a) RA, DEC and Position_angle of instrument pointing direction.
b) Axis information describing the pixel size in each dimension.
and a second file has the same information, then the program can work out the offsets of the second file from the first and rotate the second so that it has the same orientation as the first.
Note: for this to work, the units of the axes must be the same ! There is no checking of axis units in the program.
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