Star sensor is imaging the space weak star point. The background noise from stray light will badly affect the extracting accuracy of image. So baffle design is the key technology of the star sensor. In this paper, the principle of baffle design is listed , the formula of the optimum length for two-stage baffle is deduced and an example of baffle design of a star sensor is cited. It is indicated that the new design has shorter length and better performance in stray light attenuation than the traditional barrel baffle.
At present, using star sensors to measure the attitude of E-star has high accuracy and is the main satellite positioning method. Due to the fact that star sensors detect weak signals, the stray light in the optical system has a significant impact on the star image points of the star sensor CCD camera. This greatly reduces the contrast of the image plane and deteriorates its clarity. In severe cases, the signal light is completely submerged by stray radiation noise. Therefore, the design of stray light suppression is one of the key technologies of star sensors.
Star sensors are mainly used to detect stars in deep black space. Table l provides the technical parameters of the camera.
In CCD star sensors, the background noise of the CCD image plane seriously affects the accuracy of star point extraction. To reduce the background noise, it is necessary to effectively eliminate stray light. Therefore, the light shield in CCD star sensors plays an important role. Further analysis and research have been conducted on the design of the light shield to address this issue.
CCD star sensor is an attitude sensor that uses stars as a reference for measurement. The optical system images stars on the CCD photosensitive surface, and the CCD performs photoelectric conversion. The output electrical signal is processed by subsequent circuits and inputted into a computer. The computer calculates the coordinates of each star in the sensor body coordinate system, and then matches it with the memory navigation star catalog to complete the recognition of stars in the field of view, thereby determining the sensor direction at the current time. However, due to the entry of stray light (such as terrestrial light and solar light), the background noise of CCD increases, which affects the accuracy of star point extraction. With the continuous development of aerospace technology, the requirements for the accuracy and precision of star sensors are becoming increasingly high, and reducing the background noise of CCD photosensitive surfaces has become increasingly important. The design of the light shield is one of the key factors in reducing this type of noise. In the past, the size of light shields designed was often large, and the extinction effect was not particularly ideal. To address this issue, it is imperative to design a miniaturized and effective light shield. This requires changing the previous design plan and re exploring a reasonable design approach.
(1) Old design scheme for light shield
In the past, the design of light shields mainly focused on absorbing and eliminating stray light by blocking leaves, as shown in Figure 1. Design a series of shading leaves inside the shading tube, and the stray light entering the shading cover will be reflected back and forth between these shading leaves and absorbed. However, due to the large number of obscuring leaves in this design, the scattering direction of light is uncertain. From the figure, it can be seen that both the obscuring leaves and the inner wall of the obscuring tube will cause the scattered light to directly reach the outgoing image surface, resulting in unsatisfactory extinction effect. Moreover, the size of the light shields designed by this scheme is relatively large, indicating that the old scheme can no longer keep up with the development of miniaturized star sensors.
Figure 1 Schematic diagram of the old scheme for the light shield
(2) A New Design Scheme for Light Shield
From the old plan, it can be seen that the scattering of light is an important factor affecting the effectiveness of the light shield. Therefore, in the new design, it should be minimized to prevent light from scattering onto the image surface. Based on this idea, the primary extinction part of the light shield is designed as a reflecting surface, and the secondary extinction part is designed as an absorbing surface. The purpose of this design is to reflect most of the light in the first extinction section, and the scattered light is absorbed in the second extinction section, resulting in very little light reaching the image plane. It can be seen that the design requires a very high level of first level extinction, with high reflectivity and low scattering. The interior of the hood is no longer a barrier leaf, but a set of conical surfaces with openings pointing towards the direction of incidence, as shown in Figure 2. Light β Angular incidence, the cone inclination angle of the first extinction part is θ。 This can be determined by the angle of incident light θ Size range. In order to prevent the scattered light from the primary extinction part from directly reaching the image plane, the circular surface formed on the exit surface after the intersection of the cone extension lines of the primary extinction part should be larger than the image plane. The heights of Y1 and Y2 must ensure that the field of view is not obstructed.
