Tycho 2 catalog astrometry

[Andrew Bennett <andrew.bennett@ns.sympatico.ca>]

I have just come across the Tycho 2 catalog
http://archive.eso.org/skycat/servers/ASTROM

This differs from the Tycho catalog I was using
in having a lot more stars (it is more complete for
the fainter stars) and in having proper motions. The
catalog positions are corected to 2000.0.

This makes quite a big difference. For example, for
file H3R1659.840 (CD15) my program using Tycho:
Matched 235, accepted 215 sd 0.918 pe 0.745 arc sec
For Tycho 2:
Matched 352, accepted 324 sd 0.686 pe 0.541 arc sec

Winding Tycho 2 back to the date of observation gives
astrometric errors essentially identical to Tycho: the
improvement is all the result of proper motions. Wow!
Stars move!!

Magnitudes come out a little worse;
Tycho pe 0.1062  Tycho 2 pe 0.1384 mags
This is probably just the higher proportion of
fainter stars. Tycho 2 tries to cope with close
doubles too so there may be some cleaning up to
do to match the MkIV resolution ...

["matched" means just that, including apparently saturated
sources:
"accepted" is after pruning outliers.]

Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.