A novel high-accuracy star simulator with simple structure and the function of optical feedback was designed to satisfy the practical requirements of high-accuracy test and calibration of star sensor,which was a device used for celestial navigation.LED was selected as the light source.The parallel output beam was formed by means of focusing first and then collimating. The optical feedback system of the star simulator automatically adjusted the luminous intensity of its light source to keep the output intensity stable within a quite long time when the output intensity was changing.The parallelism of output beam of the star simulator reached 8″,the uniformity of its output beam achieved 80%and the stability of output intensity of the star simulator reached 89%under the continuous working condition of at least 8hours.Because the stable output intensity reduces the influence of magnitude change of the star simulator,it is very useful for improving the accuracy of test and calibration of star sensor.
With the rapid development of China’s aerospace industry, deep space exploration technology is also constantly advancing. Autonomous navigation technology, as a key technology in deep space exploration, has also made significant progress. Astronomical navigation is based on stellar reference frames, which are widely used in fields such as aerospace and deep space exploration due to their direct, natural, reliable, and precise advantages. In astronomical navigation, the most widely used navigation equipment is star sensors. With the continuous development of astronomical navigation equipment towards all-weather and high-precision requirements, the accuracy requirements for detection and calibration of navigation equipment are also increasing. There are two main methods for detecting and calibrating astronomical navigation equipment: external field and internal field. The application of field detection is limited due to its high cost, low efficiency, and significant impact from external environmental conditions; On the other hand, infield detection has low cost, high efficiency, and is basically unaffected by the external environment, mainly limited by the accuracy of the detection equipment. Therefore, it is necessary to develop high-precision detection equipment.
Star simulators have a wide range of applications, mainly used for the detection and calibration of various instruments and equipment, including the infield detection and calibration of star sensors. The continuous development and progress of star sensor technology have put forward higher requirements for the detection and calibration accuracy of star sensors, which also puts forward higher requirements for star simulator technology, such as large field of view, high accuracy, good dynamic performance, and miniaturization of equipment.
Star simulators are mainly divided into large field of view star map simulators and small field of view single star simulators according to their functions. The star map simulator with a large field of view has the advantages of relatively simple equipment and convenient star map implementation, but there is a problem of low simulation accuracy. The single star simulator has high simulation accuracy, but there is a problem of relatively limited functionality.
This article designs a high-precision small single star simulator with optical feedback function based on the actual engineering requirements of the field detection and calibration of star sensors in astronomical navigation equipment. The optical feedback system of the star simulator can automatically adjust the luminous intensity of the star light source based on the feedback signal, so that the output star light intensity remains stable for a considerable period of time, which is beneficial for improving the detection and calibration accuracy of the star sensor.
The star simulator device mainly consists of a star light source, ground glass sheet, focusing optical system, splitter prism, star hole, collimation optical system, optical feedback system, and control circuit. The schematic diagram of the system is shown in Figure 1.
Ground glass sheets are mainly used to make the light intensity emitted by light-emitting diodes uniform, because in previous experiments, we found that the light intensity emitted by light-emitting diodes is not uniform, but has obvious circular dark bands. If left untreated, it will seriously affect the uniformity of the output light beam of the star simulator. The focusing optical system is mainly used to focus the beam of light emitted by the starlight source, forming the smallest possible spot of light. The splitter prism is mainly used to separate a portion of the light intensity for optical feedback adjustment. Star point holes are used to form point light sources. The collimating optical system is mainly used to convert the beam passing through the star hole into a parallel light output that meets the requirements. The optical feedback system mainly consists of a photoelectric transistor and a feedback circuit. The feedback system feeds back the strength of the received optical signal to the control circuit in the form of an electrical signal. The control circuit automatically adjusts the luminous intensity of the star light source based on the feedback signal, thereby maintaining a stable output light intensity of the star simulator.
According to the design requirements of the star simulator, the main parameters of the optical system are shown in Table 1.
