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 off-pointing the telescope from Sun center.
The basic idea of the Kuhn-Lin-Loranz technique for calibrating spatially non-uniform image
array detectors is to compute the horizontal and vertical derivatives of the
logarithm of the gain function from multiple displaced full-disk images. Once
the gradient of the log-gain function is determined, integration of this
function leads to the logarithm of the gain within an additive constant. Finally,
an iterative least-square algorithm is used to solve the pixel gain function for various
displacement vectors.
Computing the gain table from the full-sized flat-field 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 full-sized versions. |
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