Methodology For In-flight Flat-field Calibration Of The Lyman-alpha Solar Telescope(LST)

Flat-filed 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-Semicondector(CMOS).The recorded im-ages of a telescope must be corrected for the flat-field to obtain images closer to the traget.The common flat-field calibration for telescope using CCD and CMOS as detec-tors can be done via the uniform light source or stable light source at different exposure times within the linear range of the detector's response.It is often difficult to obtain a uniform or stable light source,so in 1991,Kuhn,Lin and Loranz proposed the so-called KLL method to obtain the flat-field of a solar telescope,which uses relatively stable non-uniform images as input.Flat-field calibration is crucial to both ground-based and space-born solar telescope/instrument and sometimes even determines the usability of observational data.The Lyman-alpha Solar Telescope(LST)is one of the payload for the Advanced Space-based Solar Observatory(ASO-S)mission,and it consists of a White-light Solar Telescope(WST),a Solar Disk Imager(SDI),and a Solar Corona Imager(SCI).And one method of its in-flight flat-field calibration is the so-called KLL method.In order to study the influence of the time interval between two successive offset positions on the derived flat-fields of the WST and SDI instruments,we use the full-disk images from the Helioseismic and Magnetic Imager(HMI)and the Atmospheric Imaging Assemble(AIA)onboard the Solar Dynamics Observatory(SDO)satellites to mimic the images of WST and SDI observations,and use them as test images for the KLL method calculation.Based on our calculation and analysis,we got the following conclusions:when we adopt the KLL method to obtain the flat-field,the shorter the time interval is,the better the flat-field accuracy is.This puts some requirements for the spacecraft platform.Considering the time required for the satellite's attitude adjustment and stabilization as well as image acquisition,the time interval for SDI flat-field needs to be within 240 seconds.The WST flat-field accuracy is not sensitive to the time interval between two successive offset positions.This conclusion also limits the time required for the satellite's attitude adjustment and stabilization.The KLL method is suitable for flat-field calibration of telescopes working in white light and infrared wavelbands.When the working wavelength is extended to UV or EUV wavebands,many difficulties and uncertainties reduce the accuracy of the ob-tained flat-field.We use images taken by the 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 spa-tial resolution down to about 15" allow us to obtain flat-fields with required accuracy of 2%of the SDI.We have to determine the in-flight flat-field calibration of LST's three instruments and four detector systems.Through simulation and analysis,the in-flight flat-field cal-ibration methods of the LST are as follows:the SDI has three in-flight flat-field cal-ibration mehtods:the defocus KLL method that with off-pointing the instrument by the spacecraft with relatively large displacements and the KLL method with small dis-placements with the Piezoelectric Transducer(PZT)activators,as well as using its own stable light-emitting diode(LED)in front of the filter to illuminate the detector system,which is referred to as the LED method.The WST has two kinds of in-flight flat-field calibration methods,one is using the KLL method and another is the LED method.The SCI has two kinds of in-flight flat-field calibration methods,one method is the dif-fuser mehtod,which inserts a transmissive diffuser between the primary mirror and the secondary mirror.The diffuser is far from the focal plane of the primary mirror and therefore is illuminated by the Sun almost uniformly.Another method is to obtain ob-servation data by means of satellite rotation and carry out flat-field calibration of the SCI via the KLL method,which is referred as rotating KLL method.Finding suitable in-flight flat-field calibration method for LST is very important,and the defocus KLL method and local correlation tracking(LCT)method proposed by us can well solve the problem of low accuracy of the flat-field calibration of UV and EUV solar telescope.It is expected to contribute to the production of high-quality scientific data for the LST payload of the ASO-S mission.