In order to realize the ground test of the high-precision star sensor，a high precision static star simulator was designed and required that the star angular distance of star simulator was better than 20 ″ . The star reticle was made by laser direct technology，the score precision is better than 1μm. Combining the technical specifications of the system，the optical system with high imaging quality was designed by ZEMAX software. The design results of collimation optical system show that the entrance pupil diameter is 80mm；the focus length is 500mm；the wavelength range is 480~900nm and distortion less than 0.02%；MTF reaches diffraction limit within the full field. A method to measure the system with theodolite was presented；and the calculation formula of the theoretical star angular distance and the actual star angular distance are conducted .
The ability to obtain accurate spatial attitude information is the foundation of autonomous navigation for space vehicles and the most important element in aircraft control systems. When a spacecraft operates in space, it uses space navigation sensors to capture and extract its flight attitude information. Common attitude sensors include earth sensors, sun sensors, magnetometers, star sensors, etc. Among them, star sensors have the highest attitude measurement accuracy, which can reach 1 second. With the development of space technology and the continuous improvement of attitude positioning requirements for spacecraft such as satellites and manned spacecraft, higher requirements have been put forward for the technical specifications of star sensors. In order to achieve ground detection of star sensors, a star simulator was developed for ground testing and calibration. In order to match the testing requirements of the star sensor, the indicator requirements of the star simulator should also be correspondingly increased.
Static star simulators are mainly used to simulate the spectra, magnitude, and inter star angular distance of single or fixed star maps. This type of star simulator requires high accuracy in simulating the star position, magnitude, and single star angle of the star map, without the need to provide real-time star maps, and has a simple structure. According to the requirements of various indicators, this article designs an optical system with high imaging quality, which has the characteristics of wide spectrum and small distortion.
The static star simulator belongs to the calibration type star simulator, which provides relatively static star point images. It mainly consists of a control circuit, a light source, a star point reticle, a filter, and a collimating optical system, as shown in Figure 1
Place a star point dividing plate with several small holes on the focal plane of the collimating optical system, and the light source corrects the spectrum to illuminate the star point dividing plate through a filter. After being imaged by the collimating optical system to simulate the position of the star at infinity, a complete star map is formed at the entrance pupil of the star sensor. The simulation of magnitude is achieved by adjusting the brightness of the light source through a control circuit.
Determine the design indicators of the static star simulator based on the usage requirements of the star sensor to be tested, as shown in Table 1.
The collimating optical system is an important component of the static star simulator, which directly affects the simulation accuracy of the star map. Therefore, the designed optical system must have high imaging quality. In order to ensure that the outgoing light flux of the star simulator is equal to the incoming light flux of the star sensor, the outgoing pupil of the collimating optical system should coincide with the incoming pupil of the star sensor, which conforms to the principle of pupil connection. Due to the high accuracy requirements for star point position simulation in star simulators, the optical system should have high imaging position accuracy. The main factor affecting imaging position accuracy is system distortion, so the relative distortion of the system should be strictly controlled; At the same time, considering that the star map reading method of the star sensor is to capture the energy center of each star point, it is also important to focus on controlling the deviation between the energy center of the optical system and the main light.
This article adopts a transmission design. The transmission system has a simple structure, low light energy loss, and is easy to install and adjust. The optimized optical system has good image quality, and the optical path is shown in Figure 2.
(1) Optical Transfer Function (MTF)
The modulation transfer function is the most comprehensive evaluation index in image quality evaluation, which can intuitively reflect the excellence of optical systems.
(2) Distortion
Distortion has no impact on the imaging clarity of the optical system, but it directly affects the positional accuracy of the star points. Therefore, eliminating system distortion is the focus of design.
(3) Point plot
In practical design of optical systems, scattered patterns are formed due to aberrations and lack of concentration in imaging, known as point plots, which can reflect the energy distribution of the system.
(4) Wavefront aberration
According to the Rayleigh criterion, the maximum wave aberration does not exceed λ/ At 4 o’clock, the system can be considered flawless.
Using laser direct writing technology, mark the star point board according to the simulated star map, and the position of the star point on the star point board is determined based on the azimuth angle obtained from the star catalog（ α）、 Pitch angle（ β） Calculate the theoretical inter star angular distance of each star point relative to the central star point using the system focal length (f ‘).
The Leica theodolite is used for experimental testing of the designed high-precision static star simulator, and the TM5100A theodolite is proposed to be used with an accuracy of 0.5 seconds. Record the azimuth angle of the star point to be tested during testing α N and pitch angle β n. The measured inter satellite angular distance is calculated using a formula. The error between the actual measurement of inter satellite angular distance and the theoretical inter satellite angular distance is compared between the actual test results and the theoretical value to verify whether it meets the design index requirements.
According to the testing requirements of high-precision star sensors, a wide spectrum and small distortion collimation optical system was designed using ZEMAX software. We have determined the experimental detection and calculation methods to verify whether the inter star angular distance error of simulated star points can meet the testing needs of high-precision star sensors.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
