Selection of in-flight flat-field calibration dither pattern for the Solar Disk Imager of the Lyman-alpha Solar Telescope

Abstract

The Solar Disk Imager (SDI), one of the instruments of the Lyman-alpha Solar Telescope (LST), aims to obtain full-disk images of the Sun in the Lyman-alpha waveband with high spatial and temporal resolution. In order to obtain high quality scientific data, various calibrations (including dark current, flat-field, radiometry, optical geometry, etc.) are required both on ground and in-flight. As other solar imagers, the flat-field calibration of SDI generally adopts the so-called KLL method to derive the flat-field from a set of relatively displaced non-uniform solar images. The KLL method is suitable for flat-field calibration of telescopes working in white light and infrared wavebands. When the working wavelength is extended to ultraviolet (UV) or extreme ultraviolet (EUV) wavebands, many difficulties and uncertainties reduce the accuracy of the obtained flat-field. We use images taken by the Atmospheric Imaging Assembly (AIA) to simulate the input images for the flat-field computations with the KLL algorithm and different dither patterns to find a suitable one for the in-flight flat-field calibration of SDI. Our results show that a circular dither pattern with 21 offset positions properly arranged and images with reduced spatial resolution down to about 15 allow us to obtain flat-fields with required accuracy of 2% of SDI.