Facing the urgent requirement of stray light suppression for the star sensor in orbit, stray light irradiance at the exit of the lens hood of a star sensor was determined quantitatively. Based on micro fiber optic spectrometer, two-dimensional high-precision displacement table and solar simulator,a stray light test platform which scans point-by-point was established. With the effective integration of radiation measurement software and motion control software, system integration test software was developed.Therefore,real-time quantitative measurement on irradiance distribution of the stray light at the exit of the lens hood was completed. Regarding of the measured values at the exit of the lens hood under different incident angles of irradiance, the irradiance distribution of the stray light at the exit of the lens hood was calculated according to the integration time set, dark background data and calibration data of spectrograph applied in the measurement. The data processing result shows:the irradiance of the stray light at the exit of the lens hood is about 10²W/cm² and it is distributed unevenly. Based on the error analysis theory,a measurement error model is built with a calculated error of 4.87%,which shows the reasonableness and feasibility of the measurement scheme.The measurement data can provide important technical support for design, demonstration, testing, shaping and application of the star sensor.
With the development of the aerospace industry, the application of star sensors in spacecraft attitude determination has received widespread attention from countries around the world. Star sensor is a high-precision attitude measurement instrument that uses the celestial coordinate system as the reference frame and stars as the detection targets. It images several stars in the instantaneous field of view on a photodetector, and based on template images, obtains the instantaneous attitude error of the aircraft in the space inertial coordinate through image processing, completing the attitude measurement of the aircraft in the space inertial coordinate system, thereby providing high-precision attitude information for various spacecraft such as satellites and deep space detectors.
Star sensors belong to weak light photodetectors and are susceptible to interference from background stray light such as sunlight, terrestrial light, and moonlight when working in orbit. These stray lights will reduce the contrast and modulation transfer function (MTF) of the image plane, resulting in reduced clarity, color distortion, saturation, reduced hierarchy, and irregular energy distribution of the entire image plane. This will result in the formation of spots on the image plane, and in severe cases, the target signal will be completely submerged by stray radiation noise. Due to the lack of consideration of the influence of stray light during design or processing, many optical systems’ performance indicators cannot meet the requirements, such as the Meteosat-5/7 series imagers from the European Union and the GOES-I/M from the United States, which have also been forced to temporarily shut down due to the influence of stray light. The geostationary orbit spin stable meteorological satellite FY-2, developed by China, obtains raw image quality of each channel that is comparable to similar foreign satellites. However, there is significant stray light in the current observed images, which has become a bottleneck restricting its data quantification application. The design of the onboard camera of a certain satellite launched by China has met the performance requirements. However, due to incomplete measures to suppress stray light, the stray light is two orders of magnitude higher than the target, and the target image is submerged by stray noise. It is only after using a light mask to eliminate stray light that it can barely work. The light shield plays a very important role in suppressing stray light, and is currently the most commonly used method for suppressing stray light. It undertakes the task of suppressing stray light in star sensors. In terms of evaluating the stray light suppression performance of light masks, there have been many simulations and tests conducted on Point Spread Transmittance (PST) and stray light distribution coefficient (VGDI) in the past, but they are not sufficient to comprehensively evaluate the stray light suppression level of light masks. In order to comprehensively evaluate the stray light suppression performance of the sunshade and consider the urgent need for star map simulation, this paper conducted research on the testing method of stray light irradiance distribution at the exit of the star sensor sunshade. The uniformity of stray light irradiance distribution at the outlet of the star sensor hood is an important indicator for evaluating the stray light suppression performance of the hood. Currently, there have been no reports on experimental measurements of this indicator. Therefore, it is necessary to design a reasonable experimental measurement plan and platform for the stray light suppression performance of the hood to obtain reliable experimental testing data, in order to design, demonstrate, and test the star sensor system scheme Provide theoretical basis and technical methods for finalization and application.
