1. From a background region within the source image via the backexp parameter. In this case the task normalises for the different source and background region areas. NB: This normalisation does not take account of bad pixels, bad columns or chip gaps.
2. From a background image in which case the background is taken from the source region within the image specified in the bkgimageset parameter.
3. As a single value entered in counts per second via the bckval parameter.
