In this paper, the photometric characteristics of passively marked corner-cube-reflector-class cooperative targets are studied. The imaging characteristics of corner cube reflector(CCR) are discussed theoretically, and the influencing factors such as surface accuracy, angle error and diffraction effect are analyzed. The reflection uniformity and diffraction characteristics of CCRs with different positions and different precision are simulated and verified. In addition, the influencing factors and control methods of CCR precision are proposed. To the photometric characteristics of cooperative targets, especially the influence of aperture size of cube-corner prism on comprehensive aberration, the transmission surface (bottom surface) on comprehensive aberration was concerned firstly, besides the three reflection surfaces. The conclusion was been drawn that the transmission surface and the three reflection surfaces had most effect on the parallel of emerging beams, and the minimum error was the minimum algebra sum of above four surfaces. It can provide theoretical support for on-orbit services such as companion flight, autonomous rendezvous and docking (RVD).
With the continuous development of astronomy theory and space exploration technology, searching for extrasolar planets has become one of the most active research topics in astronomy. In recent decades, countries around the world have invested a lot of ground and space projects in this search field and obtained abundant results. Firstly, this paper summarizes the mainstream exoplanet detection methods such as radial velocity, transit and direct imaging method with the outline of the principles and features. Then, several instruments for obtaining the spectrum of exoplanets are introduced, focusing on the optical system parameters of telescopes and spectrometers. Finally, according to the comprehensive discussion above, the future development trend of exploration missions and instrument design in this field is predicted, and it is recognized that these survey missions for detecting and characterizing exoplanets are of great significance for searching biological signals outside the solar system.
Dual-band imaging system can effectively improve the detection and identification capability for airborne camera. To ensure the system with compact structure and without increasing the complication of the design, a catadioptric system is proposed as an alternative where the visible light and long-wave infrared (LWIR) share the mutual aperture and adding a dichroic beam splitter between the primary and secondary mirrors enables simultaneous imaging of the two bands. The common aperture is achieved by sharing a modified Cassegrain reflection structure in both bands. The entrance pupil diameter for the two bands is 148mm and their field of view (FOV) is 3°. The visible light works in the range of 0.5∼0.8μm and the focal length is 304.2mm with 2048×2048 detector, and the pixel size is 5.5μm. The LWIR works in the range of 8μm∼14μm and the focal length is 205.3mm with 1024×768 infrared detector, and the pixel size is 14μm. The system aberration is well corrected in two bands and for the performance analysis of the system in the temperature range of 20±5‡C, the design results can meet the requirements.
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.
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.
With the development of related technology gradually mature in the field of optoelectronic information, it is a great demand to design an optical system with high resolution and wide field of view(FOV). However, as it is illustrated in conventional Applied Optics, there is a contradiction between these two characteristics. Namely, the FOV and imaging resolution are limited by each other. Here, based on the study of typical wide-FOV optical system design, we propose the monocentric multi-scale system design method to solve this problem. Consisting of a concentric spherical lens and a series of micro-lens array, this system has effective improvement on its imaging quality. As an example, we designed a typical imaging system, which has a focal length of 35mm and a instantaneous field angle of 14.7”, as well as the FOV set to be 120°. By analyzing the imaging quality, we demonstrate that in different FOV, all the values of MTF at 200lp/mm are higher than 0.4 when the sampling frequency of the Nyquist is 200lp/mm, which shows a good accordance with our design.
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