The measurement of irradiance distribution at the outlet of the hood under different light incidence angles is planned to adopt a point by point scanning measurement method. The measurement platform mainly consists of a fiber optic spectrometer, a two-dimensional displacement table, and a solar simulator. As shown in Figure 1, the parallel light emitted by the solar simulator is incident onto the inlet of the hood. At the outlet of the hood, the irradiance is measured using the fiber optic probe of the fiber optic spectrometer. The fiber optic probe of the fiber optic spectrometer is placed on a two-dimensional displacement table, and the irradiance at different positions of the hood’s outlet is measured through the horizontal and vertical movements of the two-dimensional displacement table. At the same time, the light shield and two-dimensional displacement table are placed on a precision electronic control turntable, which can perform circular motion on the horizontal plane to measure the irradiance distribution at the light outlet of the light shield at different incident angles.
The uniformity of the distribution of stray light irradiance at the outlet of the light shield is an important indicator of the ability of the light shield to suppress stray light. This indicator is used to characterize the stray light suppression level of the light mask and provide reliable experimental data for the product design of the light mask.
The measurement equipment for irradiance distribution at the outlet of the hood mainly includes lighting systems, mechanical angle and displacement systems, detection and reception systems, and data acquisition and control systems.
Based on a measurement platform built in the laboratory, the irradiance distribution of stray light at the outlet of the light shield is measured. When the incident zenith angle is less than the protection angle of the light shield, bright light spots can be seen at the outlet of the light shield; When the incident zenith angle is greater than the protective angle of the light shield, the bright spots at the outlet of the light shield disappear, fully verifying the rationality and importance of the design of the protective angle of the light shield.
Measure the irradiance distribution at the exit of the hood at different incident zenith angles, collect data from 10000 points at each incident zenith angle, and establish a database of irradiance distribution at the exit of the hood. Due to the large amount of data, this article only lists the experimental measurement results under the protection angle, namely the spectrometer measurement values of the light shield under the protection angle, as shown in Table 5.
Tab.5 Measurement results of exit irradiance distribution of hood at 30° incident zenith angle
According to the above data processing method, based on the measurement data of the spectrometer, the distribution of outlet radiation illuminance of the light shield at the protection angle is calculated, as shown in Figure 9.
Fig.9 Irish distribution at exit of hood at 30 ° incident zenith angle
From the magnitude change of the irradiance distribution measurement data at the outlet of the hood, it can be seen that when the incident zenith angle of light is less than the protection angle of the hood, the irradiance magnitude change at the outlet of the hood is significant, with about two magnitude changes; When the incident zenith angle of light is greater than the protective angle of the hood, the magnitude of the irradiance at the exit of the hood varies slightly, all of which are of the same order of magnitude. In addition, from the distribution of the irradiance data at the outlet of the hood, it can be seen that the irradiance distribution at the outlet of the hood is uneven and shows a periodic trend of change with its position.
In order to quantitatively describe the energy distribution characteristics of the hood outlet and scientifically evaluate the stray light suppression performance of the hood, it is necessary to establish evaluation indicators for the energy distribution data of the hood outlet. Based on the stray light transmission characteristics of the hood, this article uses energy concentration to characterize the stray light distribution characteristics at the hood outlet.
This article focuses on the practical application requirements of star sensor system scheme design, demonstration, testing, and finalization, and conducts research on the quantitative testing of stray light suppression ability of star sensor masks. Firstly, a point by point scanning ground measurement scheme was designed, and a testing platform was built based on a micro fiber optic spectrometer, a two-dimensional high-precision displacement platform, and a solar simulator. The radiation measurement software and motion control software were effectively integrated, and a system integration testing software was developed to collect real-time energy distribution data at the outlet of the hood. The irradiance distribution at the outlet of the hood is calculated based on the integration time set during measurement, the saved dark background data, and the calibration data of the spectrometer, taking into account the measurement values obtained at different light incidence angles. The data processing results indicate that the irradiance of the hood at the exit is approximately 10 ² W/cm ², And the distribution is uneven. Finally, a measurement error model was constructed based on error analysis theory, and the error calculation result of stray light testing was 4.87%, which verified the rationality and feasibility of the measurement scheme. The above research results provide important theoretical basis and technical path for the design and optimization of the next generation star sensor products.
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