Research on baffle detection technology for star sensor (invited)

  Objective  Star sensor are mainly used to achieve star attitude and perceive non-cooperative targets. When the equivalent magnitude of a constant star or non-cooperative target is large, the detection and capture of stars or sensing targets will cease to be effective due to the influence of stray light. Therefore, the suppression performance of stray light is a key technical indicator of situational awareness sensors. Engineering experience has shown that a first-order sunshade can reduce sunlight to 10⁻⁵-10⁻⁶, effectively reducing sunlight pollution on the detector image surface and improving the probability of target capture. In the design stage of the baffle, the scattering characteristics of the extinction coating on the inner surface of the baffle are often modeled as Lambert bodies, resulting in significant differences between the actual and expected values of the extinction performance; During the processing stage of the light retaining ring, the thickness of the cutting edge is often ignored, resulting in significant differences between the actual and expected values of the extinction performance of the light retaining ring.

  Methods  To solve the technical problem of projects mentioned above, this article conducts theoretical and engineering research from two technical approaches of measurement and modeling of BRDF, and cutting edge thickness measurement. Firstly, based on the commonly used fitting formula of BRDF, the visualized model for the scattering characteristics of Magic black extinction coating on the aluminum alloy base of 1.0 μm thickness is built. The blue box represents the measurement data and the white box represents the fitting data (Fig.4). The figures demonstrate the scattering characteristics of the coating surface BRDF values changing with different incident zenith angles and reflected zenith angles when the detection azimuth angle is 180° and the incident light wavelengths are 500 nm, 550 nm, 600 nm, 650 nm, and 700 nm, respectively; Secondly, edge thickness detection is carried out (Fig.5). Considering the particularity of the position of the edge structure, this paper proposes a dual telecentric coaxial light illumination scheme for edge thickness detection. The detection optical path is shown (Fig.6). The basic principle is that the collimated light source is incident onto the right side optical path through a semi-transparent and semi-reflective mirror, and is reflected onto the edge of the light blocking ring through a reflector. Due to the use of a coaxial telecentric optical path, the edge imaging is clear and sharp. The beam scatters return to the optical path through the edge. After passing through a semi-transparent and semi-reflective mirror, the imaging side's telecentric optical path is incident onto the detector's image plane. As shown in Fig.9, calibration with the imaging instrument, the measurement accuracy is better than 1.2 μm. The measurement data of the cutting edge is shown (Fig.12), 30 points are collected from each light blocking ring cutting edge, evenly divided into one circle. The figures show that the thickness of the cutting edge at each point is different, while the thickness variation range is controlled within 20-27 μm.

  Results and Discussions   In the article, quantitative testing of the extinction ratio of the light shield and outdoor stray light observation testing are used to evaluate the stray light suppression performance of the baffle. The quantitative testing plan and equipment used for extinction ratio are shown (Fig.13, Tab.2), and the test data is shown (Fig.15). The three extinction ratio curves represent the theoretical values, measured values, and Lambert scattering model values after using the BRDF model. The starting angle of the test is 24° and the ending angle is 72°. From the graph, it can be seen that within the range of 24° to 60°, the test value is close to the theoretical value, with a deviation of about 12%, and the data trend is consistent; The dark room stray light test (photoelectric whole machine test) is shown (Fig.16), and the test data is shown (Fig.17). When the sunlight and the optical axis of the situation sensing sensor are incident at 24°, the average gray level of the image plane is 55.80, the maximum gray level is 134, the RMS is 10.73°, and the entropy is 6.18. As the testing angle gradually increases, the average gray level is 51.35 at 12°; The field stray light star observation test site is shown (Fig.18), and the experimental data is shown (Fig.19). Under stray light irradiation conditions, the background gray level of the image increases, and the signal-to-noise ratio between the original star target and the deep space background decreases. The stray light contaminated area may produce false star point targets or equivalent dark current noise or detector electronic noise, resulting in errors in attitude recognition process.

  Conclusions  This article demonstrates the detection result of key components of the baffle in the process of suppressing stray light for situational awareness sensors from two different stages of design and processing. The design stage mainly tests the bidirectional reflection distribution function based on the scattering characteristics of the Magic black extinction coating on the process components, and fits visual data suitable for optical simulation analysis; In the processing stage, based on the key characteristics of the cutting edge, a coaxial dual telecentric optical path is proposed to detect its thickness, a testing platform was built, and the measured data of the cutting edge were obtained; In the verification section of the article, quantitative testing of the extinction ratio of the baffle was carried out, and its extinction performance was further verified by testing the stray light of the entire machine. The data shows that the deviation between the quantitative test value and the theoretical simulation value of the extinction ratio was 12%, and the average gray level of the image caused by sunlight incident at 24° on the hood was 55.80; The accuracy of star point positioning is changed from 2.5″ (3σ) without stray light pollution to 3.0" (3σ). This article provides a theoretical basis and technical support for the detection method of the situation perception sensor cover for other optoelectronic sensors.