  Flat-field reflects the non-uniformity of the photometric response at the
focal plane of an instrument, which uses digital image sensors, such as Charge
Coupled Device (CCD) and Complementary Metal-Oxide-Semiconductor (CMOS). This
non-uniformity must corrected before being used for scientific research. In
this paper, we assess various candidate methods via simulation using available
data so as to figure the in-flight flat-field calibration methodology for the
Lyman-alpha Solar Telescope (LST). LST is one of the payloads for the Advanced
Space-based Solar Observatory (ASO-S) mission and consists of three
instruments: a White-light Solar Telescope (WST), a Solar Disk Imager (SDI) and
a dual-waveband Solar Corona Imager (SCI). In our simulations, data from the
Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly (AIA)
onboard the Solar Dynamics Observatory (SDO) mission are used. Our results show
that the normal KLL method is appropriate for in-flight flat-field calibration
of WST and implementing a transmissive diffuser is applicable for SCI. For the
in-flight flat-field calibration of SDI, we recommend the KLL method with
off-pointing images with defocused resolution of around 18", and use the local
correlation tracking (LCT) algorithm instead of limb-fitting to determine the
relative dis-placements between different images.
