[vsnet-chat 5614] Re: USNO-B1.0 paper preprint

[Brian Skiff <Brian.Skiff@lowell.edu>]

>>  I would like to ask if there is any major difference between
>>  the currently released USNO B1.0 and GSC II?

     The answer may depend on what you consider to be a "major" difference.
GSC-2.2 has no proper motions.  For large numbers of stars GSC-2.2 is
based on only a only a single plate (easy to see where there is only a
red magnitude for an entry---where there is only a blue magnitude, usually
that entry is nothing more than a copy of Tycho-2).  USNO-B includes
detections from the far-red plate series, which might be important in
identifying very red stars that don't appear on other plates.  USNO-B shows
photometry (such as it is) from all the plates scanned, which might be
useful for assessing variability.  GSC-2.2 appears not to have done merging
of coincident detections:  I have found numerous cases where there are
half a dozen entries for the same mag. 13-14 star (single, uncrowded, etc)---
no wonder GSC-2.2 has a billion stars in it!
     GSC-2.2 does a pretty good job of resolving crowded stars, pairs, etc.,
that is the positions are not strongly skewed by crowding.  I have not 
tested USNO-B for this, but will do so once it becomes available from VizieR.
     In terms of astrometric precision, the catalogues are probably
similar, but thorough tests await VizieR.
     It is clear already that the photometry in both catalogues is rather
poor, and in any case, USNO-B is strictly on the native color systems of
the plates, not in standard passbands (BVRI...).  Again more thorough tests
of the systematic problems awaits VizieR.
     A lot of people like to have actually images rather than catalogue lists.
The USNO-Flagstaff pixel-server provides all of these, not just the single
red or blue plate-scans, but often 8 or 10 or 11 images---again often useful
for looking at variability, flaws, etc.

>>  What is the
>>  need for producing two catalogues from similar plate
>>  collections?

     Actually at least four such sets of scans have been done:  USNO, GSC-x.x,
Minnesota APM catalogue, and Edinburgh SuperCOSMOS are ones you can
readily access on-line.  There are others, such as from the Cambridge (UK)
and Paris MAMA scans that are more hidden.  The APM catalogue was built mainly
to look at galaxies, so doesn't cover any part of the Milky Way; the
SuperCOSMOS data only cover parts of the southern hemisphere away from the
Milky Way, and so on.  Some of the datasets are based on glass copies of
the originals, whereas USNO-B is mostly from the original plates.
     All these factors mean the differences may be subtle, but others may
be of specific interest.

\Brian

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