3:45 PM - 4:00 PM
[PCG18-14] Development of the autofocusing subsystem (AFS) onboard the Raman spectrometer for MMX (RAX): Delivery of the AFS flight model
Keywords:MMX, Raman spectroscopy, Rover, Autofocusing mechanism, Optics
MMX Rover and RAX: JAXA’s Martian Moons Exploration (MMX) mission carries a small rover that will explore the surface of Phobos [Michel et al. 2022]. The Raman Spectrometer for MMX (RAX) is one of the four scientific instruments onboard the rover. RAX will determine the mineral composition of the Phobos surface to reveal the origin of the moon and the heterogeneity of the material distribution on its surface [Hagelschuer et al. 2019]. A study using a RAX-compatible breadboard model (BBM) demonstrated that the Raman spectra of Phobos-relevant minerals can be obtained with a signal-to-noise ratio (SNR) of up to 100, depending on mineral species [Cho et al. 2021]. Note that the flight model of RAX will allow for even higher SNRs with more sophisticated optics.
RAX Development: RAX is a compact Raman spectrometer equipped with a focus adjustment mechanism and a CW excitation laser (wavelength 532 nm). Japan and DLR (Germany) develop the autofocusing system and the spectrometer module, while INTA and University of Valladolid (Spain) are in charge of the laser assembly. Because the distance from RAX to Phobos soil will be unknown, fine-focusing laser beam onto a sample will be critical to obtain optimum Raman spectra. To achieve this, the Japanese team develops a high-quality light collection lens and laser focusing mechanism (Autofocusing Subsystem, AFS) as a part of the spectrometer module [Rodd-Routley et al. 2021].
Design of Autofocusing subsystem (AFS): The AFS is an opto-mechanical assembly capable to focus a collimated laser beam, collect Raman-scattered photons, and modify the focus position by scanning an objective lens unit in the vertical direction. The Raman-scattered light is collimated by the lens and sent to the spectrometer in the module. The translation mechanism consists of a small stepping motor, gears, lead screw, nut, and linear guide on which the lens is attached. The stroke of the translation mechanism is 13 mm with a positioning precision better than 50 μm. The laser beam diameter is designed to be < 50 μm at the focus. The distance from the lens to the Phobos surface (i.e., working distance) is approximately 80 mm. The tolerance to harsh environmental conditions was also required to AFS. For example, the AFS is required to work at a design temperature as low as -40℃ without any heaters.
Development of AFS: Two AFS engineering models (demonstration model DM; engineering-qualification model EQM) and one proto-flight model (PFM) were manufactured. We conducted a series of qualification tests (QT) using the EQM, including thermal vacuum test at a temperature range between -60 and +60℃, sine vibration test up to 30 G, random vibration test up to 32 G-rms, and shock test for 1000 G at 1000 Hz. After the QT, the motor gear head was replaced with a higher-torque type and lens assembling procedure was reconsidered for the manufacturing of the PFM. The AFS EQM was delivered to DLR in November 2021. The proto-flight tests of the PFM were conducted in December 2021. In the verification campaign, we confirmed that the optical performance of the lens satisfied all its requirements, the lens stroke was more than 13 mm, the lens positioning reproducibility was better than 50 μm, and the motor torque margin was higher than a factor of 2 even at -40℃. Such a compact, lightweight (< 250 g), power-saving (< 1 W), highly robust opto-mechanical device with a high-performance optics can be widely used not only for Raman spectroscopy but for other planetary lander missions that involve light-focusing mechanisms. As of February 2022, the integration and tests of the RAX FM are in progress at DLR [Schroeder et al. 2022]. RAX will be delivered to the rover for integration in May 2022.
[1] Michel P. et al. (2022) Earth, Planets and Space, 74, 2. [2] Cho, Y. et al. (2021) Earth, Planets and Space, 73, 232 [3] Hagelschuer, T. et al. (2019) 70th Int. Astronautical Congress, 2019. [4] Rodd-Routley, S. et al. (2021) 52nd Lunar and Planetary Science Conference, Abstr. #2548 [5] Schroeder et al. (2022) 53rd Lunar and Planetary Science Conference, Abstr. #1915
RAX Development: RAX is a compact Raman spectrometer equipped with a focus adjustment mechanism and a CW excitation laser (wavelength 532 nm). Japan and DLR (Germany) develop the autofocusing system and the spectrometer module, while INTA and University of Valladolid (Spain) are in charge of the laser assembly. Because the distance from RAX to Phobos soil will be unknown, fine-focusing laser beam onto a sample will be critical to obtain optimum Raman spectra. To achieve this, the Japanese team develops a high-quality light collection lens and laser focusing mechanism (Autofocusing Subsystem, AFS) as a part of the spectrometer module [Rodd-Routley et al. 2021].
Design of Autofocusing subsystem (AFS): The AFS is an opto-mechanical assembly capable to focus a collimated laser beam, collect Raman-scattered photons, and modify the focus position by scanning an objective lens unit in the vertical direction. The Raman-scattered light is collimated by the lens and sent to the spectrometer in the module. The translation mechanism consists of a small stepping motor, gears, lead screw, nut, and linear guide on which the lens is attached. The stroke of the translation mechanism is 13 mm with a positioning precision better than 50 μm. The laser beam diameter is designed to be < 50 μm at the focus. The distance from the lens to the Phobos surface (i.e., working distance) is approximately 80 mm. The tolerance to harsh environmental conditions was also required to AFS. For example, the AFS is required to work at a design temperature as low as -40℃ without any heaters.
Development of AFS: Two AFS engineering models (demonstration model DM; engineering-qualification model EQM) and one proto-flight model (PFM) were manufactured. We conducted a series of qualification tests (QT) using the EQM, including thermal vacuum test at a temperature range between -60 and +60℃, sine vibration test up to 30 G, random vibration test up to 32 G-rms, and shock test for 1000 G at 1000 Hz. After the QT, the motor gear head was replaced with a higher-torque type and lens assembling procedure was reconsidered for the manufacturing of the PFM. The AFS EQM was delivered to DLR in November 2021. The proto-flight tests of the PFM were conducted in December 2021. In the verification campaign, we confirmed that the optical performance of the lens satisfied all its requirements, the lens stroke was more than 13 mm, the lens positioning reproducibility was better than 50 μm, and the motor torque margin was higher than a factor of 2 even at -40℃. Such a compact, lightweight (< 250 g), power-saving (< 1 W), highly robust opto-mechanical device with a high-performance optics can be widely used not only for Raman spectroscopy but for other planetary lander missions that involve light-focusing mechanisms. As of February 2022, the integration and tests of the RAX FM are in progress at DLR [Schroeder et al. 2022]. RAX will be delivered to the rover for integration in May 2022.
[1] Michel P. et al. (2022) Earth, Planets and Space, 74, 2. [2] Cho, Y. et al. (2021) Earth, Planets and Space, 73, 232 [3] Hagelschuer, T. et al. (2019) 70th Int. Astronautical Congress, 2019. [4] Rodd-Routley, S. et al. (2021) 52nd Lunar and Planetary Science Conference, Abstr. #2548 [5] Schroeder et al. (2022) 53rd Lunar and Planetary Science Conference, Abstr. #1915