JANUS (Jovis Amorum Ac Natorum Undique Scrutator) is a high-resolution camera operating in the spectral range 340-1080 nm and designed for the ESA space mission JUICE1 (Jupiter Icy moons Explorer) planned for launch in 2023 and arrival at Jupiter in 2031. The main scientific goal of the mission is the detailed investigation of Jupiter and its Galileian moons: after three years in Jupiter orbit and many fly-bys with the icy moons, JUICE will be the first spacecraft to be inserted in orbit around Ganymede in 2032. During the final stage of the mission, JANUS is expected to provide images of the moon surface with ground sampling up to 7.5 m/pixel in both panchromatic and narrow band spectral ranges. Leonardo Spa is the JANUS prime contractor and is in charge, on behalf of the Italian Space Agency (ASI) and in collaboration with the science team led by Parthenope University and the Italian Institute of Astrophysics (INAF), of developing and integrating the Opto-Mechanical Structure of JANUS Optical Head Unit (OHU). The present paper will discuss the procedure adopted for the integration of the OHU and the results in terms of optical quality of the system in flight conditions. Ensuring a Modulation Transfer Function (MTF) close to the diffraction limit at the Nyquist frequency of 71.4 cy/mm constitutes the main challenge for the telescope integration and sets the maximum acceptable transmitted wavefront error to be at most a few tens of nm over the whole field of view.
MAJIS (Moons And Jupiter Imaging Spectrometer) is the VIS-IR imaging spectrometer of the ESA/JUICE mission that will explore the Jovian system. It covers the spectral range 0.5 to 2.35 μm and 2.26 to 5.56 μm using two channels. The MAJIS Optical Head (OH) consists in a TMA telescope shared between the two channels, as well as a slit and collimator, then a dichroic filter that splits the light between the channels, each one being equipped with its own grating and focusing lens. The same type of detector is used for both spectrometers (apart from the order sorting filter), being a 400 pixels × 508 spectels with 36 µm pitch. We will present the characterization of the OH performed at Leonardo Company at cold operational temperatures (from 110K to 150K). The test set-up with a specific emphasis of the Optical Ground Support Equipment (OGSE) used during that characterization campaign will be shortly described. Then, the main performances (spatial, spectral) of the OH will be presented and a comparison with the requirements will be provided.
The STereo Imaging Channel (STC) is one of the three channels of the SIMBIO-SYS instrument on board the BepiColombo ESA spacecraft. The design of the camera consists in a double wide-angle camera with two sub-channels looking at ±20° with respect to the nadir direction. Each sub-channel can acquire three quasi-contiguous areas of the Mercury surface in different colours determined by the filters mounted on the detector. The filters are divided in two categories: 4 broad band filters (20 nm of bandwidth and centred at 420, 550, 750 and 920 nm respectively) necessary to the chemical analysis of the Hermean surface; 2 panchromatic (PAN) filters with 200 nm of bandwidth and centred at 600 nm, designed for the stereo acquisition. The nominal Field of View (FoV) of each sub-channel is 5:38°x4:8°. The in-flight stellar calibration will be performed during the nominal mission using stellar fields images. To effectively plan this calibration activity, two analyses have been performed: the first one consists in simulating the observation of stars having different apparent magnitude to derive the best integration time needed to reach a specific Signal to Noise Ratio. Considering the characteristics of the STC camera and of its CMOS detector, the threshold magnitude needed for a star to be detectable will also be determined. The second part consists in selecting the stellar fields from the ESA GAIA archive and Tycho stellar catalogue that contain a pre-defined minimum number of stars required to perform the in-flight geometrical calibration. This selection have been performed taking into account stars brighter than the threshold defined in the first part.
