Star simulator,as the ground calibration system of star sensor,can be used to simulate the star involving its size,magnitude,spectrum,color temperature,position and the angular distance between stars,etc.With the continuous development of space technology,the requirement of the star simulator itself has become higher,and then the design of collimation optical system for star simulator important component has become the key factor.Based on the characteristics of off-axis reflective optical system which has no chromatic aberration,small volume, high light efficiency and the center without blocking,an off-axis parabolic collimation optical system was proposed,and the assembly and calibration process was discussed.The system consists of an off-axis parabolic mirror and a plane mirror,which has a clear aperture of Φ300mm,focal length of 3 000mm,and viewing angle of 30′.The image quality analysis shows when the distortion in field of view is 0.006 2%(
The traditional star sensor is a widely used aerospace product, which is an optical attitude sensor based on stars as observation objects. It is mainly used for attitude measurement tasks of satellites, spacecraft, and other spacecraft during in orbit flight. The current star sensor in the usual sense belongs to the star mapper method of star sensors. It first needs to capture an image of a certain area of the star sky, then detects the star image in the image through image processing algorithms, and uses star recognition technology to confirm the “identity” of the observed star, ultimately completing the attitude measurement task.
In order to test the performance of the star sensor, it is necessary to provide a certain observation target as input excitation in ground testing, so that the star sensor can produce a certain output. This excitation source is a star simulator, which can usually generate multiple simulated starlights with certain spatial positional relationships to form a specific simulated star map. Star simulators can generally be divided into dynamic star simulators and static star simulators. The simulated star map generated by the dynamic star simulator is variable; The simulated star map generated by a static star simulator cannot be changed. The star simulator introduced in this article belongs to the static star simulator, which is a component of the ground testing equipment of the satellite control subsystem. It is used to test the function and performance of the star sensor. It is used to simulate the angular distance of the star as the observation target source of the star sensor. In the article, an off axis reflective optical system is proposed. Compared with traditional refractive optical systems, reflective optical systems have no color difference and therefore do not have the problem of secondary spectra. It is easy to achieve lightweight design and meet the quality requirements of optical systems for space applications. However, the field of view of coaxial reflection optical systems is small, and the presence of central obstruction seriously affects the imaging quality. Off axis reflection optical systems do not have central obstruction, which increases the field of view of the optical system and greatly improves the imaging quality of the system.
A static star simulator is an experimental device that simulates the position and brightness of stars in the sky on the ground to complete functional testing of star sensors and attitude recognition systems. It mainly consists of a collimating optical system, star point reticle components, light sources (backlight plates), power supplies, and mounting brackets. The magnitude of stars is simulated by illuminating the light source and adjusting the brightness of the light source. The spectrum of stars is corrected by filters, and the simulation of stars at infinity is achieved by a collimating optical system.
The structural diagram of the static star simulator is shown in Figure 1. The working principle of a static star simulator is to place a standard star point reticle plate with several transparent micropores on the focal plane of its collimating optical system. The light source corrects the star light spectrum through a filter, illuminates the star point reticle plate, and forms a simulated star point. The light transmitted by the simulated star point is parallel emitted through the collimating objective lens group, forming a complete star map at the entrance pupil of the star sensor, thus achieving the simulation of stars at infinity. When the system is working, the power supply supplies power to the simulator and simulates the magnitude by adjusting the brightness of the light source; The static star simulator needs to simulate stars at infinity, so this article uses a collimating optical system to design and project the simulated stars to infinity. The main basis for designing the collimating objective of a static star simulator is to ensure energy conservation and to match the optical system parameters of the star sensor.
The collimation optical system of the static star simulator should ensure the accurate position of the star image points. The collimating optical system of a static star simulator is equivalent to a collimator, which images the simulated star points on the focal plane of the object at infinity. The determination of the radius of the parallel beam emitted from it should be considered in conjunction with the optical system parameters of the star sensor. The exit pupil of the star simulator should coincide with the entrance pupil of the star sensor to ensure that all star map information of the star simulator can be transmitted to the star sensor. Therefore, the collimating objective system of the star simulator is an optical system with the exit pupil outside. To ensure 100% coverage of the field of view of the star sensor, the field of view of the star simulator should be slightly larger than that of the star sensor. According to the requirements for the use of star sensors, a large aperture and long focal length optical system with high imaging quality has been designed. The main parameters of the system are shown in Table 1.