Figure 2 Schematic diagram of the structure of the light shield
Stray light is the non target light that reaches the detector in an optical system, that is, the target light that reaches the CCD through an abnormal optical path. The stray light of star sensors mainly refers to strong light outside the field of view, mainly solar light, terrestrial light, etc. Compared with sixth class stars, their intensity is above 10 ^ 13. Therefore, it is required that star sensors have excellent stray light suppression ability. At present, star sensor masks designed with a single bucket structure generally have a longer length, but the suppression effect is not very ideal. The two section type hood has a compact structure, which avoids the direct entry of strong light scattered by the inner wall into the lens, and has obvious advantages compared to the single section type.
This article provides the basic principles for the suppression design of a two section sunshade, derives the basic formula for the optimal length design of a two section sunshade, and finally conducts simulation verification using a star sensor as an example.
A typical sunshade design mainly ensures direct entry of the target light path, so it generally adopts a barrel structure, with the sunshade opening angle equal to or slightly greater than the field of view angle. For the absence of strong stray radiation outside the field of view, this design can achieve excellent hood volume. However, a major drawback of this design is that it cannot limit the scattering from the edges of the light shield or light ring directly into the lens. As shown in Figure 1, the sun’s light will scatter at the edge of the light barrier, and the more edges of the light barrier, the more light will enter the lens.
Fig.1 Edge sea ttering of van of barrel muffle
The design of the two section light shield (Figure 2) is based on the principle of controlling strong stray radiation outside the field of view. It ensures direct entry of the target light path through an I level hood, and suppresses direct entry of strong stray radiation outside the field of view through an II level hood. The Level II hood utilizes multiple reflections between the barrel walls to return strong incident light to the human space or attenuate energy multiple times. Finally, the light entering the Level I hood is generally outside the target field of view and has very little energy.
Fig.2 Edge scattering of van of two-stage baffle
Therefore, it can be determined that the general principles for the design of star sensor masks are: 1) Level I masks limit the imaging beam entering the lens; 2) Strong light emitted by people outside the field of view cannot directly enter the Level I light shield; 3) The scattered (reflected) light on the inner wall of the Level II hood cannot directly enter the lens.
In order to achieve higher stray light suppression effect of strong light outside the field of view, a multi-level light mask can also be designed, and the multi-level light mask also follows the above principles. For example, in the design of a Level III hood, the light scattered (reflected) from the inner wall of the Level III hood cannot directly enter the Level II hood.
Based on the principles of sunshade design, the schematic diagram of the optimal two-stage sunshade design is shown in Figure 3. Strong light outside the field of view from A to suppress angle d human radiation; Intersect at the edge B of the Level II hood and diffuse reflection occurs there; Scattered edge light enters the first level hood along the wall of the second level hood barrel and intersects with the edge C of the first level hood.
Fig.3 Optimal length design of two stage baffle
From the formula, it can be seen that the volume of the star sensor mask is related to the aperture, field of view angle, and suppression angle of the optical lens, but not to F # (F number, F #=focal length/person’s sleep), but F # affects the energy entering the image plane. Therefore, if conditions permit, using a smaller aperture in the optical system will greatly reduce the volume of the light shield.
When the aperture and suppression angle of the optical system are fixed, the length L, opening diameter D, and volume V of the two section type light shield are tangent to the half opening angle tan of the light shield θ The function curve of is generally shown in Figure 4. It is not difficult to see that only a well-designed hood angle can meet the spatial size requirements of the camera.
Fig.4 Baffle length (a) baffle diameter (b) and baffle volume (c) curves for different tan θ
Through the above research on the design of star sensor masks, several key elements that need to be noted in this optimization design can be obtained:
The light shield is an important component of a star sensor to resist stray light. Optimizing its design can help improve the overall working efficiency of the star sensor.
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