• Acta Photonica Sinica
  • Vol. 51, Issue 6, 0612004 (2022)
Kunyan WANG1、2, Yichun DAI1、*, and Zhenyu JIN1
Author Affiliations
  • 1Astronomical Technology Laboratory,Yunnan Observatory,Chinese Academy of Sciences,Kunming 650216,China
  • 2University of Chinese Academy of Sciences,Beijing 100049,China
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    DOI: 10.3788/gzxb20225106.0612004 Cite this Article
    Kunyan WANG, Yichun DAI, Zhenyu JIN. Attitude Alignment Method of Solar Telescope Secondary Mirror Based on Aberration Detection[J]. Acta Photonica Sinica, 2022, 51(6): 0612004 Copy Citation Text show less

    Abstract

    Ground-based solar telescopes take high-resolution observations as their main goal, and have stricter image quality requirements. Due to the direct observation of the sun, solar telescopes are more susceptible to heat and other factors during operation. Gravity and temperature will cause changes in the positional relationship between the primary and secondary mirrors, resulting in time-varying aberrations. If not improved, it will affect the effect of high-resolution observation. In order to better realize high-resolution imaging observation, it is necessary to strictly control the image quality of the telescope during operation. How obtain the misalignment of the secondary mirror relative to the primary mirror is a key link in the attitude alignment of the secondary mirror. In the attitude alignment problem of the secondary mirror based on aberration detection, due to the coupling relationship between the translation and the tilt of the optical axis of the telescope, they will both lead to the coma aberration of the system. Therefore, the aberration measurement cannot completely distinguish decenter and tilt, which will have a certain impact on the attitude adjustment accuracy of the secondary mirror. For telescopes with a large field of view, aberration detection can be performed in different fields of view, and decenter and tilt can be decoupled through aberrations in different fields of view. Small field telescopes cannot achieve decoupling through multi-field wavefront aberration detection. Taking the attitude alignment system of the secondary mirror of the 2 m Ring Solar Telescope as an example, the attitude alignment method of the secondary mirror based on the measurement of the wavefront aberration of the small field of view is studied. To reduce the influence of coupling on the accuracy of misalignment solution, there are usually two schemes. The first scheme is to directly constrain the low-sensitivity misalignment, that is, not to adjust the low-sensitivity misalignment; the second scheme is to use a regular factor to constrain the low-sensitivity misalignment. In the correction scheme of using the regular factor to constrain low-sensitivity misalignment, two implementations are proposed on this basis: "alignment based on stable 0-point position" and "alignment based on relative position". Due to the real-time nature of the attitude alignment of the secondary mirror, multiple corrections are required during the observation time. Based on the misalignment calculation accuracy and image quality compensation effect, the concept of "accumulated error" in the control system is proposed. According to the sensitivity matrix model of the misalignment correction, the measurement accuracy of the wavefront detection system and the execution error of Hexapod, different correction methods are simulated from three aspects: image quality compensation effect, misalignment solution accuracy and cumulative error. The simulation results show that although the scheme of directly constraining the low-sensitivity misalignment has a certain compensation ability, the root mean square error of the corrected wavefront is greater than λ/14, which cannot meet the target accuracy requirement of 2mRST. The solution accuracy of the misalignment is not as good as that of using a regular factor to constrain the low-sensitivity misalignment, and since the decenter does not participate in the control, the accumulated error on the decenter will also exacerbate the deviation of the misalignment from the adjustment range of Hexapod. In the scheme in which the low-sensitivity misalignment is constrained by the regular factor, the misalignment adjustment scheme of the relative position is adopted. With the increase of the adjustment times, there is a problem of accumulation of errors, and the coupling between the decenter and the tilt leads to a low solution accuracy of the decenter and the tilt, the cumulative error range is larger. And as the accuracy of the detection and execution links decreases and the correction frequency increases, the speed of error accumulation will increase, and the execution of the misalignment will exceed the travel range of the Hexapod, which may cause damage to the mechanism. A correction scheme that uses a certain regular factor to make an overall constraint on the misalignment based on the stable 0-point position, can achieve better results in solving the misalignment and improving the image quality of the system. This method can improve the wavefront root mean square error from 0.495 9λ to 0.006 2λ, and will not introduce a large cumulative error in the continuous adjustment process. This scheme is more suitable for the attitude alignment system of the secondary mirror based on the measurement of small field of view aberrations.
    Kunyan WANG, Yichun DAI, Zhenyu JIN. Attitude Alignment Method of Solar Telescope Secondary Mirror Based on Aberration Detection[J]. Acta Photonica Sinica, 2022, 51(6): 0612004
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