The system is a large aperture and long focal length structure, usually using an aspherical reflector objective, and a parabolic reflector is directly processed into the desired surface shape using a single block grinding method. This method is technically difficult, has a long cycle, high processing accuracy, and the use and processing status are consistent. At the same time, the collimating optical system is a high-precision testing system, and excessive distortion can seriously affect the testing accuracy. Therefore, it should meet the requirements of distortion elimination performance. The collimating objective optical system mainly consists of an off axis primary mirror and a secondary mirror. The primary mirror is a parabolic reflector, and the secondary mirror is a planar reflector, used to bend the optical path and reduce the length of the system.
The working principle of a collimating optical system is that the light source is placed at the focal plane, and the light beam emitted by the backlight plate passes through the subplane mirror and reaches the parabolic mirror. After that, it is reflected by the main mirror parabolic mirror and emitted as a parallel beam, achieving the function of the collimating optical system. As other parameters of the collimating optical system have been determined, the structural parameters to be determined for the off axis reflection collimating optical system are the off axis quantity h and the position of the planar reflector. When designing the system, the collimating optical system can be considered as an imaging optical system for stars at infinity. In the same field of view, the larger the off-axis h, the larger the dispersion spot of the collimating optical system simulating star point imaging, which will have a certain impact on the imaging quality of the collimating optical system simulating star points; The increase of the off axis amount h will also increase the asphericity of the parabolic reflector and the radius of the parent parabolic reflector, thereby increasing the difficulty of processing and prolonging the processing period. Therefore, in the design of optical systems, the off axis amount h should be considered as small as possible, but too small an off axis amount h can cause obstruction in the central area of the collimated optical system. Therefore, the flat reflector should be placed at a position that does not obstruct the emitted beam from the edge of the field of view, and at a certain distance from the backlight plate.
Use Zemax software for off axis design, adjust and optimize the data during the design process. After multiple attempts to obtain the off axis value h, select an off axis value h=220mm, a distance of 2500mm between the parabolic mirror and the plane mirror, and a diameter of 165.5mm for the plane mirror. The structure diagram of the collimating optical system is shown in Figure 3.
The main structural part of an off axis reflective collimating optical system is the frame, which carries all the optical components of the system. The frame is made of cast aluminum alloy, which reduces quality while ensuring stability. The exterior of the frame is covered with iron sheet to prevent dust from entering, and the interior is coated with black matte paint to eliminate interference from stray light and further ensure the uniformity of the light emitted from the light tube. The bottom of the frame comes with four leveling foot screws for easy system leveling, and the foot screws can be connected to the optical isolation platform. The system is planned to be installed and calibrated using the following process:
1) Firstly, use the gravity symmetry method to determine the axis of symmetry of the parabolic reflector, mark it, adjust the base, and use a theodolite to calibrate the installation reference in the horizontal direction.
2) Then, place the parabolic reflector on the mirror base, ensuring that the axis of symmetry is in a horizontal position. Install a self aligning flat mirror and a knife edge gauge, and make the optical axis and knife edge gauge have the same height by trimming the gasket. Observe with a knife edge instrument to prevent mirror deformation.
3) Install the secondary mirror, adjust the pitch and yaw positions of the secondary mirror, and use the principle of self alignment to adjust the position of the secondary mirror.
4) Install the reticle assembly, which is composed of a reticle and a sliding tube focusing mechanism. Install a Gaussian eyepiece and visible light source behind the reticle, observe through the Gaussian eyepiece, adjust the position of the switching reflector using self alignment principle, and trim the parallax circle. The installation and calibration diagram of the parabolic reflector is shown in Figure 8, where 1 is the parabolic reflector, 2 is the self aligning plane mirror, and 3 is the knife edge instrument.
In order to simulate stars at infinity, the main parameters of the star simulator are determined based on the optical system parameters of the star sensor. A parabolic reflector and a planar reflector are selected for design, and the system off-axis method is used to avoid central obstruction and improve image quality while maintaining the miniaturization and lightweight characteristics of the reflector system. The off-axis reflection type collimation optical system is designed using Zemax software, with a field of view angle of 30 ‘, Working band 0.5 μ M~0.9 μ m. RMS value is better than λ/ 10. Through image quality analysis, it can be seen that the system meets the design requirements.
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