Combining Images

A number of the above steps require the combination of a number of IR images. This can be a sensitive matter because of the continuously varying sky background, and care must be taken to ensure that a suitable multiplicative scaling, or additive shift, is applied to match the sky background in all those frames being combined. Whether shifting or scaling is most appropriate unfortunately depends on the prevailing conditions, and you should experiment to find out which is best.

The command COMBINE/CCD is used to combine several images. The command itself has a number of options to use DO tables or catalogues, but the simplest version takes the form:

COMBINE/CCD type images output

where type is one of BS (for bias), FF (for flat field), DK (for dark), SK (for sky) and OT (for other) and is used to indicate what type of image you are combining; images is a comma separated list of images you wish to combine; output is the name of the resulting output file. Scaling and/or shifting options can be specified for each of these image types. These parameters can be checked using SHOW/CCD type, and be set using SET/CCD type, where type, using the codes listed above, specifies the type of image whose combination parameters you are interested in. An example is displayed below:

COMBINE/CCD FF kflat1,kflat2,kflat3 kflat

will combine the images kflat1, kflat2 and kflat3 into an output image kflat using the combination options specified for the FF image type.