As part of leading research for future space missions, we have been conducting a feasibility study on an optical imager system assumed to be mounted on a geostationary satellite for instantaneous Earth observation as needed. The target for ground sampling resolution was less than 10 m from geostationary orbit, and the primary mirror was set to a diameter of 3.5 m based on our previous conceptual study. Moreover, the primary mirror was conceptually designed with cuttingedge technologies such as segmented mirror technology for scalability to larger sizes in the future. The main technical challenges in achieving such a large optical system entailed reducing the primary mirror weight and minimizing dimensional changes in the space environment. Optical material selection was a particularly key consideration in defining the optical system performance. Therefore, a trade-off study was conducted on the selection of materials for the segmented primary mirror. The thermal deformation behaviors of certain low thermal expansion materials as mirror substrates were analytically compared under the assumed unsteady and inhomogeneous thermal conditions in geostationary orbit, in consideration of the deterioration induced by radiation.
In recent years, JAXA has been conducting a technical survey for a geostationary Earth observation satellite using a 3.5 m diameter aperture with a segmented primary mirror. One of the problems associated with such a large optical observing satellite is a reduction in image quality due to thermal deformation of the optical elements and the metering structure. In this paper, we present our first conceptual structural design and thermal analysis of that design. We also propose a solar light incident avoidance maneuver for this satellite and show the validity of that maneuver.
A feasibility study was conducted for an optical imager system assumed to be mounted on a geostationary orbit satellite for Earth observation. The targeted spatial resolution was less than 10 meters for panchromatic mode at nadir observation conditions, and the observation area was assumed to 100 × 100 square kilometers. The optical system was designed based on a Korsch three mirror anastigmat; the primary mirror was 3.5 meters in diameter, and the focal length was approximately 45 meters. The worst wavefront error was estimated at less than 0.017 λrms in the field of view. As the next step, the primary mirror was segmented, and a trade-off study was conducted on two types of segmented mirror configurations. The optical performance of each configuration was compared in terms of PSF and MTF. Moreover, the deterioration of optical performance due to the misalignment and distortion of the segmented mirror was discussed and numerically estimated by using the Monte Carlo method. The sensitivity of the wavefront error was consequently estimated for the segmented mirror assembly.
The ASTRO-H mission was designed and developed through an international collaboration of JAXA, NASA, ESA, and the CSA. It was successfully launched on February 17, 2016, and then named Hitomi. During the in-orbit verification phase, the on-board observational instruments functioned as expected. The intricate coolant and refrigeration systems for soft X-ray spectrometer (SXS, a quantum micro-calorimeter) and soft X-ray imager (SXI, an X-ray CCD) also functioned as expected. However, on March 26, 2016, operations were prematurely terminated by a series of abnormal events and mishaps triggered by the attitude control system. These errors led to a fatal event: the loss of the solar panels on the Hitomi mission. The X-ray Astronomy Recovery Mission (or, XARM) is proposed to regain the key scientific advances anticipated by the international collaboration behind Hitomi. XARM will recover this science in the shortest time possible by focusing on one of the main science goals of Hitomi,“Resolving astrophysical problems by precise high-resolution X-ray spectroscopy”.1 This decision was reached after evaluating the performance of the instruments aboard Hitomi and the mission’s initial scientific results, and considering the landscape of planned international X-ray astrophysics missions in 2020’s and 2030’s. Hitomi opened the door to high-resolution spectroscopy in the X-ray universe. It revealed a number of discrepancies between new observational results and prior theoretical predictions. Yet, the resolution pioneered by Hitomi is also the key to answering these and other fundamental questions. The high spectral resolution realized by XARM will not offer mere refinements; rather, it will enable qualitative leaps in astrophysics and plasma physics. XARM has therefore been given a broad scientific charge: “Revealing material circulation and energy transfer in cosmic plasmas and elucidating evolution of cosmic structures and objects”. To fulfill this charge, four categories of science objectives that were defined for Hitomi will also be pursued by XARM; these include (1) Structure formation of the Universe and evolution of clusters of galaxies; (2) Circulation history of baryonic matters in the Universe; (3) Transport and circulation of energy in the Universe; (4) New science with unprecedented high resolution X-ray spectroscopy. In order to achieve these scientific objectives, XARM will carry a 6 × 6 pixelized X-ray micro-calorimeter on the focal plane of an X-ray mirror assembly, and an aligned X-ray CCD camera covering the same energy band and a wider field of view. This paper introduces the science objectives, mission concept, and observing plan of XARM.
The soft x-ray spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from microvibration from cryocoolers mounted on the dewar. This is mitigated for the flight model (FM) by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the FM was verified before launch of the spacecraft in both ambient condition and thermal-vacuum condition, showing no detectable degradation in energy resolution. The in-orbit detector spectral performance and cryocooler cooling performance were also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the microvibration could degrade the cryogenic detector is shown. Lessons learned from the development to mitigate unexpected issues are also described.
Optical mirrors for space telescopes, which require high precision and high thermal stability, have commonly
been made of glass materials such as ultra low expansion glass (e.g. ULE®) or extremely low expansion glassceramic
(e.g. ZERODUR® or CLEARCERAM®). These materials have been well-known for their reliability
due to their long history of achievements in many space applications.
Soft X-ray Spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from micro-vibration from cryocoolers mounted on the dewar. This is mitigated for the flight model by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the flight model was verified before launch of the spacecraft in both ambient condition and thermal-vac condition, showing no detectable degradation in energy resolution. The in-orbit performance was also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the micro-vibration could degrade the cryogenic detector is shown.
Small-JASMINE program (Japan Astrometry Satellite Mission for INfrared Exploration) is one of applicants for JAXA (Japan Aerospace Exploration Agency) space science missions launched by Epsilon Launch Vehicles, and now being reviewed in the Science Committee of ISAS (Institute of Space and Astronautical Science), JAXA. Telescope of 300 mm aperture diameter will focus to the central region of the Milky Way Galactic. The target of Small-JASMINE is to obtain reliable measurements of extremely small stellar motions with the highest accuracy of 10 μ arcseconds and to provide precise distances and velocities of multitudes of stars up to 30,000 light years. Preliminary Structure design of Small- JASMINE has been done and indicates to satisfy all of requirements from the mission requirement, the system requirement, Epsilon Launch conditions and interfaces of the small science satellite standard bus. High margin of weight for the mission allows using all super invar structure that may reduce unforeseen thermal distortion risk especially caused by connection of different materials. Thermal stability of the telescope is a key issue and should be verified in a real model at early stage of the development.