Definition evaluation method for real-time auto-focusing of wide-angle telescope

Jia-hui YUAN; Hong-bo CAI; Qi LIU; Jian-yan WEI

doi:10.3788/OPE.20172505.1368

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Definition evaluation method for real-time auto-focusing of wide-angle telescope

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- Definition evaluation method for real-time auto-focusing of wide-angle telescope
- Optics and Precision Engineering Vol. 25, Issue 5, Pages: 1368-1377(2017)
- 作者机构：
  
  1.中国科学院国家天文台空间天文与技术重点实验室, 北京 100012
  2.中国科学院大学, 北京 100049
- 作者简介：
- 基金信息：
- DOI：10.3788/OPE.20172505.1368
  CLC： TP391.4
- Received：13 December 2016，
  
  Accepted：24 January 2017，
  
  Published：25 May 2017
- 稿件说明：
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Jia-hui YUAN, Hong-bo CAI, Qi LIU, et al. Definition evaluation method for real-time auto-focusing of wide-angle telescope[J]. Optics and precision engineering, 2017, 25(5): 1368-1377.
DOI：

Jia-hui YUAN, Hong-bo CAI, Qi LIU, et al. Definition evaluation method for real-time auto-focusing of wide-angle telescope[J]. Optics and precision engineering, 2017, 25(5): 1368-1377. DOI： 10.3788/OPE.20172505.1368.

摘要

SVOM天文卫星附属的地基广角相机阵（GWAC）含有36个广角望远镜，其短时标的高精度自动观测建立在实时自动调焦的基础上，本文将对广角望远镜实时自动调焦的图像清晰度评价方法进行研究和实现。文章首先比较研究了常用望远镜图像清晰度评价方法的原理及在GWAC系统上的适用性，得出基于星像能量集中度的两种方法：星像50%能量半径、PSF的FWHM值适用于GWAC系统。区别于常用天文软件包IRAF费时的算法，本文提出了基于点源强度分布进行PSF拟合计算FWHM的方法，并进一步探究了诸如拟合模型、选星标准、定心精度、拟合半径、插值方法、插值间隔、FWHM后处理等关键方法参数对FWHM计算精度与速度的影响。最终得到一套适用于GWAC实时自动调焦的清晰度评价方法，并用C++编程实现。本文中方法FWHM计算误差为0.046 pixel，精度与IRAF相当，计算焦点位置一致；单图（挑选后约300颗星）计算时间为0.67 s，约为IRAF计算时间的1/20，满足GWAC系统自动调焦的精度与实时性需求。研究结果在GWAC系统中得到应用，并可为其他自动化望远镜观测系统提供参考。

Abstract

Ground Wide Angle Cameras (GWAC) of the astronomy satellite SVOM contains 36 wide-angle telescopes

and its high-precision automatic observation of short exposure is established on real-time auto-focusing

in this paper

we focus on the research and realization of image definition evaluation for real-time auto-focusing on GWAC. At first

principles of image definition evaluation methods of common telescope and applicability on GWAC system were researched

thus two kinds of methods based on encircled energy of star image were obtained

namely radius of 50% energy of stars and the full width of half maximum (FWHM) of point spread function (PSF)

were suitable for GWAC system. Different from time-consuming algorithm of common astronomy software package IRAF

method of calculating FWHM via PSF fitting calculation based on source intensity distribution was proposed

and further influence of key method parameters such as fitting model

star-choosing standard

centring precision

fitting radius

interpolation method

imterpolation interval

FWHM after-treatment

etc on calcu-lation precision and speed of FWHM was researched. A set of definition evaluation methods that are applicable to GWAC real-time automatic focusing were acquired in the last

and it is realized by C++ programming. Calculation error of FWHM method in the paper is 0.046 pixel

the precision is equal to IRAF approximately and calculated focus location is the same; calculation time of single graph (about 300 stars after selection) is 0.67 s which is 1/20 of calculation time for IRAF. It satisfies precision and real-time requirements of automatic focusing for GWAC system. The research results havebeen applied in GWAC system and provide reference for other automatic observing system of telescope.

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