Hi Pete,

> UCAC-3 seems to be the optimum catalog for astronomical plate solves...

Be warned that there is some controversy over this. UCAC-3 has some
very nice properties; it's a big leap forward over GSC, its magnitudes
are decent, and it doesn't have as many "fake stars" (or missing stars)
as other catalogs. This is, in part, because it's based on CCD
imaging, and matched to 2MASS. So you don't get the plate scratches
and other artifacts common to GSC or Ax.0 or B1.0.

Also, the match to 2MASS means there's some passable color data
available. And if a star doesn't have a 2MASS match, it's probably
not "real". (Guide 9 will omit the 1.5% of UCAC-3 stars that lack a
2MASS match; almost all of them are bogus.)

So what's the down side? The positions and proper motions have some
errors, almost entirely negligible for most purposes. For a printed
or on-screen chart, for example, the errors will usually be about
.001 pixel or less; nobody would ever know. But for plate solving,
you really need to pick nits. Something appears to have resulted in
anomalously high proper motions for some stars. (The usual cases I've
seen involve close pairs of stars; in UCAC-3, it looks as if they're
zooming apart from one another.) There's also concern about a systematic
offset of about 20 milliarcseconds.

There were some other concerns about UCAC-3. For example, there are
some missing brighter stars. The really bright ones get picked up in
Tycho-2 anyway, but some around mag 11 or 12 or fainter are just plain
missing. There are also some spurious stars and false doubles. Good
news there is that they can be immediately spotted by the fact that
they weren't matched to 2MASS; drop them, and the problem goes away.

You would never notice this under most circumstances, which is why
I'm reasonably content with UCAC-3 as the base catalog for Guide 9.
But for plate solving, if you're really trying to get the sort of
precision required for asteroid astrometry and such, you may be better
off with B1.0. (Especially if you're going down to mag 20; at that
point, I'd expect a lot of UCAC-3 stars to be saturated.)

Also, be aware that the above should not be taken as gospel. There
was some rather heated debate about all this on the Minor Planet Mailing
List a while back. Nothing conclusive, but one does need to be aware
that UCAC-3 has strengths and weaknesses (just like any other star
catalog.)

> ...The star display still allows me to turn on the SAO numbers on,
> but they don't display any longer. Right clicking only provides the UCAC-3 info.

There are some odd things there, because UCAC-3 isn't linked to the
other catalogs. In Guide 9, I'll be able to know which UCAC-3 stars
appear in Tycho-2; that lets Guide show bright stars from Tycho-2 (taking
advantage of the fact that Tycho-2 has SAO and PPM and other numbers),
and shows fainter stars from UCAC-3. But that requires the specially
processed dataset that's on the Guide 9 disk. At present, if you show
UCAC-3 stars, then the SAO and PPM numbers will be suppressed.

(At least, they will be until you zoom out a bit and UCAC-3 isn't
shown any more. At that point, they'll snap back into view, and you'll
also see them when you right-click on a star.)

-- Bill