Reduction procedure

Infrared images of target objects are dominated by the large, additive sky background. Various reduction schemes are possible. One common approach is as follows.

1. subtract the dark current from all the frames (both flat fields and target objects),

2. create a master flat field,

3. apply the flat field to the target frames,

4. resample the target images onto the same pixel grid and assemble them into a mosaic,

5. optionally any remaining residual non-zero sky can be subtracted later, if required.

Obviously observations made with different filters and on different nights are reduced separately. An alternative, though less usual, approach is to subtract the sky background before making the flat field and dark correction:

$${\rm Final~target~frame} = \frac {{\rm Raw~target~frame} - {\rm Sky~frame}} {{\rm Flat~frame} - {\rm Dark~frame}}$$ (5)

The flat field will usually be normalised. The sky frame should have been acquired at a similar time to the target frame, have the same co-adds and be an average of several frames taken before and after the target observation. Median filtering and pixel masking can be used to remove cosmic-ray hits and bad pixels, respectively.

The dark frame includes the bias offset. It should have the same exposure time and co-adds as the flat field and both should be averaged from numerous individual frames. Although in principle it is possible to combine exposures of different duration by scaling, there may be subtle effects which do not scale, so it is better to ensure that the exposures are of the same duration.

For some further details see Section 6.3, IR data reduction, of SUN/139[10].