Taking fixed stars as a reference frame, star sensors solve the attitude algorithm by exploring fixed stars at different positions of the celestial sphere, through which accurate spatial orientation and datum can be provided for spacecraft. An on-orbit star sensor is usually interfered with by stray light, primarily by sunlight. The illumination of sunlight in a low earth orbit is approximately 1350 W/m2, while that of a sixth-magnitude star is approximately 1.26 × 10-10 W/m2 in the same condition. The ratio of the above two is approximately 1013. Thus, star sensors are extremely demanding for the technology of stray light suppression. In the presence of stray light, the pixel in calculation receives starlight and stray light simultaneously. The energy of stray light affects the gray scale of the pixel, which degrades the accuracy of star sensors or even causes the failure of stellar target acquisition in severe cases. Therefore, the function of stray light suppression is necessary for star sensors.
During the stray light suppression by a star sensor, a baffle is employed to effectively eliminate stray light pollution in the working field of view. With suppressing stray light down to the sixth-magnitude stars' level as an example, this paper focuses on the specification demonstration, scheme design, simulation of light beam tracing, and stray light test of the star sensor baffle from perspectives of both theory and engineering application. As a start, depending on the design parameters of the optical lens, the paper specifies the extinction ratio and further clarifies the technical requirement applicable to the extinction ratio of the optical system (Equation 1). Moreover, the paper demonstrates the initial design of a baffle with such information as the effective entrance pupil aperture of the first lens (Fig. 1), the exit pupil aperture of the baffle (Fig. 2), the field of view of the baffle, and the angle of stray light suppression. Finally, the paper explains the detailed design of the baffle structure (Fig. 6), the position of vanes (Fig. 7), the critical scattering surface, and the thickness, the chamfer angle, and the direction of the edge. In the meantime, the design of a secondary or multi-level baffle is suggested for detecting highly sensitive stars so that the impact of stray light from single scattering can be avoided.
Firstly, the simulation in the paper shows the influence of edge thickness on stray light suppression (Fig. 11). The finding is only applicable to the baffle mentioned in this paper. For other baffles, edge thickness should be analyzed according to the simulation method herein. Regarding edge thickness, the following situations are discussed: 1) when the edge thickness is zero, single scattering does not occur, and multiple scattering is the main source of stray light; 2) when the edge thickness is 10 μm, the energy of single scattering is less than that of multiple scattering, and thus multiple scattering is still the main source of stray light; 3) when the edge thickness is 30 μm, the energy of single scattering is stronger than that of multiple scattering, and single scattering becomes the main source of stray light.Secondly, the performance of the star sensor in stray light suppression is verified through experimental testing (Fig. 12). Under the conditions of a sunlight suppression angle of 30° and one solar constant, such necessary conditions to distinguish a sixth-magnitude star can be fulfilled as the mean value, maximum value, and standard deviation of gray scale (61, 130, and 21, respectively). When the sixth-magnitude star is in the field interfered with by stray light enormously, the maximum gray scale of the star point is 72. When the threshold offset is 20, the centroid of the star point can be detected according to Equation 2.Finally, the performance of the baffle in stray light suppression is verified through an outfield test. The accuracy of the star sensor without stray light is 2.13″ and 2.34″; that of the star sensor equipped with a regular baffle in the presence of stray light is 8.07″ and 7.66″; that of the star sensor equipped with the proposed baffle under the interference of stray light is 3.89″ and 4.01″. In short, the performance of the designed baffle is better in stray light suppression, which can control the deviation of accuracy within 2″.
Mechanism research, simulation analysis, and experimental testing verify that the above-mentioned design method for stray light suppression of star sensors is rational and feasible. On the basis of this method, the suppression function of baffles is improved further within limited overall dimensions. This approach enables the design of baffles applicable to different star sensors and extreme magnitudes. This design could be commonly used to obtain baffles with desired extinction ratios for meeting different needs of stray light suppression. Meanwhile, the matching of different adaptability in the optomechanical link is sufficiently considered. As a result, the phenomena of vignetting and stray light in the field of view do not occur. The design method in this paper regarding stray light suppression can provide reference for other designs of photoelectric sensors.
