Which reference catalog should I use?
We recommend against USNO UCAC4 for plate solving for telescope pointing because it lacks stars fainter than mag-16, for only about one hundred million stars, where the smaller USNO A2.0 has five times as many stars and includes many more faint (down to mag-18) stars. This can make plate solving with A2.0 more reliable. The small errors in a few individual stars (proper motions) in A2.0 are irrelevant for telescope pointing.
The short story: For plate solving in ACP, MaxIm, FocusMax, etc. which use PinPoint to get hyper-accurate image center point coordinates, use either the GSC or the USNO-A2.0. The choice depends on your field of view. If it's more than 20 arc minutes, you can use the GSC. If less, you should consider the A2.0. The disadvantage of the A2.0 is that you have to download and store 6 gigabytes. The disk space costs essentially nothing. You only have to download it once, but it'll take hours at a minimum. Bottom line: use A2.0 if you can.
For precision astrometry of individual objects in an image, the choice of catalog is more dependent on what you're doing, but in general the USNO UCAC4 catalog is a great choice. It is the one that is recommended by the IAU Minor Planet Center and others for precision astrometry.
How do I get the Catalogs?
Note: Do not attempt to use Internet Explorer for FTP access. It is just too lame. Create a "network place" for the FTP location and use that. Look in the Windows Help and Support Center file for "network places" for details. Or use an FTP application like the free FileZilla FTP Client. If you do use an FTP program like Filezilla, be sure to set your FTP program to "binary" transfer mode. Auto can result in corrupt catalog files form some of the catalogs. Binary always works.
Guide Star Catalog:
As of October 2003, the STScI GSC in its original format (FITS tables) is no longer available from NASA ADC. In order to support our customers, we have placed this catalog online at http://gsc.dc3.com/. Detailed instructions are in the readme.txt file at the above address. If you have a reliable net connection, download the self-contained installer (300+ MB) then run it. You can trash the installer after that.
The most common problem with the GSC is having the wrong directory layout. The 2-CD set from ASP, as well as the online distribution at http://GSC.DC3.com/ has the catalog files in a subdirectory called GSC and the table files in a subdirectory called TABLES. PinPoint is designed to work with the CD-ROM root/drive as the path for all of its catalogs, and GSC is no exception. Set PinPoint's path to the parent directory of the GSC and TABLES directories, wherever it is.
If you are using the 2-CD (uncompressed) version of the GSC/1.1 as obtained from (e.g.) the Astronomical Society of the Pacific, you must have two CD-ROM drives (or copy the contents of both CD-ROM into a common directory on hard disk). When using 2 CD-ROM drives, enter the drive letters for the north disk and south disk separated by a semicolon (e.g., "N:;S:") This will also work if you choose to copy the North and South CDs into separate directories on (possibly 2 different) hard disk(s); separate the north and south directory paths with a semicolon.
This is a large catalog (6Gb), and while old, this is still the "one to use" for common plate-solved telescope pointing applications because it goes deeper than the others, and includes bright stars. The fact that some nearby stars' proper motions are out of date has an insignificant effect on the centering accuracy of the solution. You can download it in bits and pieces at night if needed. Get it from http://usnoa2.dc3.com/. Read the HOW TO document by clicking on it with your browser. Now back up and using those directions, copy the zonexxxx.acc and zonexxxx.cat.zip files in this directory to a directory on your system. Unzip the cat files and throw away the .cat.zip files. Set your catalog type to USNO A2.0. Set your catalog path to the directory into which you copied the filies from USNO.
Alternatively (and more painfully), you can get this catalog from http://usnoa2.dc3.com/. You'll have to right-click and save link as... or save target as... for each file, then after that you have to unzip all of the .CAT files. Time consuming. I don't have FTP capability at the high-speed service on which this resides, and I don't want to clog up the local office net service as we use it for our phones etc. It's a one-time project...
USNO UCAC4 (PinPoint 6 or later only)
This is a large catalog just over 8Gb), but it is "the one to use" if you are doing precision astrometry with PinPoint 6 or later. You can download it in bits and pieces at night if needed. Get it from CDS at ftp://cdsarc.u-strasbg.fr/pub/cats/I/322A/UCAC4. When you go to this page, you will see subfolders u4b and u4i. Create a folder on your drive called UCAC4, then create subfolders u4b and u4i under it. Now copy the contents of these folders from the CDS site into the corresponding folders on your hard drive. Run the Catalog Checker to validate your installation.
Additionally, the US Naval Observatory will send anyone a free 9GB (double sided) DVD of the UCAC4 database. To obtain a copy, send an email request to firstname.lastname@example.org with UCAC4 in the subject line. Be sure to include your US Postal mailing address. When you get the DVD, create a folder somewhere, say My Documents\Catalogs\UCAC4. Then create two sub-folders "u4b" and "u4i" under your new folder. Copy all of the stuff from the u4i folder on side 1 of the dvd to your u4i folder. Then copy the stuff from the u4b folders on both side 1 and side 2 into your u4b folder. Finally run the Catalog Checker to validate your installation.
USNO UCAC2 and UCAC3 are superseded by UCAC4.
Tycho-2 (Special applications only!)
As of April 2011, the Tycho 2 catalog is no longer available in its original format. In order to support our customers, we have placed this catalog online at http://tycho2.dc3.com/ Copy all of the files in this directory as well as all subdirectories, preserving the subdirectory structure exactly, to a directory on your system or to a CD-ROM at the root. Set your catalog type to Tycho-2. Set your catalog path to the directory into which you copied the tree. See the README.txt file at the root of the catalog data.