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.
Multiband optical cameras have played important roles in science observations for many planetary explorations. To ensure high-quality imaging observations using a camera, it is crucial to reduce stray light, which is defined as light reaching an imaging detector via an irregular path in this study. Multiple reflections between a detector and a bandpass filter are often the major sources of stray light in planetary camera optics. In this article, we present a study for camera optics using tilted bandpass filters to reduce stray light caused by multiple reflections between a detector and bandpass filters. A comparative study for four cases of optical design, which have different configurations of the bandpass filter, was conducted. A case of optical design adopting a combination of X- and Y-tilted filters provided high performance. The results of this study are potentially applicable not only for cameras for planetary exploration but also for various other optics.
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 Raman Spectrometer for MMX (RAX) as part of the JAXAs Martian Moons eXploration (MMX) mission, to be launched in 2024, is designed for in-situ science on the Martian moon Phobos. It is installed on the MMX rover to investigate the Phobos surface mineralogy complementary to the anticipated sample return mission of MMX reaching earth in 2029 [1]. To ensure high Raman signals with the RAX instrument we utilize a volume phase holographic (VPH) grating as diffracting element. The VPH grating diffracts light by refractive index modulations within a thin layer of transmissive gelatin sandwiched between two glass substrates. Optimized VPH grating parameters combined with a small spectral bandwidth lead to peak efficiencies approaching up to 100 % [2]. Due to the rather small Raman scattering efficiency they are particular suitable for space instrumentation, where initial laser intensity is relatively limited [3]. We have designed an optical setup for the characterization of 1st order diffraction efficiency and wave front aberration evaluation. A laser source similar in emission characteristics to the RAX laser (Nd:YAG at 532 nm) is widened to 14.2 mm beam diameter before illuminating the VPH grating. The VPH grating is installed axis-centered on a rotation platform within a second outer rotational platform mounting a camera for optical verification or a laser power meter for the diffraction efficiency measurement. The VPH gratings reach diffraction efficiencies up to 87 % within their specified spectral range with diffraction limited patterns nearly identical to the undisturbed reference beam and dispersed only due to the laser band width.
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