JANUS (Jupiter Amorum ac Natorum Undique Scrutator) is a high-resolution camera to be flown on board JUICE Spacecraft, devoted to investigate the atmosphere of Jupiter and the surfaces of his icy moons (Europa, Ganymede and Callisto), in the frame of ESA “Cosmic Vision” program. The scientific objectives that JANUS will reach constrained the design of JANUS Optical Head Unit (OHU), and in particular the specific measurement of Ganymede Libration, imposes highly stringent requirement on the Line of Sight (LoS) knowledge of the instrument. The differential thermal environment conditions of the mission orbits, as well as the instrument heat dissipation timelines, induce optical elements translation and rotations that correspond to a Line of Sight variation. During the mission, the LoS can be characterized with a stellar field or single star observation but none direct measurement of its variation can be retrieved during the scientific imaging sessions. To recover the LoS knowledge, a Structural Thermal Optical and Performance Analysis (STOP) is implemented. The optics and the instrument structure are Finit Element Modeled and processed (NASTRAN) imposing the temperature distributions obtained by the OHU Thermal Model (ESATAN-TMS). The obtained thermo-elastic deformations are then considered in the OHU Optical Model (ZEMAX). The resulting LoS and the dominant OHU temperature gradients are finally entangled with a proportionality relation, as well as its direction uncertainty. The indirect estimation of the LoS variation, and its uncertainty, can be establish, thanks to STOP analysis, in real time during operations as function of installed temperature sensor measurements.
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.
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.
The Stereo Channel (STC) is a double wide-angle camera developed to be one of the channels of the SIMBIO-SYS instrument onboard of the ESA BepiColombo mission to Mercury. STC main goal is to map in 3D the whole Mercury surface.
The geometric and radiometric responses of the STC Proto Flight model have been characterized on-ground during the calibration campaign. The derived responses will be used to calibrate the STC images that will be acquired in flight. The aim is to derive the functions that link the detected signal in digital number to the radiance of the target surface in physical units.
The result of the radiometric calibration consists in the determination of well-defined quantities: i) the dark current as a function of the integration time and of the detector temperature, nominally fixed at 268 K; ii) the Read Out Noise, which is associated with the noise signal of the read-out electronic; iii) the Fixed Pattern Noise, which is generated by the different response of each pixel; iv) once these quantities are known, the photon response and the Photo Response Non-uniformity, which represent the variation of the photon-responsivity of a pixel in an array, can be derived.
The final result of the radiometric calibration is the relation between the radiance of an accurately known and uniform source, and the digital numbers measured by the detector.
JANUS is the camera of the ESA mission JUICE, dedicated to high-resolution imaging in the extended-visible wavelength region (340 – 1080nm). The camera will observe Jupiter and its satellites providing detailed maps of their surfaces and atmospheres. During the mission, the camera will face a huge variety of observing scenarios ranging from the imaging of the surfaces of the satellites under varying illumination conditions to limb observation of the atmospheres. The stray-light performance of JANUS has been studied through non-sequential ray-tracing simulations with the aim to characterize and optimize the design. The simulations include scattering effects produced by micro-roughness and particulate contamination of the optical surfaces, the diffusion from mechanical surfaces and ghost reflections from refractive elements. The results have been used to derive the expected stray-light performance of the instrument and to validate the instrument design.
The optical design of the Moons And Jupiter Imaging Spectrometer (MAJIS), is discussed. MAJIS is a compact visible and near-infrared imaging spectrometer covering the spectral range from 0.5 to 5.54 μm (split into two channels), designed for the Jupiter Icy moons Explorer (JUICE) mission. The MAJIS optical layout is constituted by a TMA telescope shared between the two channels, as well as the slit and a collimator, a dichroic filter that splits the light between the channels (VIS-NIR and IR), each one endowed with its own grating, objective and detector. A flat mirror mounted in a Scan Unit before the telescope allows scanning the line of sight in a direction perpendicular to the slit. The collimator has a Schmidt off-axis configuration, with a specular correcting plate for each channel (the dichroic is inserted between the collimator primary mirror and the correcting plate). With the same conceptual layout in both channels, the collimated light is reflected by a flat ruled grating and crosses a completely dioptric objective. The objectives have the same focal length of the collimator, so both spectrometers have unitary magnification. A linear variable order rejection filter is placed in front of the detector so to reject the higher orders dispersed by the grating. A calibration unit allows radiometric and spectral calibration of both channels, with an incandescent lamp and a black body illuminating a common diffuser. Calibration is realized thanks to an extra-rotation of the Scan Unit. The developed design is optimized to work at cryogenic temperatures, with a good optical quality along the whole FOV and a good correction for transverse chromatic aberration and distortions.
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