大视场程控自主望远镜的平场改正及其平场自主获取策略

For modern observational astronomy, flat-field correction is a necessary and critical step in data processing, which directly affects the photometric accuracy of astronomical images, but there is still no detailed study on accurate flat-field correction for large field-of-view. A flat-field correction aims to correct the non-uniform response of the optical system and the detector, so the uniformity of the light source (a screen illuminated by a lamp, or twilight/dark night sky) is crucial for the effectiveness of the flat-field correction. However, for wide-field robotic autonomous telescope, due to the large field of view, the gradient in the surface brightness of twilight/dark-night sky becomes a big problem that the difference of the gradients between multiple images affects the combined master flat-field precision, and leaves some large-scale patterns in the master flat-field. These large-scale patterns introduce systematic errors into the photometry. On the other hand, obtaining a dome flat-field for robotic telescopes automatically is technically costly, therefore we need to discuss its necessity and effectiveness. Based on the requirements of the photometry precision for the Antarctic Survey Telescope (AST3; with a field-of-view of 4.3 square degrees), we will study the flat-field data and find practical solutions, including: 1) investigating the uniformity of sky, the spatial distribution of the sky brightness and its variation with time, 2) analyzing and comparing various components in Twilight/Supersky flat-field and their impact on photometric accuracy, 3) monitoring the long-term stability of flat-field, and 4) researching and optimizing the flat-field acquisition strategies and the automatic observation procedures. This project will not only improve photometric accuracy so as to achieve the scientific goals of AST3, but also provide scientific evidence and guidance of flat-fielding for other wide-field robotic autonomous telescopes.

平场改正是现代天文观测数据处理中的关键环节，但对大视场平场精确改正的研究目前并不深入。平场改正的目的在于改正望远镜光学系统和探测器的非均匀响应，所以光源的均匀性至关重要。然而，对于大视场程控自主望远镜，由于视场较大，在Twilight/Supersky平场中天空的面亮度梯度问题将突显，多幅单次曝光图像间的梯度差异将影响合并后主平场的精度，主平场中残留的大尺度结构将引入测光时的系统误差；此外，无人干预下取得Dome平场的技术成本较高，其效果也需讨论。本项目将基于南极巡天望远镜AST3（视场4.3平方度）的精确测光需求和数据：1）研究天光的不均匀性及其随时间的变化，2）分析Twilight/Supersky平场中存在的各种成分及其对测光的影响，3）监测平场长期稳定性，4）优化平场获取策略和自动观测程序。研究结果将不仅能够保证AST3的测光精度，也将为其他大视场望远镜的平场获取提供依据和指导。

针对大视场程控自主望远镜的特点，通常采用Twilight/Supersky天光平场来改正望远镜光学系统和探测器的非均匀响应，然而由于这种望远镜的视场较大，其平场中天空的面亮度梯度等不均匀结构的问题突显。.按照原申请计划，本项目完成了基于AST3望远镜平场改正的一系列工作：处理并分析了历年来AST3望远镜的大量天光平场图像；我们推导出天光亮度梯度对平场结构和平场精度的影响，我们还分析了平场图像中存在其他各种不均匀结构（渐晕、通道增益变化等）；为了消除减小这些平场结构对后续的测光精度和测光的稳定性的影响，本项目测试了多种方法以去除平场中的各种不均匀结构，改进了平场数据处理流程，进而将主平场中的空间相关误差减小到可忽略的水平，经相应改正后的平场数据具有均匀、高精度的特点，达到了预期的效果；我们检验并细化了当地太阳地平位置，望远镜指向方向，天光流量和天光亮度梯度之间的关系，实现对实时天光亮度分布的预测，进而提出可优化的自动平场观测策略，以观测更加稳定和均匀的天光；基于此策略我们开发了一个自动观测平场图像的程序，自动化程序大大提高了每天有限的晨昏蒙影时间窗口内平场观测的效率和准确性，实现了本项目的预期目标。.我们优化后的平场改正不仅有助于AST3望远镜实现其科学目标，而且为其他宽视场程控自主望远镜的平场获取和平场改正提供了科学依据和指导。