Sky Illumination Corrections

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 does not 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 uses 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 frames 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 3.9: Keywords for making the illumination frame

Keyword	Value	Description
CCD_IN	name	input sky frame
IL_TYP	SKY	exposure type
IL_XBOX	5.0,0.25	smoothing box in x
IL_YBOX	5.0,0.25	smoothing box in y
IL_CLIP	yes	clipping the input pixels
IL_SIGMA	2.5,2.5	low and high clipping sigma

 

Both in the pipe line reduction and in the manual reduction the illumination corrections to the science frames will be done provided the keyword SC_ILCOR is set to `yes'. In both cases it is assumed that the illumination frames are available. In addition, in the case of pipe line processing the names of the illumination corrections must be stored in the Association Table. If an illumination correction frame is absent an error message will be issued and the illumination correction for the associated science frame will not be done.