Sander, You ask a very good question. I've closely read Bill Gray's summary about the GSC-ACT at [ http://www.projectpluto.com/gsc_act.htm ]. The answer to your question depends on what you want to use the GSC-ACT for? Reading the thread of your post tonight hasn't offered me any insight about that? My primary use for CDC is making charts for AAVSO visual variable star estimates.
Since around 1994-95 the AAVSO has used GSC astrometric positions to create a star field (print black star dots on a sheet of paper)for their charts. Although the GSC is considered to be inadequate for photometric magitude data and the AAVSO mostly uses it only on preliminary charts for a nova or supernova and GSC magitude estimates can be later revised using adequate photometery. Sander from reading your post you find the Tycho-2 Catalog to be indequate in astrometry and you'd like to see as many stars on your screen has your C11 telescope shows. You have only 2 options for a "CDC cat folder ready" catalogue and thats the quote, "obsolete" GSC-ACT 1.14GB. or the "dense" USNO A2.0 which is exactly 6,315,439,692 bytes and +500,000,000 stars and 11 CDs of star catalog. (I don't feel the current state of art USNO- B1.0 which is 80GB with 100 CDs of stars catalog is an option for most CDC users.) Let me quote a tiny portion of Bill Gray's "GSC-ACT files and information" URL: "The purpose of the GSC-ACT project is to recalibrate the Hubble Guide Star Catalog (GSC), version 1.1, using the ACT (Astrographic Catalog/Tycho) data from the US Naval Observatory." . . . "I expect the main use of GSC-ACT will be in asteroid astrometry. And for this purpose, the answer to the question of "How much of an improvement is GSC-ACT over GSC 1.x?" is: "It cuts down errors a lot." . . . "What about photometry?" . . . "First, there is a lack of suitable data. The Tycho dataset provides excellent photometry for stars down to about magnitude 11, and it is true that this could be used to recalibrate bright stars. But it would not necessarily help much at fainter magnitudes (i.e., for about 90% of GSC.) (But this objection may vanish. It does appear that Brian Skiff's LONEOS.PHOT photometric database could extend the range of "decent" photometry quite nicely.) . . . Second, it's not clear that the photometry could be improved very much. The raw magnitude data coming in from GSC 1.1 is sufficiently "random" that recalibration might not result in much of a benefit. And thirdly, Dave Monet has recalibrated the A1.0 dataset (both photometrically and astrometrically), using ACT. This resulted in a much better photometric dataset than GSC-ACT could hope to be. Sander, is that as clear as "MUD"? Personally, I feel Bill Gray knows what he is talking about on the topic of star catalogues. This is because any optical telescope has a certain degree of image field curvature and when the image forms on a flat CCD chip or photographic plate it distorts a tiny bit. When all the image data of a whole sky survey is combined to create a star catalog all those many optically distortioned plates joined together creates two kinds of "errors". First - the stars in the center of the field are in better focus the those towards the edge, this makes errors in star magitudes (photometric errors). Second - the positions of the stars in the center of the field are more accurate that those towards the edge this makes star positions distorted (astrometric errors). Brian Skiff's LONEOS.PHOT photometric database seems to be some sort of "software voo-doo" to focus those somewhat out of focus stars at the edge of a CCD or photo plate and reduce magitude errors. To offer a reply to your question: I would recommend the GSC-ACT for 2 reasons. First- I think you'll learn to deal with an astrometric error of .3 arcseconds (.3") on a computer screen, just set the CDC (HST GSC original fits) field at min 0 & max 1. Second-The magitude errors (for example: Star GSC4766. 1330 - Magnitude: 14.39+/-0.40). This star's magitude could be as bright as 13.99 or as faint as 14.79 and I kinda doubt something like that is going to give you or very many amateur astronomers any heartburn! Best Wishes, Danny Mudcreek Observarory Parker County Texas Latitude: +32 deg 34 min 11 sec - north Longitude : -97 deg 46 min 43 sec - west ===================================================================== --- In [email protected], Sander Pool <[EMAIL PROTECTED]> wrote: > > > Yes, I saw that catalog but it doesn't appear to be available for > download anywhere. Some sites hold information files and code but no > data. If you know of a location please let me know. > > I requested a CD set. > > Sander > > starryeyeguy wrote: > > > > UCAC-2 covers the south pole to about +30 dec from 8th to 16th > > magnitude. There is a supplement for the brighter stars derived from > > Tycho-2. The complete catalog should be available next year. > > > > Cheers, > > > > Geoff Chester > > > > --- In [email protected] > > <mailto:skychart-discussion%40yahoogroups.com>, Sander Pool <sander@> > > wrote: > > > > > > > > > I'm seeing more stars in my C11 than the Tycho-2 catalog contains. > > There > > > is the GSC-ACT catalog but its homepage on projectpluto says it's > > > obsolete. USNO is nice but huge and I don't need mag-20 (I wish :). > > > UCAC-2 sounds promising and contains the range of stars I care > > about. If > > > I'm reading http://ad.usno.navy.mil/ucac/ > > <http://ad.usno.navy.mil/ucac/> correctly it's mostly geared > > > towards the Southern hemisphere though, right? > > > > > > So, which catalog do you recommend for dim stars in CdC? > > > > > > Thanks, > > > > > > Sander > > > > > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/skychart-discussion/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/skychart-discussion/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
