... 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. ... 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. ...