ASPHERIC TWO-MIRROR TELESCOPES OF NANOSATELLITES FOR REMOTE EARTH SENSING AND SEARCH OF MINERALS

Abstract

The optical system is one of the important components of any space satellite used to observe the Earth's surface or astronomical objects. Due to the small overall size of nanosatellites, lens objectives with the relatively short focal length are often used. The latter provides the sufficiently wide field of view, but limits the spatial resolution of obtained images. In addition, to address various problems in agriculture and geodesy, it is important to obtain images of the earth's surface in different spectral ranges. This fact prevents the application of widespread commercial photographic lenses corrected for the visible spectral range. The article presents the results of the automated parametric synthesis of centered and decentered aspheric optical systems of mirror telescopes based on the Ritchie-Chretien scheme. The obtained optical systems have the focal length of 547 mm, the square entrance aperture of 80×80 mm, and the axial length that does not exceed the size of two CubeSat units. The angular field of view of the telescopes, which is equal to 0.7° in diagonal, enables to apply modern matrix image sensors having a diagonal of the sensitive area up to 6.7 mm. For an axial beam, 90% of the radiation energy falls into a square area of the image with a side of 2.5 μm. The maximum value of relative distortion does not exceed 0.4 %. The considered mirror systems do not have chromatic aberrations, which makes it possible to obtain high-quality images not only in the visible spectral range, but also in several infrared sub-bands. The given results of the aberration analysis testify to the high image quality achieved in both variants. The presented optical systems can observe a rectangular area of the Earth's surface with the diagonal size of about 7.9 km from the satellite’s track height of 650 km. When using a multi-element image detectors with the pixel size of 3 μm, the geometric projection of one pixel on the Earth's surface will be equal to 3.6 m.