In order to satisfy the high precision calibration requirement of the star sensor’s optical signal, a star sensor ground calibration system with spatial background light is proposed. It solves the problem of calibration accuracy of the star sensor’s optical signal due to the spatial background light, improves the accuracy of star sensor calibration. The influence of spatial background light on the calibration accuracy of star sensor is analyzed. The scheme of star simulator with spatial background light is given. A set of star simulator optical system which can simulate spatial background light and star point position at the same time is designed. The results show that the system distortion is smaller than 0.1 %, MTF is larger than 0.5 with 80 lp-mm-1. The system satisfies the high-precision requirements. A set of spatial background light system is designed through the control of the current and the variable iris. The background brightness adjustment is up to 26 times. Finally, a method of correct star point position with aberration compensation is proposed, and the correction model is presented. The star position error and spatial background brightness of the star simulator are tested by using the Leica T6100 theodolite and the illuminometer. The result shows that the spatial background brightness adjustment is up to 64 times and the star position accuracy is better than 10″. The star simulator can meet the ground calibration requirement of star sensor.
Star sensors, as one of the most accurate space attitude optical sensors, have been widely used in the field of space. The continuous development of space science and technology has put forward increasing demands for the measurement accuracy and testing conditions of star sensors. The working conditions of star sensors in the cosmic space environment are very strict, among which spatial stray light seriously affects the imaging quality of star sensors, This is because stray light increases the background noise of the star sensor, reduces the contrast of the image plane and the signal-to-noise ratio of the optical system. Therefore, the star sensor not only needs to have high attitude measurement accuracy, but also has strong recognition ability for star background spatial light radiation information
When the star sensor operates in orbit, it is disturbed by spatial light radiation information, which in turn affects the imaging of the detection surface. The background energy distribution of the originally uniform star map shows a significant change, and the overall increase in grayscale values seriously affects the star point extraction rate, leading to the loss of star tracking and ineffective star attitude data. Therefore, It is necessary to establish an effective ground calibration system to evaluate the star sensor’s star map recognition ability and star point extraction ability. As a ground calibration equipment for star sensors, star simulators should have star map and background light simulation functions. Currently, whether static or dynamic star simulators, they usually only focus on the accuracy of star map simulation, Without considering the background simulation of the star map, the star sensor did not perform star map recognition testing under the condition of simultaneous background light and star points during the ground calibration stage. This paper studied a star simulator with spatial background light simulation function.
The simulator with a spatial background light star mainly consists of a stellar light source system, a spatial background light system, and a collimating optical system. The stellar optical system provides simulation of the stellar spectrum and magnitude for the star simulator, mainly consisting of warning lights, filters, integrating spheres, and control systems; The space background light system provides a simulation of the brightness of stars in space for star simulators, consisting of alarm lights, filters, integrating spheres, electrically controlled variable apertures, control systems, and feedback systems.
The main working principle of a stellar light source system is to correct the light emitted by the alarm light source with a filter to obtain a spectrum range that meets the simulated stellar spectrum. Then, the beam that meets the stellar spectrum is evenly distributed in the integrating sphere, and the required magnitude is adjusted by the control system. The working principle of a spatial background light system is similar to that of a stellar optical system, where the alarm light spectrum is corrected with a filter to obtain the required spectral integrating sphere for spatial background light, The brightness of the spatial background light entering the star simulator is controlled by an electrically controlled variable aperture. Finally, the light from the star light source system and the spatial background light are combined through a beam combining prism. The optical system of the star simulator simulates a star map with spatial background light, and the overall structure of the star simulator with spatial background light is shown in Figure 1
The star simulator has spatial background light information and stellar information. In order to ensure that the optical system of the star simulator has a large working distance, a beam merging prism is installed, and the system adopts a structure similar to an anti far-field system
The optical system of the star simulator has a spectral range of 400-900nm, which belongs to a thousand wide spectral optical system. The star simulator belongs to a thousand high precision calibration equipment, so it is necessary to ensure that the color difference is corrected to a reasonable range. In order to achieve high-precision simulation of the star point position in the collimation optical system of the star simulator, suitable lens optical materials are selected to achromatize the optical system. By combining the achromatic formula with the relative dispersion coefficient and Abbe constant plot of glass materials, the solution for glass that satisfies achromatic conditions can be obtained. Three types of glass are selected here: School N-SK2, KZFS8, and SF66. Design a collimating optical system with spatial background light with a focal length of 500mm, a field of view of 70, a spectral range of 400-900nm, an exit pupil aperture of 100mm, and an exit pupil distance of 100mm The total length of the optimized optical system is 660mm The two-dimensional optical path of the optical system is shown in Figure 2.
