Design of Abnormal Light Shield for Satellite Star Sensors

The baffle is an important part of the star sensor to resist stray light, which is generally a lightweight thin-walled structure. In addition to ensuring the realization of optical indicators, it also needs to pay attention to its mechanical properties. The baffle shell of a star sensor was fractured during the qualification level random vibration test. The problem was analyzed and investigated. Through finite element simulation analysis, the maximum stress of the baffle exceeds the allowable stress of the material under the excitation of the test load, and the position is consistent with the actual fracture position. The problem phenomenon can be repeated by retesting, which fully explains that the insufficient safety margin of design is the direct cause of the problem. On this basis, the safety margin is effectively improved by optimizing and simulating the structure, and finally passes the vibration test.

Star sensor is a precision instrument that uses stars as a benchmark and achieves attitude measurement through star map recognition. It is currently the most accurate attitude sensor in the space field. Star sensors are designed for low light detection and operate continuously in orbit for a long time. They are inevitably affected by all or part of the interference from stray light sources such as the sun, moon, terrestrial light, and the scattering of external surfaces and components of spacecraft. The light shield is the first line of defense against stray light interference. By designing a reasonable sunshade structure and coating the internal cavity with a high absorption rate, the ability to suppress stray light at a certain angle can be achieved, thereby improving the ability of the star sensor to work normally in orbit for a long time. The design of the sunshade has always been one of the key technologies in the development of star sensors. The smaller the stray light suppression angle index, the greater the difficulty in designing the light shield. In order to improve the stray light suppression ability and cope with complex track lighting environments, the aperture and length dimensions of the light shield need to be correspondingly increased. At this time, thin-walled structures are generally used to achieve lightweight design. In order to improve the structural strength, some load-bearing space cameras often use high-strength and low-density carbon fiber composite materials to process the light shield, while small light shields for star sensors usually use metal materials with higher strength, such as aluminum alloy or magnesium alloy, which can reduce production costs and facilitate the coating of extinction layers. In order to avoid the mechanical risks in the impact and vibration environment during the rocket launch phase, it is necessary to pass the mechanical environment test assessment at the appraisal level on the ground before finalizing the design of the hood, and verify the design results.

1. Design of the light shield

The design of the light shield is to ensure that the level of stray light entering the optical system meets our specifications. Therefore, the length, aperture, and size of the shielding ring of the light shield are determined by specific design indicators. The following two schemes are used to design the light shield, as shown in Figure 1.

Fig.1 T he structure of the lens hood

The first structure designs the structure of the light shield as a cylindrical outer layer, with a circular frustum formed between the shielding rings on the inner layer, and an incident surface on the right. The advantage of this design is that it deepens the depth of the occlusion ring of the secondary extinction part (as shown in Figure 2 with a length of x), which increases the number of reflections of light between the two occlusion rings and absorbs more energy, thus achieving the goal of improving extinction efficiency.

Fig.2 Sketch for the lens hood design

The second structure mainly increases the aperture of the shading cylinder of the first stage extinction part (as shown in Figure 2 with a length of y), which relatively deepens the depth of the shading ring in this part. Due to processing reasons, light scattering can occur at the edges of the occlusion ring, seriously affecting the imaging quality of CCD. Therefore, there should be a certain inclination angle at the edges during design, as shown in Figure 1 (a). The inclination angle is α， Its size is determined by specific design indicators.

2. Simulation

With the development of aerospace technology, the accuracy requirements for star sensors on satellites are becoming higher and higher. However, the influence of stray light (also known as stray light or stray radiation) in optical systems on star sensor CCD camera star point extraction is very obvious. If the stray light cannot be effectively eliminated, the background noise of the CCD camera will be high, and the impact on star point extraction is very significant. To obtain high-resolution star image points, it is necessary to effectively eliminate stray light, thereby improving the reliability of star sensor star image point extraction. Stray light (stray light) is the non target light that reaches the detector (CCD) in an optical system, that is, the target light that reaches the CCD through an abnormal optical path. These non target rays include external stray light and internal stray light. The former refers to sunlight, terrestrial light, etc., while the latter is mainly generated by the optical system itself due to changes in temperature and other factors. For star sensors, external stray light accounts for the main part, which greatly reduces the contrast of the CCD image plane and deteriorates its clarity. So effectively eliminating stray light is essential for star sensor design. For the analysis of stray light, we mainly use the MONT E-CARLO algorithm, which is a random analysis method based on mathematical statistics and probability analysis. The basic idea is to consider stray radiation energy as composed of a large number of independent energy beam rays, the emission position and direction of the rays, the absorption or reflection upon reaching the reflecting surface, and the absorption, refraction or transmission upon entering the semi transparent component, And a series of transfer processes such as diffraction are determined by corresponding probability models. Track each beam of light until it is absorbed or escaped from the system. By tracking a certain number of rays, stable results can be obtained. By using OptiCAD, we can track and calculate the reflection, anisotropic emission, and anisotropic reflection of each incident light beam in the hood, and accurately calculate the extinction ratio (the ratio of the energy of the light reaching the CCD to the energy of the incident light) to meet our design requirements.

3. Experimental analysis

Through the above design and simulation, we can also roughly understand some key aspects of designing light shields, mainly reflected in the following:

1) When designing a sunshade, the number and spacing of the shading rings, as well as the length of the sunshade, are directly related to the extinction efficiency of the sunshade. These should be analyzed and designed according to specific design indicators.

2) The direction of the edge inclination angle of the occlusion ring must be designed according to the principle of avoiding light scattering at its edges as much as possible.

3) The surface finish of the light shield is also a very important issue. When designing the light shield, we try to make it have a high finish as much as possible, because we can see from the simulation results above that when the proportion of reflection and scattering of light inside the light shield is 6:4, the extinction ratio is 1.171 × 10-6, when the ratio is 5:5, the extinction ratio is 2.073 × 10-6, so we know that when light propagates inside the hood, the larger the proportion of reflection, the more times the light is reflected (the scattering direction is random), and most of it is absorbed before reaching the CCD. This way, the less energy emitted and the better the extinction effect. We can also see this from Table 2, so the surfaces of the light shield we designed should have appropriate smoothness, and also use materials with high absorption rate.

Table 2 Different angles of incidence vs. conflicting extrapolation rates

The measurement results have verified the rationality and feasibility of the measurement scheme, providing a reliable technical means for the testing and evaluation of existing star sensor products, and providing important technical support for the design and optimization of the next generation star sensor products.