It's hard to directly compare two catalogs since they may have been made for different purposes. Most of the deep catalogs are for precise astrometry (position), while some catalogs are for precise photometry (brightness).
Neither of these will matter for most amateur use, unless you do some "real science" with your scope. If you just want a deeper catalog, then yes, UCAC3 goes more than a magnitude deeper than GSC and has about 5 times as many stars. Your question about whether "UCAC3 catalog replaces HST GSC catalog because of the more precise magnitudes of UCAC3 (the limiting magnitude is fainter with ~m16.5 anyway)" is hard to answer. Both of these catalogs were made for astrometry, and just because a catalog goes fainter doesn't mean it's more precise, especially if you're talking about using an astrometric catalog for photometry. For a blatant example, see the astrometric catalog USNO-A1.0, the first "very deep" catalog showing most of the stars in the Palomar Schmidt all-sky survey plates, which goes much fainter (with half a billion stars to about mag 20, compared to about 20 million stars to mag 15, for GSC, or 100 million to mag 16.5 for UCAC3) than any of these catalogs, but where the developer admitted that "The photometric calibration of USNO-A1.0 is about as poor as one can have and still claim that the magnitudes mean something". When you go very deep, there aren't many references to use for calibration. Or to use a completely different analogy, the early maps of the "new world" made by Columbus and other early European explorers went much "further" than any earlier map, but weren't very accurate. -John ----- Original Message ---- > From: babarosa6 <[email protected]> > To: [email protected] > Sent: Sun, October 4, 2009 3:44:05 PM > Subject: [skychart-discussion] UCAC3 versus HST GSC > > Hi! > > I have a (maybe sheepish) question: > > Am I right that UCAC3 catalog replaces HST GSC catalog because of the more > precise magnitudes of UCAC3 (the limiting magnitude is fainter with ~m16.5 > anyway)? > > Installing and using it with skychart_3.1-1232_i386.deb works fine. > > Greetings > Michael > > > > ------------------------------------ > > Yahoo! Groups Links > > >