When the star sensor is in orbit, it is mainly affected by three types of spatial radiation information: terrestrial gas, moonlight, and sunlight. As both terrestrial gas and moonlight reflect sunlight, the space background light simulation spectrum of the star simulator is mainly composed of a solar spectrum space background light simulation system, which is mainly composed of a warning lamp, a filter, an electric variable aperture, and an integrating sphere. The warning lamp and filter mainly provide a simulated solar spectrum for the space background light, In theory, it can be considered that the luminous intensity of the alarm light is proportional to the current. Therefore, the current in the circuit is controlled by adjusting the adjustable potentiometer in the circuit. However, in actual testing of spatial background light, the luminous intensity and current of the alarm light are not strictly proportional to the luminous characteristics of the alarm light itself. Only when the current reaches a certain level can the alarm light turn on, and when the current is too low, the luminous spectrum curve of the alarm light will change, The luminous intensity of the star sensor can be adjusted up to 5 times by the current. As the star sensor test requires a spatial background light brightness adjustment multiple of 64, in order to ensure the adjustment accuracy and stability of the light source emission, a control and variable aperture are used to simultaneously control the brightness of the spatial background light simulation system. The electric variable aperture is used to adjust the luminous flux entering the integrating sphere, achieving the simulation of the spatial background light brightness.
(1) Correction of star position error
The star position correction method mainly corrects the displayed position of the star based on the measured star position error. Due to the strong subjectivity of the star position error measurement process and the high level of contingency in multi-point testing, the consistency with the elevation test cannot be guaranteed. The star position correction function usually adopts a univariate multiple function, which will result in inconsistent star positions in both directions during the correction process and the inability to accurately determine the correction boundary, Causing some star point position accuracy to exceed the standard
(2) Space Background System Testing
The key to simulating spatial background light is radiation uniformity and continuously adjustable radiation brightness. By adjusting the star magnitude simulation system through the star point light source controller, the corresponding star magnitude simulation is achieved. The current illuminance value is monitored in real-time using a weak light illuminometer, and compared with the theoretical value of the star magnitude to obtain the simulation error column.
(3) Star point position accuracy test
Apply the Kaka T6100 theodolite to test the star position accuracy of the star simulator. There are 12 stars in the target star map, which are tested for the highest and lowest spatial background light energy. In each case, 10 sets of tests are conducted to eliminate gross errors and determine the star position accuracy of the star simulator; Experiments have shown that star sensors can clearly recognize star maps and extract star point positions when there is no spatial background light. However, when the spatial background light of star points increases to a certain extent, it will have a certain impact on the star recognition of star sensors
This article analyzes the impact of spatial background light on the calibration accuracy of star sensor star light signals, proposes a simulation method for star simulators with spatial background light, designs a high-precision collimation optical system with spatial background light, and tests the system using a T-6100 theodolite and illuminometer. The spatial background light system can effectively simulate the spatial light radiation brightness around the star when the star sensor is in orbit, solving the problem of spatial light radiation affecting the accuracy calibration of the star sensor, and improving the ground calibration accuracy of the star sensor
Send us a message,we will answer your email shortly!
TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
