The Solar Corona Imager is an internally occulted coronagraph on board the ASO-S mission, which has the advantage of imaging the inner corona in H I Lyman-α (Lyα) and white-light (WL) wavebands. However, scattering of solar disk light by the primary mirror (M1) becomes the main source of stray light. To study the methods of stray light suppression, three scattering models are used to model M1 scattering in Zemax OpticStudio. The ratio of coronal emission to predicted stray light decrease along field of view in both channels. The stray light in Lyαchannel is generally lower than coronal emission, but the stray light in WL channel tends to be one order of magnitude higher than coronal signal at 2:5Rꙩ. Optimized parameter combinations that suppress the stray light to required level are obtained, which put some limitations on the M1 manufacture. Besides, K-correlation model is recommended to simulate surface scattering.
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.
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