  The extreme ultraviolet (EUV) observations are widely used in solar activity
research and space weather forecasting since they can observe both the solar
eruptions and the source regions of the solar wind. Flat field processing is
indispensable to remove the instrumental non-uniformity of a solar EUV imager
in producing high-quality scientific data from original observed data.
Fengyun-3E (FY-3E) is a meteorological satellite operated in Sun-synchronous
orbit, and the routine EUV imaging data from the Solar X-ray and Extreme
Ultraviolet Imager (X-EUVI) onboard FY-3E has the characteristics of concentric
rotation. Taking advantage of the concentric rotation, we propose a post-hoc
flat field measurement method for its EUV 195 channel in this paper. This
method removes small-scale and time-varying component of the coronal activities
by taking the median value for each pixel along the time axis of a concentric
rotation data cube, and then derives large-scale and invariable component of
the quiet coronal radiation, and finally generates a flat field image. Analysis
shows that our method is able to measure the instrumental spot-like
non-uniformity possibly caused by contamination on the detector, which mostly
disappears after the in-orbit self-cleaning process. It can also measure the
quasi-periodic grid-like non-uniformity, possibly from the obscuration of the
support mesh on the rear filter. After flat field correction, these
instrumental non-uniformities from the original data are effectively removed.
X-EUVI 195 data after dark and flat field corrections are consistent with the
193 channel data from SDO/AIA, verifying the suitability of the method. Our
method is not only suitable for FY-3E/X-EUVI but also a candidate method for
the flat field measurement of future solar EUV telescopes.
