Re: Tycho 2 catalog astrometry

[Stupendous Man <richmond@a188-l009.rit.edu>]

  John Phillips asks:

> Michael, when you used the Tycho-1 catalog, did you apply the proper motions
> given in Tycho-1?  How about the parallax values?  I haven't found a write
> up on the parallax, so I haven't been using the information, but ignoring
> the information probably puts a systematic error in my astrometric solutions
> ... right?

  Did I apply the proper motions?   No, unlike Andrew, I ignored them;
I just used the positions listed in the catalog (which were appropriate
for 1991.25).  As Andrew discovered, the stars _do_ move by a significant
amount over the nine-year period between then and now.

  Did I apply the parallax values?  Again, no.  Most of the stars have
such small parallaxes (less than 0.1 arcsec) that its effect on 
astrometric calibration of Mark IV images is negligible.  My method
of measuring the position of each star, for example, yields a position
which is good to only 0.03 pixels; that corresponds to 0.2 arcseconds,
much larger than the parallax effect.

  Moreover, if one takes a picture only 4 degrees on a side, the
_difference_ in parallax across the field of view will be much, much
smaller than the absolute parallax.  I don't think it could even
be detected in Mark IV images ...

                                               Michael Richmond