There is a clear need for miniaturized, lightweight, accurate and inexpensive star tracker for spacecraft with large anglar rate. To face these new constraints, the Beijing Institute of Space Long March Vehicle has designed, built and flown a low cost miniaturized star tracker that provides autonomous (“Lost in Space”) inertial attitude determination, 2 Hz 3-axis star tracking, and digital imaging with embedded compression. Detector with high sensitivity is adopted to meet the dynamic and miniature requirement. A Sun and Moon avoiding method based on the calculation of Sun and Moon’s vector by astronomical theory is proposed. The produced prototype weight 0.84kg, and can be used for a spacecraft with 6°/s anglar rate. The average angle measure error is less than 43 arc second. The ground verification and application of the star tracker during the pick-up flight test showed that the capability of the product meet the requirement.
The mechanism of the image degradation due to the opaque metallic wire grid is analyzed and a degradation mode is built based on the theory of Fourier optics. The effect of different opaque wire grid on the image quality is simulated. We define the manner of using wire grid that will meet system requirements for both optical and shielding efficiency. A gray non-uniformity correction model based on two-point correction method is built, and the way to estimate the degradation function and the restoration process are proposed. Both the experimental results and simulation show that the non-uniformity of the image after correction is less than 1/10 of that of before correction and the relative error between the original image and restored image is 0.56%.
In view of the lack of precision and small dynamic range in one or two-point temperature nonuniformity correction method which is applied in the Space Infrared Image, this paper will introduce the technique of discrete wavelet transform which is applied to the non-uniformity correction. In addition, it is used the real-time infrared image processing method of removing blind pixels, invalid pixels and additive noise with using ground calibration parameters and deep space images. It is designed the infrared image processing method applied to the flight that the multiplicative noise in infrared image is turned into additive noise using the logarithm aimed at the characteristic of noise distribution.
Blackbody is a crucial device for performance test and radiometric calibration of infrared system. This paper put forward a low-temperature surface blackbody with variable temperature. It works under vacuum and low temperature environment, and can realize the variable temperature control within the range from 130K to 450K by the dual control of liquid nitrogen and electric heating. This paper gave a detailed introduction to the system composition and structure of surface blackbody, introduced the design processes of radiator, temperature controller, temperature control system and other important parts, and analyzed the temperature field distribution of blackbody radiator through finite element software. And test method was used to test performance of the surface blackbody. The results show that the blackbody has good temperature uniformity and temperature stability performance, and is able to provide a benchmark for low temperature test of infrared system.
An optical model to describe the reconstructed image of the collinear holographic data storage system is presented. The 2-D shape of the recording spot in the medium is simulated. The system structure parameters that influence the signal-to-noise ratio (SNR) of the reconstructed image are investigated in order to choose proper optical components to improve data storage density of the system. The role of the random binary phase mask (RBPM) utilized in the multilayer collinear holographic system is investigated. It is proved that the inter-layer crosstalk can be effectively suppressed by using different RBPMs. Different layers of data pages can be recorded with the SNR increased by a factor of six at least and the shift selectivity along z-axis can also be improved significantly.
A transmission type of collinear holographic data storage system (CHDS) based on photorefractive LiNbO<sub>3</sub> crystal is constructed. The polarization states of the coherent beams in the crystal are optimized for a larger dynamic range based on the coupled mode theory and the linearized band transport model of photorefractive effect. The optimization predicts that the photorefractive crystal will have a larger dynamic range by use of extraordinary light than using ordinary light for CHDS system when the grating wave vector is parallel to the c-axis of the crystal. It also predicts that the dynamic range for CHDS can be larger than that for traditional 90 degree holographic recording geometry. In the experimental results, data pages are recorded with the shifting multiplexing method in the LiNbO<sub>3</sub> crystal by using the extraordinary light.
Proc. SPIE. 8420, 6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Optical System Technologies for Manufacturing and Testing
KEYWORDS: Holography, 3D image reconstruction, Data modeling, Data storage, Image restoration, Optical testing, Image quality, Holographic data storage systems, Optics manufacturing, Current controlled current source
An image reconstruction model for the collinear holographic data storage (CHDS) system is proposed and analyzed. The analytical solution for the reconstructed image by the CHDS system is derived based on the first Born approximation and scalar diffraction theory. This expression shows the CHDS system is not a linear space-invariant system. A page-oriented simulation method is desired for a rigorous analysis. By applying the fast Fourier transform method, a page-oriented simulation method for the CHDS system is presented. The influence of the reference pattern and the media thickness on the quality of the image reconstructed from the holograph is investigated. The fill factor of the reference pattern is the key parameter for the quality of the reconstructed image.
The wavelength and defocus margins for collinear holographic data storage system are theoretically analyzed based on
the first Born approximation and the scalar diffraction theory. Explicit expressions for the decay of diffracted signal in
the center of the detector plane with the shift of the reading wavelength and with the defocus of the disc are presented.
The expressions predict that the defocus margin is independent of the media thickness while a thicker disc leads to a
narrower wavelength margin. Simulation results show that the wavelength margin of collinear holographic scheme is
larger than that of the conventional 2-axis holographic scheme. The influences of the properties of reference pattern on
both margins are also discussed.