Once a day  usually early in the morning  a dark current frame is
taken and a flat field frame is computed from one centered solar image
and 7 to 8 solar images shifted with respect to the center of the camera chip
by applying the method of Kuhn, Lin and Loranz (1991).
The shifted frames are obtained by offpointing the telescope from Sun center.
The basic idea of the KuhnLinLoranz technique for calibrating spatially nonuniform image
array detectors is to compute the horizontal and vertical derivatives of the
logarithm of the gain function from multiple displaced fulldisk images. Once
the gradient of the loggain function is determined, integration of this
function leads to the logarithm of the gain within an additive constant. Finally,
an iterative leastsquare algorithm is used to solve the pixel gain function for various
displacement vectors.
Computing the gain table from the fullsized flatfield frames would take
a lot of computer time. Therefore, we reduce the size of the centered and
shifted calibration frames, which are corrected for dark current, by
a factor of 8 before computing the flat field
image. The resulting miniature gain table is subsequently enlarged to
original frame size.
Example: 
Dark and flat field frames for June 19, 2000 ... 

Dark Frame 
Centered 
Shifted 1 
Shifted 2 
Shifted 3 







Shifted 4 
Shifted 5 
Shifted 6 
Shifted 7 
Shifted 8 







... and the resulting flat field, showing the
pattern of the shutter: 


Please click on the images to view the fullsized versions. 
