Hi Pat,

There has been a lot of discussion of GSC-2.2 on the Minor Planet
Mailing List (MPML) over the last few days. The story goes as follows:

Shortly after GSC-1.1 was released around 1992, the Space Telescope
Science Institute folks (STScI) started planning on GSC-2. It's been
"just around the corner" for some years now (yes, I know... sounds
like another updated version of something we've been discussing
recently.) However, prompted by Owen Brazell's post on this list,
I dug up some info on GSC-2.2. (Earlier versions, 2.0 and I think
2.1, were never released.) That resulted in my making the following
post on MPML... which, in hindsight, I ought to have mentioned on
this list as well.

--------- My post to MPML begins ----------
Hi folks,

After waiting expectantly for about six years or so, I finally
got a look at GSC-2.2 last night. Some info about it is at

http://www-gsss.stsci.edu/gsc/Copy%20of%20gsc2/GSC2home.htm

From here, you can click on "Data Access" to get a form that
makes it reasonably simple to download small areas of GSC-2.2. I
haven't given it a careful going-over yet (and would be interested
in comments from those who have), but it looks promising. I
downloaded some chunks to display in comparison to A2.0 data,
with the following results:

The GSC-2.2 data was, in general, more dense than the A2.0 data.
(No big surprise there. GSC-2.2 claims almost a billion objects,
almost twice as many as A2.0.) Bright stars are patched in from
Tycho-2; proper motions are given for them. For everything else,
the proper motion field is set to zero (for the moment). F, J, V,
and N magnitudes are given, though exactly which mags are set to
non-null values depends on where you are in the sky. V and N are (as
far as I've seen so far) always identical. Sigmas are given for
everything, along with an epoch date. The result is about 200
bytes/object, so it does take a while to download much data.

Show it with A2.0 data in the background, and the wonders of
proper motion become apparent. The first area I checked had
mostly GSC-2.2 epochs around 1995, while the A2.0 data is all
around 1950-1955. The brighter stars all appeared as close
doubles, with sufficiently random distribution to persuade me
that it wasn't some sort of systematic shift.

-- Bill
--------- My post to MPML ends ----------

I have good news and bad news. Bad news is that you can't simply
download GSC-2.2 data and display it as a user-added dataset; the
format is somewhat unorthodox (for astronomical datasets). Good
news is that I've written a little piece of code that enables Guide
to access the data fairly simply, and then puts it into usable form;
you'll be able to zoom in on an area of interest, hit a button,
wait, and see GSC-2.2 data in the background. (Somewhat like the
"grab DSS image via Internet" option I added to Guide a while back,
and have long wished I could do with A2.0 data.)

I will be posting an update on the Web site shortly to provide this
new feature, at which point we can all start going over GSC-2.2 with
a fine-tooth comb and get some idea as to its characteristics.

-- Bill