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that was first set up in Chile to observe the southern half of the sky, then in Arizona to observe the northern half. UCAC-3 data is usually good to the error values mentioned in "More Info" for a given star. But be warned that several issues have turned up since the catalog was released. There are some small systematic errors causing some areas to be off by a fraction of an arcsecond. The data for certain stars is almost certainly wrong, for reasons that haven't been adequately explained yet; for example, the proper motion values indicate faster motion than seems likely. And all star catalogs suffer from "completeness" problems: you'll sometimes see a faint star in the catalog right next to a bright star that was omitted. For most users of this software, these flaws won't be apparent. (The errors are far below the level where a visual observer could ever notice, for example.) But if your project really requires extreme precision -- asteroid astrometry, for example, or figuring out asteroid occultations -- you should proceed with caution. While the Hipparcos/Tycho Catalogs and UCAC-3 are used for display of stars, when you click for "More Info" on a star, you may get data from many other catalogs. Guide will attempt to cross- reference to the SAO (Smithsonian Astrophysical Observatory), HD (Henry Draper), Yale, GCVS (General Catalog of Variable Stars), WDS (Washington Double Star), PPM (Position and Proper Motion), and NSV (New Suspected Variable) catalogs. Each will either provide additional data, or a cross-check on the data you've seen in other catalogs, or both. There are five galaxy catalogs on the Guide DVD. The PGC (Principal Galaxy Catalog 1996) of over 100,000 galaxies forms the basis for display of small fields. Data is also shown from the RC3 (Third Revised Catalog), the Uppsala General Catalog (UGC), the Morphological Catalog of Galaxies (MCG), and the European Southern Observatory extension to the UGC (ESO/Uppsala). As with any other problem you may find with Guide, please let us know if you find any problems in the data. At the very least, error reports can be passed on to those maintaining the data. 23: ACCESSING GUIDE'S DATA Quite a few programmers have asked about accessing the data on the Guide DVD from their own programs. In certain cases, this can be easy to do; some of the data (for example, all of the "user datasets") are in their original ASCII form, and are well-documented. In others, it can be a difficult undertaking (e.g., the GSC, UCAC-3, Tycho, and PPM data). The compression really is not intended to frustrate people. But a quick check will show you that the DVD is almost completely full; unless many larger datasets were highly compressed, something would have been omitted. (This was even more of a concern with earlier versions, which
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