Documentation on offset

Key: offset
	A number of Miriad tasks either take or return celestial coordinates
	as offsets from some reference position -- invariably the observing
	center or the reference pixel. Although the `offset' keyword is
	generally used, the fitting tasks include offsets in their
	"spar" keyword, whereas IMPOS uses the "coord" keyword.

	Offset celestial coordinates are always given in arcseconds on the
	sky. The offset, (x,y), is related to a true sky coordinate, (RA,DEC),
	by

	  x = (RA - RA0) * cos(DEC0)
	  y = DEC - DEC0

	(These equations should be viewed as a definition, rather than a
	small angle approximation). Tasks invariably convert the offset
	coordinates to true sky coordinates before further converting them
	to something more computationally convenient (e.g. pixel coordinates
	or direction cosines).