The high-precision calibrated star simulator developed in this article is used for testing the performance of star sensors based on the requirements of star position indicators. We focused on designing a collimating optical system and provided a detailed explanation of the accuracy calculation method for key parameters such as single star pointing and inter star angular distance. Practical testing experiments were conducted to analyze and verify the feasibility of the system.
In order to successfully complete the ground calibration of the key parameters of the star sensor, a high-precision calibration system is needed to accurately reflect the star position information observed by the star sensor during orbital operation, thereby achieving simulation of the real sky. A star simulator is an important ground calibration equipment for star sensors that can complete the above tasks. Star simulators can be divided into calibration type star simulators and functional detection type star simulators according to the detection parameter requirements. The calibration type star simulator has high requirements for simulating the position and magnitude of star points. Generally, it only simulates a few constant-star maps in a fixed sky area, so it is also known as a static star simulator. Usually, a star point plate with high-precision transparent micropores etched on the surface is used to simulate the star position relationship, highlighting system performance; Functional detection type star simulators can output real-time simulated star maps, so they are also known as dynamic star simulators. Generally, optical modulation devices such as TFT-LCD and LCOS are selected as star map display components, emphasizing system functions.
The calibration type star simulator has a simple structure and is easy to install and adjust. The conventional calibration type star simulator mainly consists of a control power supply, a high uniformity light source, a bandpass filter, a laser direct writing microporous star point plate, a high-precision collimation optical system, etc. The structural composition is shown in Figure 1.
The working principle of a calibration type star simulator is as follows: a transparent microporous star point reticle plate with star point position distribution made using laser direct writing technology is placed on the focal plane of a high-precision collimating optical system. The light source illuminates the star point reticle plate and has high uniformity in the star map display area. The star spectrum is corrected through a neutral bandpass filter to form a simulated star point that meets spectral requirements. Due to the placement of the star point reticle on the focal plane of the collimating optical system, the light passing through the star point micropores is emitted in parallel through the collimating optical system. The star sensor receives the parallel light and converges on its image plane to form a complete fixed celestial simulated star map, thus achieving the simulation of stars at infinity by the simulator and the observation of simulated star light from infinity by the sensor. To ensure the safe operation of the simulator, control the power supply to supply power to the light source, and adjust the brightness of the light source within a certain range to change the simulated star level.
The collimating optical system is a key factor in achieving the ground testing ability of star sensors in a calibrated star simulator. It can accurately simulate the position of star points in a fixed sky area at infinity, and its performance directly affects the ground calibration accuracy of the simulator for star sensors.
(1) Design principles and parameters
Due to the need for a calibrated star simulator to have the ability to simulate large spatial size star maps, the designed collimating optical system is essentially a large field of view collimator; Considering that the docking method between the star sensor and the simulator in the testing experiment is pupil connection, it is required that the designed collimating optical system has an external pupil and must have high-quality imaging characteristics such as small distortion and apochromatic aberration. According to the ground calibration requirements of high-precision star sensors, the design parameters of the calibration type star simulator collimation optical system are shown in Table 1.
(2) Design results
According to the design requirements of the collimating optical system of the star simulator, eliminating distortion is one of the most important tasks in system design. It was found that changing the basic structural parameters of the lens cannot effectively correct distortion and cannot achieve the requirement of relative distortion less than 0.1%. Therefore, optimization operations were set to control relative distortion. Finally, according to the technical specifications, the structure of the completed star simulator is shown in Figure 2.
As shown in Figure 3, the relative distortion curve of the planned collimating optical system is shown. The maximum relative distortion error in the full field of view is not more than 0.08%, and the maximum relative distortion error in the center wavelength is not more than 0.06%.
MTF is the most comprehensive criterion among all optical system performance criteria, and the planned MTF curve of the collimated optical system is shown in Figure 4.
The main beam points of the planned collimating optical system are shown in Figure 5.
The deviation between the energy center of the collimating optical system and the main beam can be calculated through the point plot, and then the theoretical deviation of the star simulation angle generated by the optical system can be calculated. The calculation results are shown in Table 2.
The accuracy of the key parameters of the calibrated star simulator itself is the key to determining its accuracy in calibrating the star sensor.
This article designs a high-precision calibration type star simulator for the specific requirements of ground calibration of star sensors on the performance of testing equipment. Proposed the overall design scheme of the simulator, including system composition and working principle; A simulator collimation optical system with external pupil was designed, which has the advantages of large field of view, wide spectrum, and small distortion; Elaborated on the testing methods for key parameters of the simulator, built an experimental platform, and conducted actual testing on typical star maps. The results indicate that the measured results of the core parameters meet the requirements of technical indicators. Therefore, the designed calibrated star simulator can serve as an important equipment for high-precision ground performance testing of star sensors.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
