Masataka Naitoh

Long-Term Vicarious Calibration of GOSAT Short-Wave Sensors: Techniques for Error Reduction and New Estimates of Radiometric Degradation Factors

This work describes the radiometric calibration of the short-wave infrared (SWIR) bands of two instruments aboard the Greenhouse gases Observing SATellite (GOSAT), the Thermal And Near infrared Sensor for carbon Observations Fourier Transform Spectrometer (TANSO-FTS) and the Cloud and Aerosol Imager (TANSO-CAI). Four vicarious calibration campaigns (VCCs) have been performed annually since June 2009 at Railroad Valley, NV, USA, to estimate changes in the radiometric response of both sensors. While the 2009 campaign ( VCC2009) indicated significant initial degradation in the sensors compared to the prelaunch values, the results presented here show that the stability of the sensors has improved with time. The largest changes were seen in the 0.76 μm oxygen A-band for TANSO-FTS and in the 0.380 and 0.674 μm bands for TANSO-CAI. This paper describes techniques used to optimize the vicarious calibration of the GOSAT SWIR sensors. We discuss error reductions, relative to previous work, achieved by using higher quality and more comprehensive in situ measurements and proper selection of reference remote sensing products from the Moderate Resolution Imaging Spectroradiometer used in radiative transfer calculations to model top-of-the-atmosphere radiances. In addition, we present new estimates of TANSO-FTS radiometric degradation factors derived by combining the new vicarious calibration results with the time-dependent model provided by Yoshida (2012), which is based on analysis of on-board solar diffuser data. We conclude that this combined model provides a robust correction for TANSO-FTS Level 1B spectra. A detailed error budget for TANSO-FTS vicarious calibration is also provided.

Development of the Compact InfraRed Camera (CIRC) for wildfire detection

The Compact InfraRed Camera (CIRC) is a technology-demonstration payload to be carried on the Small Demonstration Satellite type-2 (SDS-2). The SDS program is a JAXA activity to demonstrate a variety of new technologies and new missions. The CIRC is an infrared camera equipped with an uncooled infrared array detector (microbolometer). The mission of the SDS-2/CIRC project is to demonstrate the potential of the microbolometer, especially for wildfire detection but also for other applications. This paper introduces the detailed design and concept of CIRC. We also discuss preliminary results of the feasibility study on wildfire detection using thermal infrared images.

Development of the Compact Infrared Camera (CIRC) for earth observation

We have developed Compact Infrared Camera (CIRC) with an uncooled infrared array detector (microbolometer) for space applications. The main mission of the CIRC is to demonstrate technology for wildfire detection. Wildfires are a major and chronic disaster that affects many countries, especially those in the Asia-Pacific region, and the situation may get worse with global warming and climate change. The CIRC detector has the largest format (640 × 480 pixels) ever used for observations of Earth from space. Microbolometers have the advantage of not requiring cooling systems such as a mechanical cooler and are suitable for resource-limited sensor systems or small satellites. In addition, the CIRC employs athermal optics and a shutter-less system, and hence, it is of a small size, is lightweight, and consumes low electrical power. The CIRC design was based on a commercial infrared camera and employs commercial-off-the-shelf (COTS) parts to reduce the cost and time for development. The CIRC will be carried as a technology demonstration payload of ALOS-2 and ISS/JEM, which will be launched in 2013 and 2014. We have developed the CIRC Proto Flight Model (PFM) and performed experiments for calibration in January 2012. In this paper, we present the verification results of the athermal characteristics and the calibration of the shutter-less system.

Optical testing activities for the SPICA telescope

SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a Japan-led infrared astronomical satellite project with a 3-m-class telescope in collaboration with Europe. The telescope is cooled down to temperature below 6 K in space by a combination of mechanical coolers with radiative cooling in space. The telescope has requirements for its total weight to be lighter than 700 kg and for the imaging performance to be diffraction-limited at 5 μm at 6 K. The mirrors will be made of silicon carbide (SiC) or its related material, which has large heritages of the AKARI and Herschel telescopes. The design of the telescope system has been studied by the Europe-Japan telescope working group led by ESA with European industries to meet the requirements. As for optical testing, responsibilities will be split between Europe and Japan so that final optical verification at temperatures below 10 K will be executed in Japan. We present our recent optical testing activities in Japan for the SPICA telescope, which include the numerical and experimental studies of stitching interferometry as well as modifications of the 6-m-diameter radiometer space chamber facility at Tsukuba Space Center in JAXA. We also show results of cryogenic optical testing of the 160-mm and 800-mm lightweight mirrors made of a C/SiC material called HBCesic, which is a candidate mirror material for the SPICA telescope.

The radiation tolerance of chalcogenide glasses

Chalcogenide glasses are compounded from chalcogen elements, such as sulphur, selenium, and tellurium. These glasses are applied to commercial applications, e.g., night vision, because they transmit infrared in the spectral range of 0.8-16μm. Chalcogenide glasses have greater advantages over germanium (Ge), i.e., their wide spectral range of high transmissivity and their small temperature dependence of the refractive index. We have developed the Compact Infrared Camera (CIRC) with an uncooled infrared array detector (microbolometer) for space applications. The CIRC has been scheduled to launch in 2013 to demonstrate the usability of a microbolometer as a space application. The optics of the CIRC adopts two different kinds of materials for athermal optics. One is germanium, and the other is GASIR1® which is a chalcogenide glass (Ge22As20Se58) developed by Umicore. However, the radiation tolerance of GASIR® has not been investigated in the past. We carried out irradiation tests to investigate the radiation tolerance of GASIR1®. We irradiated GASIR1® with gamma-rays (Co60, 1.17 MeV and 1.33 MeV) up to 3Mrad. We measured the transmissivity and refractive index in the infrared range before and after irradiation. In this paper, we report the results of the irradiation tests of GASIR1®.

