Diffractive Optical Elements (DOEs) are commonly used in the photonics community for several purposes, such as geometrical calibration of cameras,1 medical treatments, lithography, LIDAR applications. In the context of the optical alignment and integration of the RAman Spectrometer for MMX (RAX),2 a DOE was included in the test setup with the goal of providing a clear figure of merit to optimize the focusing of a dioptric lens objective on to the spectrometer detector. This Raman spectrometer will be integrated later this year in a small Rover on-board the Martian Moons eXploration (MMX) mission led by JAXA, and will operate on Phobos’ surface to characterize the different materials composing Phobos’ soil. To achieve this, the optical design of RAX is very challenging in terms of performance to reach in very limited volume and mass. As described in Ref. 2, the optical alignment and integration of RAX was a very challenging exercise, requiring several optical setups and methods. The usage of a DOE was introduced to solve a classical problem during the integration of a camera: how to integrate both the optical objective (lens assembly) and the detector to ensure that both the optical focal plane and the detector sensitive plane are co-planar. When illuminated by a collimated laser beam, the implemented DOE generates a regular pattern of collimated beams with well-known deviation angles from the input beam. It acts as a 2D diffraction grating, and generates a pattern field which covers the entire field of view of our camera. Thanks to this property, the Camera Interface Objective of RAX could be successfully positioned and oriented with respect to the detector mechanical interface. It was achieved by acquiring successive images of the DOE pattern with controlled defocused laser beam illuminating it. We were then able to compute the equivalent mechanical defocus needed to maximize the image quality. This maximizes the overall instrument performance and will ensure best possible scientific measurement on Phobos.
The Martian Moons eXploration (MMX) mission led by JAXA to Mars moons Phobos and Deimos involves a small rover developed by DLR/CNES that will be operating on Phobos’ surface. Aboard it is the Raman Spectrometer for MMX (RAX), whose main scientific objectives address Phobos surface mineralogy, its heterogeneity and relation to the Mars mineralogy. Raman spectrometers require strong suppression of straylight, since this technique operates with few nano-Watt signals that should have significant contrast to all other sources of light inside the instrument. The mission requirements involving RAX call for a compact and sophisticated optical design, precluding space for straylight suppressive elements. To optimize straylight suppression in RAX, Raman scattering, Photoluminescence and reflection were characterized for candidate coatings representing different absorbing materials and fabrication technologies over spectral ranges between 530 nm and 680 nm. This was complimented by mechanical testing to aid selection of the coatings for parts inside the RAX flight model.
The Martian Moons eXploration (MMX) mission led by JAXA will conduct remote sensing of both Martian moons Phobos and Deimos and in-situ observations and return samples from Phobos. A small rover will be operating on Phobos’ surface and perform scientific measurements, in particular with its Raman Spectrometer for MMX (RAX). The instrument is jointly developed by DLR with partners from Spain (INTA, University of Valladolid) and Japan (JAXA, University of Tokyo). With its more than 20 optical elements (e.g. laser, lenses, mirrors, grating, dichroic beam-splitters, spectral filters), the optical alignment and integration of this very compact Raman spectrometer was one of the biggest challenges of the instrument development at DLR. This article will cover the different steps of alignment with 1) the integration of the lenses in each individual lens group, 2) the alignment and integration of each lens group to build the spectrometer, and 3) the global alignment verification of the end-to-end instrument. The main goal was to integrate the optical elements in RAX’s mechanical housing providing maximized scientific performance. This meant for example that the detector’s sensitive surface had to be precisely placed at the focal plane surface of the imaging objective to optimize the spectral resolution, but also that the confocality of the laser output (and image on Phobos’ surface) with the spectrometer slit had to be very accurately adjusted to optimize the Signal to Noise Ratio of the Raman features. Aligning and integrating a state-of-the art Raman spectrometer in a very compact volume of less than 10x10x10 cm³ and a mass lower than 1.5 kg was challenging but successful. The different tests performed on the instrument presented here also showed the robustness of the design and demonstrated that RAX can perform excellent scientific measurements on Phobos.
Verification of thermal-mechanical-optical design for optical instruments in space exploration is highly significant due to large temperature variation and exposure to high shock and vibration levels. Such instruments must be completely robust to these harsh environments, as there are usually no options for realignment. The JAXA Martian Moons eXploration (MMX) Mission is set for launch in 2024 with main objectives to study the Martian moons, Deimos and Phobos. A rover will acquire for the first time Raman spectra of the Phobos surface using the Raman Spectrometer for MMX (RAX) developed at DLR. The Structural-Thermal-Model (STM) of RAX presented an early opportunity to evaluate the robustness of the instrument optical alignment to thermal and mechanical environments. An interferometric method implementing dummy objectives with cross hairs was developed to enable inline six-DOF measurements at critical places within the STM before development model (DM) optics were manufactured.
The Venus Emissivity Mapper (VEM) has a mature design with an existing laboratory prototype verifying an achievable instrument SNR of well above 1000 as well as a predicted error in the retrieval of relative emissivity of better than 1%. VEM will provide a global map of surface composition as well as redox state of the surface by observing the surface with six narrow band filters, ranging from 0.86 to 1.18 μm. Continuous observation of Venus’ thermal emission will place tight constraints on current day volcanic activity. Eight additional channels provide measurements of atmospheric water vapor abundance as well as cloud microphysics and dynamics and permit accurate correction of atmospheric interference on the surface data. A mission combining VEM with a high-resolution radar mapper such as the ESA EnVision or NASA VERITAS mission proposals will provide key insights in the divergent evolution of Venus. Here we discuss the approach and results of the evaluation of the VEM sensor’s radiometric performance by an analysis of the corresponding signal processing chain. The passage of a simulated radiometric scene signal of the planet Venus through the VEM optics, detector, analog and digital electronics has been evaluated based on a theoretical model of the system. The goal was to verify the overall system performance with respect to the scientific requirements. This is building on our preliminary evaluation of the VEM laboratory prototype and confirms that the VEM design has significant performance margins.
The VenSpec instrument suite is part of the payload for the ESA M5 mission proposal EnVision which is currently in a competitive Phase A study. VenSpec consists of three channels: VenSpec-M, VenSpec-H and VenSpec-U. VenSpec-M will provide near-global compositional data on rock types, weathering, and crustal evolution by mapping the Venus surface in five atmospheric windows. VenSpec-H will be dedicated to extremely high-resolution atmospheric measurements. The main objective of the VenSpec-H instrument is to detect and quantify SO2, H2O and HDO in the lower atmosphere, to enable characterization of volcanic plumes and other sources of gas exchange with the surface of Venus, complementing VenSAR and VenSpecM surface and SRS subsurface observations. VenSpec-U will monitor sulphured minor species (mainly SO and SO2) and the as yet unknown UV absorber in Venusian upper clouds and just above. In combination, VenSpec will provide unprecedented insights into the current state of Venus and its past evolution. VenSpec will perform a comprehensive search for volcanic activity by targeting atmospheric signatures, thermal signatures and compositional signatures, as well as a global map of surface composition.
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