KEYWORDS: Star sensors, Target detection, Stars, Sensors, Imaging systems, Signal to noise ratio, Stray light, Optical design, Signal detection, Mirrors
The star sensor is used to detect the position of the stars in space. By recognizing and analyzing star maps, satellites or spacecraft can automatically change the direction of movements to realize the navigation function. However, the strong background radiation in the sky during the day results in a low contrast of the star image. This brings great difficulties to star sensors that work on atmospheric platforms observing stars all the time. To overcome the adverse impacts of the stray lights from the sky during the whole day through the atmosphere, a catadioptric all-day star sensor optical system is presented. In comparison to Cassegrain System, the design has a smaller size of aperture of housing. Therefore, it has the advantage of superb suppression of the stray lights caused by external sky background radiation and other factors. By adopting a plane mirror to compress the light path, the size of the system is decreased, realizing a light and miniaturized design. Based on the analysis of the characteristics of sky background radiation and star radiation, the optical system parameters are selected. The system has a focal length of 800mm, an effective aperture of 70mm, and an instantaneous field of view of 2 °. Meanwhile, with a steering mirror, it can observe an area between 40° and 70° airspace at all day. Finally, the results of the analysis show that the optical system spot shape approaches to a circle in the wide spectrum of 800 nm ~ 1700 nm, and the energy of which is close to the Gaussian distribution and highly concentrated. The modulation transfer function curve is close to the diffraction limit with small chromatic aberration of magnification.
In the final step of approaching and docking proximity of space rendezvous and docking, it is necessary to measure the relative position and posture of two spacecrafts with high precision using by optical imaging sensor. The image quality of the optical system itself of optical image sensor, to a great extent, will influence the accuracy of navigation information of rendezvous and docking, and even determine the success or failure of rendezvous and docking task. The image telecentric optical system, adopted by the multi-components and retrofocus structure and designed by the hyperfocal distance theory, not only can realize clear imaging from 2 meters to infinity, but also can make sure the center position of image that is imaged by the object from 2 meters to infinity basically invariant. It used the method of "S" type correction of distortion and corrected the distortion of edged field of view (FOV) and 0.8FOV synchronously, which realized the relative distortion less than 0.028%(absolute distortion less than 0.78μm)in the range of 30° fields of view, and met the requirements of the high precision of imaging system and illuminancy uniformity of different fields of view.
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