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together as "Ax.0". The Ax.0 catalogs list about a half billion objects. Objects were found by scanning in wide-field photographic plates, some of them dating back to 1950. Usually, two plates, one red-sensitive and one blue-sensitive, were used. This means the catalog can give an indication of the object's color (not always a very accurate one, though) and that many spurious objects could be safely ignored: if an object was found on only one plate, it was assumed to be an error, or a flaw in the plate, or a passing asteroid or satellite. In this catalog, and in B1.0 and GSC-2.3, the magnitude data is of mediocre accuracy. USNO was very interested in finding positions to within a fraction of an arcsecond, and succeeded in this goal; but good magnitude measurement wasn't a priority. The documentation for A1.0 states bluntly that "the photometry is as bad as it can be while still being able to claim that the numbers mean something." The USNO B1.0 catalog was the successor to A2.0. It contains about a billion objects, with slightly better accuracy in both positions and magnitudes. In this case, up to five plates were scanned for each part of the sky. GSC-2.3 is the successor to the GSC-1.1 and GSC-ACT catalogs. It resembles B1.0, but was assembled by the Space Telescope Science Institute (STScI), in some cases from different sources, and using different methods. So it may catch some objects missed by B1.0, while missing a few that B1.0 includes. The UCAC-3 catalog is the third version of what will probably become the standard catalog of choice for stars down to about mag 15 or 16. Its positional accuracy is quite good. The version provided with Guide is necessarily a compressed one, with some of the data omitted (such as infrared magnitudes copied from 2MASS and so on). If you use the "Get UCAC-3 data from Internet" option, then Guide will be able to gather full data for that part of the sky. You may wonder why you would do this, since Guide is already showing you data from UCAC-3 already, built into the DVD. The reason is that not all data, to full precision, is provided on the Guide DVD. Certain fields that don't matter for star charting are omitted. If you download UCAC-3 data, then click on the downloaded stars and ask for "more info", you'll see much more information than before (and also much more than could have fit on the DVD, which is why Guide doesn't show all that data already). CMC-14 goes about a magnitude deeper than UCAC-3, with slightly greater astrometric and photometric accuracy. But it only covers most of a band between declinations -30 and +50, and has no proper motion data. 17c: INSTALLING TO THE HARD DRIVE
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