The STC instrument consists in a novel concept of stereocamera: two identical cameras (sub-channels) looking at ±20° from nadir which share most of the optical components and the detector. Being the detector a 2D matrix, STC is able to adopt the push-frame acquisition technique instead of the much common push-broom one.
The camera has the capability of imaging in five different spectral bands: one panchromatic and four intermediate bands, in the range between 410 and 930 nm.
To avoid mechanisms, the technical solution chosen for the filters is the single substrate stripe-butted filter in which different glass pieces, with different transmission properties, are glued together and positioned just in front of the detector.
The useful field of view (FoV) of each sub-channel, though divided in 3 strips, is about 5.3° x 3.2°. The optical design, a modified Schmidt layout, is able to guarantee that over all the FoV the diffraction Ensquared Energy inside one pixel of the detector is of the order of 70-80%.
To effectively test and calibrate the overall STC channel, an ad hoc Optical Ground Support Equipment has been developed. Each of the sub-channels has to be separately calibrated, but also the data of one sub-channel have to be easily correlated with the other one.
In this paper, the experimental results obtained by the analysis of the data acquired during the preliminary onground optical calibration campaign on the STC Flight Model will be presented.
This analysis shows a good agreement between the theoretical expected performance and the experimental results.
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