In order to complete the calibration work of the star sensor and meet the technology requirement of large field，high accuracy for dynamic star simulator，the ZEMAX software is used to complete the design of the optical system according to the dynamic star simulator working principle，which can accurately simulate the star point. The experimental results show that the focal length of the system is 110 mm，the field of view is 16°，the distortion is less than 0. 05% ，the wave range is 0. 5 ～ 0. 8 μm and the MTF is higher than 0. 7 at the frequency of 60 lp /mm. The method to determine the actual focal plane of the system after assembly is put forward，and then the precision of the optical system are analyzed. The results show that the imaging accuracy design of high-precision dynamic star simulator optical system can reach 9″，and the error between stars is less than 13″. So the whole system can meet the using need of the high-precision dynamic star simulator.
When a spacecraft operates in space, optical navigation sensors are used to capture and measure its flight attitude information, achieving real-time control of the spacecraft’s spatial trajectory and motion direction. With the development of space technology and the improvement of spacecraft attitude positioning requirements, the requirements for the technical specifications of star sensors are also becoming increasingly high.
The dynamic star simulator, as a ground calibration equipment for star sensors, has also received widespread attention, and the development of high-precision dynamic star simulators that can meet the ground calibration work of star sensors is becoming increasingly important. A dynamic star simulator is an instrument that simulates the real-time position and magnitude of stars in the sky on the ground. It mainly consists of five parts: driving circuit, light source, spatial light modulator, filter, and collimating optical system. Among them, the main factors affecting the accuracy of the star simulator are the display accuracy of the spatial light modulator and the imaging quality of the collimating optical system. Starting from these two aspects, the optimal spatial light modulator is selected to improve the display accuracy of the star simulator, ensuring that the star map information can be accurately emitted.
The main function of the optical system of the high-precision dynamic star simulator is to emit a constant star map, projecting the display content of the spatial light modulator located at the focal plane of the system to infinity. At the same time, strict aberration correction is used to ensure the energy balance and accurate position of the simulated star points. Therefore, the selection of a collimating optical system is very important. The main basis for the design of a collimating optical system is to ensure that the light emitted by each star point in the full field of view is equal to the input pupil flux of the star sensor, and that the output pupil aperture of the star simulator matches the input pupil aperture parameters of the star sensor, achieving the transmission of all star map information of the star simulator to the star sensor. Due to the fact that the star sensor’s entrance pupil is located behind the optical system, it is required that the dynamic star simulator’s optical system’s exit pupil be placed externally. As shown in Figure 1, according to the principle of pupil connection, the LXP position of the star simulator’s exit pupil should coincide with the LEP position of the star sensor’s entrance pupil, and the optical axis should be consistent.
According to the high-precision ground calibration requirements of the star sensor, the design parameter indicators of the high-precision dynamic star simulator optical system are obtained, as shown in Table 1.
Based on the design principles of the optical system of the star simulator, it is determined that the collimation optical system to be designed has an exit pupil aperture of 36 mm and an exit pupil distance of 60 mm.
Considering the usage requirements of high-precision dynamic star simulators, the optical system needs to have high imaging position accuracy, which mainly depends on the system’s distortion. The distortion of an optical system is the aberration of the principal ray at an off axis point, which is only related to the field of view, that is, the distortion value varies depending on the field of view.
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The distortion eyepiece serves as the initial structure of the optical system. During the design process, the field curvature is corrected by separating positive and negative lenses, and a negative lens is added after the flat convex lens to eliminate spherical and positional chromatic aberration through the combination of positive and negative lenses. The initial structure of the optical system is obtained as shown in Figure 2.
Eliminating distortion is one of the most important tasks in system design. Changing the basic structural parameters of the lens cannot effectively correct distortion and cannot meet the requirement of relative distortion less than 0 1% requirement, therefore set an optimized number of operations to control relative distortion. According to the technical specifications, after the design is completed, the system has a focal length of 110 mm, an exit pupil diameter of 36 mm, and an exit pupil distance of 60 mm. The optimized system optical path is shown in Figure 3.
The designed collimating optical system, along with filters and spatial light modulators, forms the optical part of the high-precision dynamic star simulator. As a high-precision dynamic star simulator display device, the spatial light modulator should be installed as much as possible at the theoretical focal plane position of the collimating optical system of the star simulator. Due to processing and assembly errors in the optical system, there is a deviation between the actual focal length of the collimated optical system and the theoretical focal length after assembly. This deviation directly leads to star position errors, so it is necessary to obtain accurate actual focal plane positions to ensure the accuracy of star light emission in practical applications and conduct accuracy analysis.
The optical system of the high-precision dynamic star simulator is similar to a collimator, which combines an adjustable front mirror adjustment method to determine the focal plane position of the star simulator optical system using the calibrated collimator. Illuminate the crosshair of the center pixel of the spatial light modulator. When the illuminated star point is clear at the same time as the collimator reticle, it is said that the star point image is imaged at the position of the reticle. At this time, it is determined that the spatial light modulator is located at the actual focal plane position of the collimating optical system of the star simulator, as shown in Figure 6.
Starting from the accuracy requirements of high-precision dynamic star simulators, this article focuses on designing a collimating optical system. Combined with specific design parameters, the design steps and results are provided, and the method of determining the focal plane using a collimator is introduced. Finally, the feasibility of the system is verified through accuracy analysis and experimental testing.
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