In the paper, we proposed a compact Korsch-type CubeSat telescope for optical remote sensing validation. The optical payload will be mounted on a 6U CubeSat named ONGLISAT. This telescope has an aperture of 92 mm and an effective focal length of 725 mm, providing a field of view of 2.2 degrees. The optical payload is equipped with a linear TDI CMOS image sensor, capable of capturing high-resolution panchromatic images with a ground sampling distance (GSD) of 2.8 meters in a low-Earth orbit at an altitude of 406 kilometers. The Korsch telescope consists of three aspheric mirrors (primary, secondary, and tertiary mirrors) as well as a roof mirror. This paper presents details of the optical design, assembly, integration, and performance test for the compact Korsch telescope. The image quality of the Korsch telescope is evaluated by the contrast transfer function (CTF). Finally, after thermal vacuum cycling testing and vibration testing, the telescope still maintains a CTF of 0.12 at 50 lp/mm line pairs. The Korsch telescope has the characteristics of a low obstruction ratio and high stray light blocking. The design of the telescope has higher resolution, a smaller size, and lower launch costs, which are beneficial for future applications in science, agriculture, commerce, and disaster relief. Furthermore, its optical quality can be quickly validated, serving as a preliminary study for larger telescopes.
The FORMOSAT-8 Program aims to develop high-resolution optical remote sensing satellites through collaboration with industry, academia, and research teams in Taiwan. In order to enhance the signal-to-noise ratio (SNR) of the images, it is crucial to understand the surface scattering characteristics and implement effective measures to suppress stray light in the optical system. In this study, a combination of bidirectional reflectance distribution function (BRDF) and total integrated scattering (TIS) measurements is utilized to identify materials that exhibit ultra diffusive-matt characteristics. All the selected materials underwent environmental tests to verify their durability for space environment usage. The databases of BRDF and TIS also facilitate the construction of mathematical models representing surface scattering characteristics. All the findings of this study were utilized to arrange the surface blackening methods for various components within the optical system and proved highly advantageous for FORMOSAT-8 satellite program.
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