Given an image covering some part of the sky, and a list of sky positions, it can be useful to know the value of the image at each of the positions in the list. The image might be a sensitivity or an exposure map for example: one might want to translate this into sensitivity or exposure at each of the positions. This is what imsample does.
The datatype of the output column is 32-bit real. If any position falls outside the image boundary, a null is written to the corresponding value in the list.
At present, the list of positions (which must be a binary table extension in a FITS file) must contain columns which give the position in image pixel coordinates. This has the advantage that image WCS keywords are not required, but it must be admitted that RA and dec values are a more universal currency. In future I'll probably modify the task to allow this option (in which event the image would of course have to contain WCS keywords).
