We describe scientific objective and project status of an astronomical 6U CubeSat mission VERTECS (Visible Extragalactic background RadiaTion Exploration by CubeSat). The scientific goal of VERTECS is to reveal the star-formation history along the evolution of the universe by measuring the extragalactic background light (EBL) in the visible wavelength. Earlier observations have shown that the near-infrared EBL is several times brighter than integrated light of individual galaxies. As candidates for the excess light, first-generation stars in the early universe or low-redshift intra-halo light have been proposed. Since these objects are expected to show different emission spectra in visible wavelengths, multi-color visible observations are crucial to reveal the origin of the excess light. Since detection sensitivity of the EBL depends on the product of the telescope aperture and the field of view, it is possible to observe it with a small but wide-field telescope system that can be mounted on the limited volume of CubeSat. In VERTECS mission, we develop a 6U CubeSat equipped with a 3U-sized telescope optimized for observation of the visible EBL. The bus system composed of onboard computer, electric power system, communication subsystem, and structure is based on heritage of series of CubeSats developed at Kyushu Institute of Technology in combination with high-precision attitude control subsystem and deployable solar array paddle required for the mission. The VERTECS mission was selected for JAXA-Small Satellite Rush Program (JAXA-SMASH Program), a new program that encourages universities, private companies and JAXA to collaborate to realize small satellite missions utilizing commercial small launch opportunities, and to diversify transportation services in Japan. We started the satellite development in December 2022 and plan to launch the satellite in FY2025.
In recent years, small satellites have been utilized for remote sensing from Low Earth Orbit (LEO) with a spatial resolution of several meters. However, improving the temporal resolution for LEO remote sensing is challenging because of the short orbital period. Observation techniques using remote sensing from a Geostationary Orbit (GEO), or its nearby orbit are becoming increasingly crucial, particularly in disaster monitoring, due to their ability to provide high-temporal resolution. To improve both temporal and spatial resolutions from GEO, it is necessary to use an optical system with a diameter of several meters due to the diffraction limit. We propose the Formation Flying Synthetic Aperture Telescope (FFSAT). One of the key issues is realizing the optical system with an accuracy of less than 1/10 of the observation wavelength to get synthesized images. We propose a method for estimating and correcting misalignment and optical aberrations using adaptive optics.
Infrared space interferometers can surpass the spatial resolution limitations of single-dish space telescopes. However, stellar interferometers from space have not been realized because of technical difficulties. Two beams coming from individual satellites separated by more than a few tens of meters should precisely interfere such that the optical-path and angular differences between the two beams are reduced at the wavelength level. Herein, we propose a unique beam combiner for space interferometers that records the spectrally resolved interferometric fringes using the densified pupil spectroscopic technique. As the detector plane is optically conjugated to a plane, on which the two beams interfere, we can directly measure the relative phase difference between the two beams. Additionally, when an object within the field of view is obtained with a modest signal-to-noise ratio, we can extract the true complex amplitude from a continuous broadband fringe (i.e., one exposure measurement), without scanning a delay line and chopping interferometry. We discovered that this spectral imaging method is validated for observing the solar system objects by simulating the reflected light from Europa with a small stellar interferometer. However, because the structure of the object spectrum may cause a systematic error in the measurement, this method may be limited in extracting the true complex amplitude for other astronomical objects. Applying this spectral imaging method to general astrophysics will facilitate further research. The beam combiner and spectral imaging method are applied to a formation flying stellar interferometer with multiple small satellites in a Sun-synchronous orbit, named Space Experiment of InfraRed Interferometric Observation Satellite (SEIRIOS), for observation of the solar system objects in visible and near-infrared. We present an overview of SEIRIOS and the optimized optical design for a limited-volume spacecraft.
Small satellites have been used for remote sensing with a spatial resolution of several meters from LEO in recent years. However, it is difficult to increase temporal resolution for LEO remote sensing due to the short orbital period. Therefore, GEO remote sensing which enables observation of high temporal resolution from GEO or its nearby orbit is getting important. In order to obtain enough spatial resolution in GEO remote sensing, an optical system having a diameter of several meters is required because of the diffraction limit. It takes huge cost to realize such a large diameter primary mirror due to manufacturability and required accuracy. To address this problem, we propose a synthetic aperture telescope by small satellites formation flying. The synthetic aperture telescope is composed of several mirror satellites constituting a primary mirror of the telescope and an imaging satellite having a focal plane assembly. By optically synthesizing the light collected by each mirror satellite with the imaging satellite, a virtual large aperture telescope is constructed. In this paper, we assume the observation at near infrared to short wavelength infrared and show the specifications of the system. The apperture telescope and the image processing method used to extract high spatial frequency information from the observed images are also described.
Although astronomers have confirmed the existence of 4,000 exoplanets to date, it is still difficult to directly compare exoplanets with the planets in our solar system because most of the known transiting exoplanets have an orbital period shorter than 1 year. Recent analyses of the 4-year data from the Kepler spacecraft revealed dozens of long-period transiting exoplanets and showed that their abundance is of order unity around Sun-like stars. However, the stars targeted by Kepler are too faint to conduct follow-up observations. The on-going all-sky survey mission TESS, with four 10.5 cm cameras with a field of view of 24 deg x 24 deg, is finding nearby transiting planets; however, the nominal observation period (1 month{1 year) is too short to find long-period planets with au-scale orbits. Herein, we propose using the LOng-period Transiting exoplanet sUrvey Satellite (LOTUS) mission, which employs a 7.5 cm wide-field (33 deg x 33 deg) camera placed on a nanosatellite, to continuously monitor the same sky region and find long-period planets transiting nearby bright stars. We present a conceptual design for the optics and bus system of LOTUS. Our optical system has a uniform point spread function over the entire field of view and a wide wavelength range (0.5{1.0 um). The bus system is designed to ensure that the pointing precision is sufficient to achieve the sub-percent photometry required for the detection of transiting exoplanets.
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