Cryogenic optical testing of an 800 mm lightweight C/SiC composite mirror mounted on a C/SiC optical bench.

We tested the optical performance at cryogenic temperatures of an 800 mm diameter lightweight mirror, consisting of carbon-fiber reinforced silicon carbide and with a mass of 11.2 kg. The ceramic composite of the mirror was HB-Cesic, developed by ECM, Germany, and Mitsubishi Electric Corporation, Japan. The test was carried out while the mirror was mounted, via Invar stress relief supports, on a lightweight optical bench also made of HB-Cesic. During the test, both the mirror and the optical bench were cooled to 18 K in a liquid-helium chamber. The test consisted of measuring the mirrors change of surface figure with an interferometer installed outside the cryo-chamber. The cryogenic deformation of the mirror was 110 nm RMS with no significant residual deformation after cooling, which is very promising for the applicability of the HB-Cesic composite to large lightweight cryogenic space optics.

Optical testing of lightweight large all-C/SiC optics

We carried out various tests of 800-mm-diameter aperture, lightweight optics that consisted wholly of carbon fiber-reinforced SiC composite, called HB-Cesic. A cryogenic optical test was performed on the primary mirror to examine any CTE irregularity as a surface change, and only small deformations were observed. The primary mirror was assembled with a convex secondary mirror into an optical system and tested under vacuum at the 6-m-diameter radiometer space chamber at Tsukuba Space Center of JAXA, where we have prepared interferometric metrological facilities to establish techniques to test large optical systems in a horizontal light-axis configuration. The wavefront difference between under vacuum and under atmosphere was confirmed to be less than 0.1 λ at λ=633 nm, if we realigned the optical axis of the interferometer and flat mirror under vacuum. We also demonstrated a stitching interferometry using the Φ800-mm optics by rotating a mask wheel of subapertures in front of the optical reference flat. The wavefront stitched from eight individual measurements of Φ275-mm subapertures differs from the full-aperture measurement without the mask by about 0.1 λ nm RMS, which showed the technique could able to be applied to test large telescopes especially for infrared wavelength region.

Experimental and numerical study of stitching interferometry for the optical testing of the SPICA Telescope

SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a Japan-led infrared astronomical satellite project with a 3.2-m lightweight cryogenic telescope. The SPICA telescope has stringent requirements such as that for the imaging performance to be diffraction-limited at the shortest core wavelength of 5 microns at the operating temperature of 6 K. The design of the telescope system has been studied by the Europe-Japan telescope working group led by ESA with the European industries, the results of which will be presented in other papers. We here present our recent optical testing activities in Japan for the SPICA telescope, focusing on the experimental and numerical studies of stitching interferometry. The full pupil of the SPICA telescope will be covered by a sub-pupil array consisting of small autocollimating flat mirrors (ACFs), which are rotated with respect to the optical axis of the telescope. For preliminary stitching experiments, we have fabricated an 800-mm lightweight telescope all made of the C/SiC called HBCesic, which is a candidate mirror material for the SPICA telescope, and started optical testing with 900-mm and 300-mm ACFs at an ambient temperature. ACFs can suffer significant surface deformation in testing a telescope at cryogenic temperatures, which is difficult to be measured directly. We therefore investigate the effects of the surface figure errors of the ACFs on stitching results by numerical simulation.

Development and tests of interferometry facility in 6-m diameter radiometer thermal vacuum chamber in Tsukuba Space Center

We present a test of optical metrology for 800-mm spaceborne optics in the 6-m radiometer thermal vacuum chamber at JAXAs Tsukuba Space Center of JAXA. Under the framework of the JAXAs large-optics study program for astronomy and Earth observations, we developed a test bench for interferometric metrology of large optics with an auto-collimation method in the chamber. The optical system was aligned in a horizontal light-axis configuration within the facility limit to handle a 3.5-m aperture telescope like SPICA. A high-speed interferometer was contained in an aluminum and titanmade pressure vessel, which was mounted on the five-axis stage. We tested the 800-mm lightweight C/SiC optics using a 900-mm diameter flat mirror. Alignment changes in tilts of about ten arcseconds were observed as pressure went down from 1 atm to vacuum. After we re-aligned the interferometer and flat mirror, the wavefronts through the optics under vacuum were observed to increase in astigmatism aberration by 0.07λRMS at λ=633nm from under atmosphere, which might be caused by a deformation in the test optics or flat mirror.

Performance of lightweight large C/SiC mirror

Very lightweight mirror will be required in the near future for both astronomical and earth science/observation missions. Silicon carbide is becoming one of the major materials applied especially to large and/or light space-borne optics, such as Herschel, GAIA, and SPICA. On the other hand, the technology of highly accurate optical measurement of large telescopes, especially in visible wavelength or cryogenic circumstances is also indispensable to realize such space-borne telescopes and hence the successful missions. We have manufactured a very lightweight Φ=800mm mirror made of carbon reinforced silicon carbide composite that can be used to evaluate the homogeneity of the mirror substrate and to master and establish the ground testing method and techniques by assembling it as the primary mirror into an optical system. All other parts of the optics model are also made of the same material as the primary mirror. The composite material was assumed to be homogeneous from the mechanical tests of samples cut out from the various areas of the 800mm mirror green-body and the cryogenic optical measurement of the mirror surface deformation of a 160mm sample mirror that is also made from the same green-body as the 800mm mirror. The circumstance and condition of the optical testing facility has been confirmed to be capable for the highly precise optical measurements of large optical systems of horizontal light axis configuration. Stitching measurement method and the algorithm for analysis of the measurement is also under study.