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.
The sources of stray light were briefly introduced and the necessity of stray light suppression was analyzed. Strong stray light often directly affected the performance of space debris detection camera and even invalidated it. Therefore, the application of stray light suppression in space debris detection camera was particularly important. The design of stray light suppression for space debris detection camera using baffle was proposed. On the basis of re-searching the source of stray light, the influences of the edge width of the baffle rings and the surfaces coating treatment scheme on the stray light suppression ability were finally determined. The stray light suppression ability of the baffle was greatly improved by exploring the limit of the mechanical processing of the baffle ring and optimizing the surface coating treatment method of the inner cavity of the baffle. At the same time, the stray light flow channel inside the optical system was optimized. The critical surfaces inside the optical system were found. After removing the critical surfaces, the stray light suppression design results showed the stray light rejection ratio reached 10-7 finally when off-axis angle was larger than 35°. The flares appearing on the detector disappeared completely in the experiments, and the signal-to-noise ratio was higher. The stray light suppression optimization obtained good results, which could better satisfy the requirements of space debris detection.
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.
The number of space debris has been increasing dramatically in the last few years, and is expected to increase as much in the future. As the orbital debris population grows, the risk of collision between debris and other orbital objects also grows. Therefore, space debris detection is a particularly important task for space environment security, and then supports for space debris modeling, protection and mitigation. This paper aims to review space debris detection systematically and completely. Firstly, the research status of space debris detection at home and abroad is presented. Then, three kinds of optical observation methods of space debris are summarized. Finally, we propose a space-based detection scheme for space debris by photometric and polarimetric characteristics.
Optical system with large depth of field and large field of view has been designed. To enforce optical system with focal length of 6 mm to imaging the object with object length of 200mmm-1200mm, accord to the equation of depth of field, in case of the CCD sensor with pixel of 5.5umx 5.5um square area, the entrance pupil diameter to ideal imaging will be 0.423mm. To enlarge the modulation transfer function (MTF) at spatial frequency of 90 lp/mm, the entrance pupil diameter is enlarged to 1mm.After design and optimization, with field of view of 80°, within object length of 200mm - 1200mm, the optical system can imaging well, the modulation transfer function (MTF) at spatial frequency of 90lp/mm is larger than 0.1, the distortion of full field of viewed is less than 3%.The optical system can be widely used in machine vision, surveillance cameras, etc.
Proc. SPIE. 9903, Seventh International Symposium on Precision Mechanical Measurements
KEYWORDS: Turbulence, Star sensors, Modulation transfer functions, Signal to noise ratio, Atmospheric turbulence, Point spread functions, Stars, Electro optical modeling, Quantum efficiency, Atmospheric sensing
All-day star sensor makes it possible to observe stars in all-day time in the atmosphere. But the detecting performance is influenced by atmospheric turbulence. According to the characteristic of turbulence in long-exposure model, the modulation transfer function, point spread function and encircled power of the imaging system have been analyzed. Combined with typical star sensor optical system, the signal to noise ratio and the detectable stellar magnitude limit affected by turbulence have been calculated. The result shows the ratio of aperture diameter to atmospheric coherence length is main basis for the evaluation of the impact of turbulence. In condition of medium turbulence in day time, signal to noise ratio of the star sensor with diameter 120mm will drop about 4dB at most in typical work environment, and the detectable stellar limit will drop 1 magnitude.