The method for aligning the focal plane assembly (FPA) with the optical structure assembly (OSA) involves finding a position with optimal focus. The experimental setup includes a collimator with an effective focal length of 10,500 mm and a free aperture of 700 mm. An electronic ground support equipment (EGSE) is responsible for capturing the sensor signals. Processing these data allows the derivation of the contrast transfer function (CTF) value using a line-pair pattern. At the beginning of the FPA alignment process, the attitude and position of the FPA are controlled with the assistance of a hexapod positioning system (a six-axis robot). After determining the attitude and the best focus position of the FPA, shimming is performed to adjust the gap between the FPA and the OSA. Based on the shim thickness required at three orientations, shim rings with the relevant thickness are installed. Due to the uncertainties from shimming, shim ring thickness manufacturing tolerance, and the residual of the tip/tilt angle between the FPA and the OSA during alignment, it is crucial to confirm and fine-tune the shim ring thickness. Once the FPA is fixed on the OSA, moving the FPA for the through-focus measurement is not feasible. However, the distance between the primary mirror and the secondary mirror of the collimator is tunable. By adjusting this distance, the collimation beam can be modified to a converging or diverging beam, thereby changing the focal plane position. Consequently, through-focus measurements can still be conducted by moving the secondary mirror of the collimator.
This paper proposes a high-precision collimator calibration method based on the principle of the vernier caliper. Collimators are crucial in the development phase of satellite telescopes, as they generate parallel light beams to adjust the focal plane of the satellite optical system, ensuring that satellites can accurately target and maintain optimal operational performance in orbit. Traditionally, laser interferometry is used to calibrate collimators. Although laser interferometry provides high-precision calibration, it is costly and time-consuming, requiring a precise five-axis motion platform to record the coordinates of the interferometer at specific focal points. Therefore, we propose a method using composite periodic patterns to enhance the accuracy of collimator calibration. We utilize an optical imaging system composed of a lens with an effective focal length of 1000 mm and an image sensor with a pixel size of 3.45 micrometers to calibrate a collimator with a focal length of 10500 mm. By analyzing the periodic patterns captured at different aperture positions of the collimator, we achieve sub-pixel level positioning accuracy. This method improves the measurement accuracy of the collimator, providing a low-cost yet highly accurate calibration solution.
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