In order to complete the testing of very high precision star sensor, a high precision static multi -star simulator which can achieve accurate simulation of star point was designed. The collimating optical system has wavelength range of 500-900 nm, at 20℃, the effective focal length of 150 mm, and distortion less than 0.02% , MTF reaches diffraction limit within the full filed of view of 7.2° . For temperature changes from -45℃ to 65℃, used ZEMAX software to analyze athermal performance of the optical system, applied ANSYS software to complet finite element analysis of the mechanic structure. The results show that, athermal performance of the optical system is fine, shape change and deformation stress on mechanical structure are small, the anti-defocus performance of the simulator can meet the requirement of technique index. Calculate theoretical accuracy of the designed simulator and provided a group of measured data. Depends on the measurement results, the error of position for single star ＜3″, the error of angular distance between stars ＜10″, are both better than required calibration accuracy of the star sensor.
Star sensors are the most accurate sensors among spacecraft attitude sensors, capable of reaching the angular second level. In recent years, both domestic and international efforts have been made to develop very high-precision star sensors. On the one hand, higher requirements have been put forward for the testing accuracy of star sensors, and on the other hand, more attention has been paid to their thermal performance under a wide range of environmental temperature changes. Therefore, it is urgent to develop supporting equipment that can meet the ground calibration work of very high-precision star sensors. Star simulators are used to generate simulated star maps at large spatial scales and serve as target sources for large field of view, high-precision star sensors under laboratory conditions, to complete tasks such as star recognition and attitude determination. A temperature controlled star simulator can also test the performance of star sensors under thermal cycling environmental testing conditions. The article mainly designs an optical system with high collimation accuracy, energy concentration, and small distortion; Analyze the anti defocus performance of the optical mechanical structure of the simulator at different temperatures, and then calculate the theoretical design accuracy; The feasibility of the static star simulator was verified by the test results obtained from the application of the system.
Static multi star simulator belongs to the calibration type star simulator, which has high accuracy requirements for single star angle and star point position error, but does not have real-time requirements for star map changes. Usually, it only completes the simulation of a fixed star map. The static multi star simulator with controllable temperature mainly consists of a collimating objective component, a star point dividing plate, a filter, a heating wire component, a light source, and a power supply, as shown in Figure 1.
The working principle of a static star simulator is to place a standard star point reticle plate with several transparent micropores on the focal plane of its collimating optical system. The light source corrects the star spectrum through a filter, illuminates the star point reticle plate, and forms a simulated star point. The light transmitted by the simulated star point is parallel emitted through the collimating objective lens group, forming a complete star map at the entrance of the very high-precision star sensor, thus achieving the simulation of stars at infinity. When the system is working, the power supply supplies power to the simulator and simulates the magnitude by adjusting the brightness of the light source; Using heating wire components for closed-loop temperature control not only ensures the working temperature requirements of the star simulator, but also ensures the normal operation of the light source and electronic components.
In order to match the optical system parameters of the star sensor and the installation characteristics of the star simulator, the design parameters of the static star simulator optical system can be preliminarily determined. As shown in Figure 2, the coordination between the star simulator and the star sensor needs to comply with the principle of pupil connection. As the entrance pupil of the very high-precision star sensor is located behind its optical system, it is required that the exit pupil of the star simulator optical system be placed outside. To ensure that the two do not collide when moving relative within the hood, the exit pupil distance of the star simulator is required to be ≥ 60mm; In order to transmit all the star map information of the star simulator to the star sensor and avoid energy loss, the exit pupil diameter of the star simulator should be slightly larger than the entrance pupil diameter of the star sensor, meeting ≥ Φ 18mm. Considering the issue of single star angle, if the diameter of the star point is too small, it may cause the star sensitivity to be unable to recognize it. Therefore, the selected system focal length is about 150mm; The distortion of the optical system determines the positional accuracy of the star point, constraining its distortion to be less than 0.02%. The design indicators of the optical system are obtained from this, as shown in Table 1.
The accuracy of star point angle simulation in multi star simulators is required to be high, and the distortion of the optical system directly affects the accuracy of star point angle position; In addition, the star map reading method of the star sensor is to capture the energy center of the star point. Therefore, the optical system design should also focus on controlling distortion and the deviation between the energy center of the star point and the position of the main ray.
Distortion is an aberration related to the field of view, and the distortion value is only affected by the field of view, that is, the distortion value varies among different fields of view. The causes of distortion include the sine difference in the position of the diaphragm and the angular magnification. Obviously, the correction of distortion is very difficult, and we can only try to make its value as small as possible to meet the requirements of the design indicators. The deviation between the energy center of the star point dispersion circle and the position of the main ray is also a major cause of star point position error. In practical optical systems, due to the presence of aberrations, the light emitted by an object no longer coincides with the ideal image point on the image plane, but forms a diffuse light spot near that point, that is, the energy center of the imaging point no longer coincides with the main light. Therefore, in design, the deviation value between the energy center of the star point’s diffuse circle and the main light should be minimized as much as possible. The spectral range of the star simulator is 500-900nm, with a central wavelength of 650nm. The dispersion has a significant impact on the near-infrared spectrum, and the chromatic aberration caused by the different refractive indices of various colors can damage the clarity of the off-axis imaging points. If the magnification color difference correction is not good, the accuracy of star point simulation in the star simulator will also be affected, so selecting appropriate materials is also a consideration in the design.
With the change of temperature, the parameters of the optical system will change, causing a decrease in its performance, and the focal plane will drift and cause defocusing. The variation of modulation transfer function relative to reference temperature at different temperatures is an important indicator for evaluating the thermal performance of a system.
When analyzing the anti defocusing performance of a static multi star simulator, not only the optical system’s thermal expansion coefficients of each component will affect the calculation results, but also the mechanical structure deformation will have an impact on the results.
The position error of star points is mainly caused by the angle error caused by the deviation between the energy center of gravity and the main ray, the collimation accuracy of the optical system, the residual distortion correction error of the optical system, and the installation and adjustment error of the star simulator.
After the installation and adjustment of the star simulator, a star map with 12 stars (center star point is star point 8) was tested to obtain the single star position error of each star point relative to the center star point. The measured results comply with theoretical calculations and meet the accuracy requirements of the technical indicators for the simulator.
Starting from the needs of ground testing of very high-precision star sensors, a high-precision static multi star simulator is designed in the article. Its collimating optical system has a simple structure, high imaging quality, and good defocusing resistance of the optomechanical structure under a wide range of temperature changes. After theoretical calculation and practical testing, it has been verified that the single star position error of the star simulator is better than 3 “, and the inter star angular distance error is better than 5”. It can be used as a detection device for testing very high-precision star sensors.
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