Aiming at the requirement of star simulator for multiple star magnitude，a kind of high imaging quality star simulator optical system was optimized and designed according to the working principle of star simulator．The energy calculation model of the star magnitude was established to realize the theoretical calculation of the energy of the star magnitude． In order to realize the high precision and continuous adjustment of multiple star magnitude，a simulation method of multiple star magnitude with two pieces of linear variable filter was proposed． The simulation results show that the simulation errors range from － 2 to 6 star magnitude are less than ± 10% ，which meets the simulation requirements of real magnitude． It provides a design idea for the real star magnitude simulation of star simulator.
Star sensor is one of the three core components of satellite attitude control (sun sensor, earth sensor, and star sensor), and its performance directly determines the accuracy of satellite orbit change, which is the fundamental guarantee of satellite orbit change control. As a ground calibration equipment for star sensors, star simulators are a hot research direction in spacecraft ground simulation calibration. Star simulators are divided into static star simulators and dynamic star simulators based on the different simulation situations of star points. Among them, dynamic star simulators can achieve real-time continuous changes in star maps, mainly used for functional testing of star sensors. Static star simulators have high simulation accuracy, but the star map display is single, mainly used for accuracy testing of star sensors. The research on conventional star simulators mainly focuses on the study of their simulation accuracy. In recent years, with the development of star sensor technology, star modeler technology has gradually transitioned to simulating the true radiation characteristics of target stars, namely the stellar spectrum and true magnitude.
Due to the extremely small apparent angle of stars, the maximum radiation angle to the Earth in a star is λ ， The star with the smallest angle is also smaller than T, so it can be regarded as a point light source at infinity. Therefore, the star simulator uses a collimating optical system to achieve the radiation of stars at infinity in the universe.
In order to better ensure the accurate position and constant magnitude of all star image points in the field of view of the simulated star map, the imaging quality of the collimating optical system is required to have characteristics such as small field curvature and small distortion. When using a star simulator in conjunction with a star sensor, in order to improve energy utilization efficiency, it is required that the exit pupil of the optical system of the star simulator coincide with the entrance pupil of the star sensor. As the star sensor generally has an internal optical system for the entrance pupil, the optical system of the star simulator is designed with an external exit pupil. At the same time, the field of view of the star simulator and star sensor should be consistent.
Based on the usage of this system, the optical system parameters are shown in Table 1.
The main structures of star simulators are divided into reflective structures and transmissive structures. The advantage of a reflective structure is that it is not prone to color difference and can be easily made into a large aperture without being troubled by secondary spectral issues. However, when correcting off axis aberrations, it will block a portion of the light in the middle, making correction relatively difficult. At the same time, as the relative aperture and field of view increase, the imaging quality will also decrease. When the blocking ratio of the system is 1:4 and the reflection ratio of the metal surface is 100%, the highest energy transmittance obtained is 75%, which has a significant impact on the energy utilization efficiency of micro and weak light source systems. The characteristic of a transmissive structure is that the lenses are arranged in order on the optical path to form a straight through structure. Coating the surface of the lens (the transmittance of a single lens can reach 99.95%) can effectively improve the light energy transmittance of the system. Therefore, a transmissive collimation system is selected.
Due to the fact that the accuracy of the imaging position of the star simulator mainly depends on the system’s distortion variables, and the distortion value is only affected by the field of view, that is, different fields of view have different distortion values.
Obviously, the correction of distortion is very difficult, and we can only try to make its value as small as possible to meet the requirements of the design indicators.
The deviation between the energy center and the main ray position is also a major cause of star position error. In practical optical systems, due to the presence of aberrations, the light emitted by an object no longer coincides with the ideal image point on the image plane, but forms a diffuse light spot near that point, that is, the energy center of the imaging point no longer coincides with the main light. Therefore, in design, the deviation value between the energy center and the main light should be minimized as much as possible.
Due to the spectral range of the star simulator being 0.35-0.9 μ m. The dispersion has a significant impact on the near-infrared spectrum, and the chromatic aberration caused by the different refractive indices of various colors of light can disrupt the clarity of off-axis imaging points. If the magnification color difference correction is not good, the accuracy of star point simulation in the star simulator will also be affected, so selecting appropriate materials is also a consideration in the design. Considering the temperature changes during the use of the star simulator, it is necessary for the star simulator to have good consistency within the temperature range of 10 to 30 ℃. The photothermal expansion coefficient of refractive lenses reflects the normalized changes in the focal length of the system when temperature changes. Usually, these effects of lenses are small, but for the high-precision requirements of star simulators, the photothermal characteristics of materials cannot be ignored.
Overall, this design adopts a four piece lens structure. In order to reduce the length of the system’s optical path, a planar reflector is added for optical path folding.
(1) Calculation model of magnitude energy
Due to the fact that the reflector in the optical system of the star simulator hardly causes energy loss, the optical system can be analyzed as a thin lens. During the calculation process, the aperture of the last side of the optical system is selected as the diameter of the optical system. The establishment of a magnitude energy calculation model is shown in Figure 7.
(2) Star magnitude simulation control method
At present, there are three main methods for magnitude simulation: one is to add multiple sets of attenuation plates to the optical system; The second is to adjust the intensity of light emitted by the star simulator light source; The third is to adjust the incident light intensity of the star simulator through a variable aperture.
The range of illumination and magnitude adjustment of the star point reticle that needs to be simulated by the star simulator requires a high ability to adjust the light source, which is difficult to achieve by adjusting the light source alone; When using attenuation plates for magnitude adjustment, the system’s robustness is poor due to the transmittance of the attenuation plate as a measure, resulting in low accuracy of magnitude control when working for a long time; The use of a variable aperture will have a certain impact on the linear adjustment of magnitude energy.
Therefore, based on the characteristics of linear gradient density filters, a preliminary adjustment and fine simulation scheme for star magnitude simulation is proposed by combining two linear gradient density filters with a stepper motor combination, thereby achieving high-precision continuous adjustment of star magnitude energy and improving the robustness and calibration of the system.
This article combines the development trend of star simulators, optimizes and designs the optical system of star simulators, studies star magnitude simulation technology and control methods, and achieves continuous and fine control of multiple stars in star simulators, providing a simulation approach for achieving true star magnitude.
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