Re: Tycho 2 catalog astrometry

[<aah@nofs.navy.mil>]

Gee, Michael, there are only 2 million Tycho2 stars;
that should be a small fraction of a night's worth of
data! <g>
  I recommend Tycho2 for the astrometry, and for BV photometry
brighter than V=10 or so.  TASS is significantly better than
Tycho fainter than this.  A better choice for astrometry,
however, is UCAC, and that is what I will be using here.
It is not currently available for the entire sky, but at
least the southern half is well covered.
  For photometry, I intend to use only Landolt standards
and calibrate the sky on photometric nights.  I am *very*
hesitant to use Tycho for all-sky stuff since (a) it has
to be transformed into the standard Johnson system and
(b) does not have I-band measures.  If ARNE shows up
here before our good Fall season, I should be able to
calibrate about 2/3 of the sky available from Flagstaff
within a couple of months; other sites can then use that
dataset for setting differential zero points and searching
for variables.
Arne