WCS question

[Doug Mink]

Patrick Wallace wrote:
> 
> Putting it another way, precession isn't involved until you want to
> point a telescope.  Even then, in the new deal there is never any reason
> to apply precession alone;  only the orientation of the Earth matters, and
> the artificial separation into secular/long-period terms (precession) and
> short-period terms (nutation) is avoided except perhaps as a computing
> convenience.

So if all we want to know is the position of an object in the image in
the
coordinate system of the reference stars, the only reasons to worry
about
the date of the observation are proper motion and conversion to another,
time-dependent coordinate system, such as FK4 or FK5?

-Doug