According to the performance parameter requirements of the star simulator optical system, we have designed a small star simulator optical system with a simple structure and very high imaging quality, as shown in Figure 2. The optical system of the star simulator mainly consists of a focusing optical system and a collimating optical system, with the first two lenses forming the focusing optical system and the last two lenses forming the collimating optical system. The star point hole is placed at the focal point of the focusing optical system, and the splitter prism is placed between the rear lens of the focusing optical system and the star point hole. Because the entire optical system only uses four lenses, the optical system structure of the star simulator is relatively simple and lightweight, and these characteristics also bring great convenience to the installation and debugging of the entire optical system.
After optimizing the optical system of the star simulator, point plots of the optical system, point plots of the infinite image space, and modulation transfer function plots can be obtained. From the dot plot of the optical system, it can be seen that the spot radius at the exit of the optical system is 30mm; The point plot of an optical system in an infinite image space reflects the divergence angle of the light emitted from the optical system. From the point plot in the infinite image space, it can be seen that the maximum divergence angle of the outgoing light from the optical system is 7.354 ″. The modulation transfer function of the optical system characterizes the imaging quality of the optical system. Therefore, we designed a modulation transfer function diagram for the optical system of the star simulator.
During the operation of the star simulator, the output light intensity also changes. The main reasons are: during long-term operation, the change in system operating temperature directly changes the output light intensity of the light-emitting diode, and the control circuit of the star light source may age after long-term operation. After long-term operation, the performance of the optical system’s optical coating will also decrease. If the output light intensity of the star simulator changes significantly during operation, it will seriously affect the detection and calibration accuracy of the star sensor. Therefore, we added an optical feedback system when designing the star simulator. When the output light intensity of the star simulator changes, the optical feedback control system of the star simulator will automatically adjust the control circuit of the star light source, change the input current, and thus change the luminous intensity of the star light source, so that the output light intensity of the star simulator remains stable. The relatively stable output light intensity reduces the impact of simulated magnitude changes on the detection results of the star simulator, which is beneficial for improving the accuracy of star sensor detection and calibration.
(1) Measurement of output beam uniformity
The instrument used to measure the uniformity of the output beam is an illuminometer. In order to measure the uniformity of the output beam of the star simulator, feature points within the effective irradiation plane are selected for measurement.
(2) Measurement of output beam parallelism
This experiment uses the pentaprism method to detect the parallelism of the output beam of the star simulator. The measuring instruments used in the measurement process mainly include pentaprism, digital display autocollimator, translation table, high-precision adjustment platform, etc. During the experiment, the intersection of the optical axis and the guide rail was used as the measurement reference point, and the position was recorded as the origin to obtain the parallelism test results of the star simulator. The parallelism of the output beam measured in the experiment is 7.92 ″, meeting the requirement of parallelism not exceeding 8 ″.
(3) Measurement of stability of output light intensity
The measurement of output beam stability is similar to the measurement of output beam uniformity. When using an illuminometer as the testing equipment, it is also necessary to select the testing feature points.
Based on the actual engineering requirements for the field detection of star sensors in astronomical navigation equipment, a small single star simulator with a simple structure and optical feedback function was designed. The article introduces the composition and working principle of the star simulator. According to the specifications of the star simulator, the optical design software ZEMAX was used to design the optical system of the star simulator, and the control circuit implementation of the optical feedback function of the star simulator was detailed. The optical feedback system of the star simulator can automatically adjust the luminous intensity of the star light source based on the light intensity feedback signal, so that the output light intensity of the star simulator can remain stable for a considerable period of time. The experimental results show that the parallelism of the output beam of the star simulator can reach 8 ″, the beam uniformity can reach 80%, and the stability of the output light intensity of the star simulator can reach 89% under continuous operation for at least 8 hours. The relatively stable simulated starlight intensity reduces the impact of changes in simulated magnitude on the star simulator, which is beneficial for improving the accuracy of star sensor detection and calibration.
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