The image is assumed NOT to have been through an inverse gnomonic
projection. The reference stars are all assumed to have positions for
the same equinox and that is the same equinox as for the plate centre,
usually J2000.0. The reference points (in the image) are in the same
order as the reference stars. As usual in images, the reference points
are measured from the origin (top, left) of the image; this fact is
allowed for in the algorithm.
Returns null in the event of errors being detected.