The flat field calibration frame may not have the same illumination pattern as the observations of the sky. In this case when the field field correction is applied to the sky data, instead of being flat there will be gradients in the background. You can check this by plotting several lines over the reduced sky frame. In case of no clear variation you can continue with the correction of your science frame(s) using the standard flat. In some cases the application of the simple flat field correction doesn't do a good job, and there may be an illumination problem. If such deviations are present one can try to correct for these by applying an illumination correction.
The illumination correction is made by smoothing the reduced blank sky frame heavily. The illumination frame is then divided into the frames during processing to correct for the illumination difference between the flat field and the objects. Like the flat field frames, the illumination correction frames may be data set dependent and hence there should be an illumination frame for each data set.
The smoothing algorithm is a moving average over a two dimensional box. The algorithm used a box size that is not fixed. The box size is increased from the specified minimum at the edges to the maximum in the middle of the frame. This permits a better estimate of the background at the edges, while retaining the very large scale smoothing in the center of the frame. Other tools in MIDAS can also be used for smoothing, but this may need more of the user and may take more processing time.
Blank sky frame may not be completely blank, so a sigma clipping algorithm may be used to detect and exclude objects from the illumination pattern. This is done by computing the rms of the frame lines relative to the smoothed background and excluding points exceeding the specified threshold factors times the rms. This is done before each frame line is added to the moving average, except for the first few lines where an iterative process is used. If this approach is not successful manual removal of objects (stars) is required.
Table: Keywords for making the illumination frame
The application of the illumination corrections to the science frames will be done automatically provided the keyword SC_ILCOR is set to `yes', and the illumination frame is available. The REDUCE/CCD command look for the illumination correction frame either in the keyword SC_ILFRM or in the CCD reduction table. In case it can't find it it will issue an error message and the illumination correction will not be done.