I want to shift a FITS image in IRAF using the imshift task. I'm confused about the xshift and yshift inputs. What are their units. Are they supposed to be physical coordinate shifts or WCS coordinate shifts. Are they just present coordinate - new coordinate or something more than that? Currently I'm giving the input as difference between present and new physical coordinates but I don't see any shift happening.
The imshift help page should answer your question:
Parameters
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xshift, yshift Fractional pixel shift in x and y such that xout = xin + xshift and yout = yin + yshift.
So: pixel shifts.