Star sensor is a high-precision space attitude optical sensor with wide applications in the aerospace field. Spacecraft and high-precision satellites often use star sensors to collect star maps and calculate real-time flight position and attitude deviations. Prior to the operation of the star sensor in orbit experiment, the system error calibration of the star sensor needs to be completed in the ground environment in advance. As an important part of ground calibration of star sensors, star simulators can provide target star sources for high-precision star sensors under laboratory conditions, simulating the position, brightness, and other characteristics of stars in the sky. The star simulator currently used in the ground verification system of star sensors is based on the principle of imaging the star point reticle located at the focal plane of the system to infinity, and then the star sensor collects the star map, extracts feature star point information, and calculates the corresponding attitude and direction. By comparing the standard star map database, the system error of the star sensor itself is calibrated to ensure its accuracy in orbit operation. The accuracy of the star simulator system directly determines the calibration accuracy of the star sensor.
Currently, there are few star simulator systems that consider the vibration factors of the ground environment, which can lead to pixel drift of star points and reduce the accuracy of the star simulator system. This further affects the ground calibration effect of the star sensor and reduces its actual performance. The fast swinging mirror (FSM) has many applications as a stabilizing element in large space telescopes. FSM can achieve beam stability by matching the corresponding detection optical path, but there are few literature reports on its application in satellite simulation systems for environmental vibration suppression. This article proposes a star simulation technology with a high-precision vibration compensation system, combined with a large aperture and long focal length collimator system, to provide a highly stable star light source for ground verification experiments of star sensors. This system achieves a large field of view, high accuracy, and multi-objective synthesis of star maps through a large aperture spectroscope, providing multiple modes of simulated star maps for star sensors. This system collects and extracts vibration information through a high-speed camera, and closed-loop controls the fast swinging mirror for vibration compensation. It can effectively suppress the impact of ground vibration on the overall optical system, provide stable star light sources for star sensors, and improve the accuracy of high-precision star sensor ground calibration experiments.
The satellite simulation system based on high-precision jitter compensation system mainly includes satellite simulation system and vibration detection and compensation system. The overall optical path diagram of the system is shown in Figure 2.3.
The propagation path of the star simulation light path is shown in Figure 2.3. The beam emitted by the integrating sphere light source is reflected by two planar mirrors to illuminate the star point dividing plate, which is pre engraved with star point holes designed according to the star map. As shown in the schematic diagram of the simulated optical path of Figure 2.4, the star point reticle is placed at the focal plane of the star collimator group, and the beam emitted from the star point passes through the first spectroscope prism, which is collimated into a parallel beam through the collimator group. This parallel beam transmits the second splitting prism to the jitter compensation mirror, which compensates for the optical axis in real time and then reflects back to the starlight convergence mirror group. The focal plane of the starlight convergence mirror group is in common with the off axis three reflection system, so the beam converged by the starlight convergence mirror group is emitted into infinitely distant parallel starlight through the off axis three reflection system. This starlight is received by the star sensor for star map recognition and calibration. This optical path is the propagation path of the simulated star map.
At present, the static star simulator can only display a single mode of simulated star map at a single time, while the dynamic star simulator is faced with the limitations of image refresh rate and uniformity of star field brightness, which can not achieve high-precision mobile star point simulation, and can not meet the new requirements of single star point tracking under the complex star map background of star sensors. This paper proposes a technology to study large field of view, high-precision, multi-target composite star map. The simulated star map optical path is shown in Figure 2.4. The output beam of the integrating sphere light source illuminates the multi star point dividing plate, while the adjustable light source illuminates the single star point dividing plate. The multi star point dividing plate and the single star dividing plate are located at the focal plane of the starlight collimating lens group. The starlight emitted from the above two star point plates is combined into the starlight collimating lens group through a large aperture dividing prism, and is received by the star sensor through the star simulation optical path.
It is worth pointing out that the above multi star reticle is installed in a reticle device with axial rotation, which can rotate the reticle 360 ° around the central optical axis and provide star map simulation of star rotation in the sky for star sensors. The single star dividing plate is installed on a high-precision translation guide rail, which can simulate the star map of a single star point moving target. The above two star maps can be simultaneously superimposed on the focal plane of the star sensor, achieving star map simulation of single star point motion in a multi star background. Multiple simulated star maps provide various operating conditions for star sensors to simulate in orbit flight.
The vibration detection and compensation optical path consists of a reference optical path and a vibration signal detection optical path.
The reference optical path for vibration detection is shown in Figure 2.5. After the light beam emitted by the light source illuminates the star point reticle, it is collimated into a parallel beam through a collimating mirror group. The parallel beam is reflected by the second beam splitter prism onto the plane reflector. After the beam reflected by the plane reflector passes through the second beam splitter prism, it converges onto the focal plane of the high-speed camera through a long focal length focusing mirror group, forming a reference star point.
The vibration detection signal optical path is shown in Figure 2.6. A small aperture planar reflector is placed at the front end of the star sensor, which automatically collimates the simulated starlight part of the star simulation optical path in section 2.3.U and returns to the off axis three reflection system. The output beam of the off axis three reflection system is a parallel beam after passing through the star convergence system. The parallel beam is reflected by the jitter compensation mirror and the second beam splitter prism into the long focal length convergence mirror group, The image is formed through a focusing mirror on the focal plane of a high-speed camera, forming a detection signal light.
The high-speed camera simultaneously receives reference light points and vibration detection signal light points. Due to disturbances such as ground vibration, the coordinates of the signal light points collected by the camera’s focal plane may change. The offset between the signal light point and the reference light point coordinates can be calculated in real-time through the image processing module. Based on this offset combined with the optical parameters of the system, the optical axis offset angle of the signal beam on the jitter compensation mirror can be calculated.
The shake compensation mirror is installed on a fast swinging mirror driven by piezoelectric ceramics. The frame diagram of the shake compensation mirror is shown in Figure 2.7. The swing mirror system is composed of a large diameter piezoelectric turntable, a driving circuit, and a fast swing mirror. By applying a pre calibrated voltage signal to the piezoelectric ceramic driver, the angle corresponding to the offset of the jitter compensation mirror can be driven, and the optical axis can be fine tuned to offset the optical axis offset caused by beam jitter, achieving stable image stabilization of the star point, thereby providing a high-quality stable constant star target source for the star sensor.
This chapter mainly proposes the project sources of a high-precision jitter compensation star simulation system and analyzes the overall design of the system. Firstly, the optical setup of the star simulation system was studied, and the implementation methods and application scenarios of key technologies for multi-objective synthesis of star maps were analyzed. Secondly, the implementation methods of vibration detection and compensation optical path were analyzed. Finally, by comparing the optical axis sensitivity of the secondary beam expansion convergence scheme and the direct convergence scheme in the vibration detection system, the direct convergence scheme was selected as the convergence mirror group for the detection convergence system, which can achieve better detection sensitivity.
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