[vsnet-chat 7618] SPM4 - a source of faint V mags for South of Declination -20 degrees?

[Ap Graph]

I came across this the other day :-

http://www.astro.yale.edu/astrom/spm4cat/spm4.html

which leads to this ftp://ftp.astro.yale.edu/pub/spm/spmcat/spm4/ and this 
ftp://ftp.astro.yale.edu/pub/spm/spmcat/spm4/readme_spm4.txt

I hadn't realised there had been yet another reprocessing, nor that so much CCD 
work had been used in the later epochs.

I downloaded the data for -70 degrees and southward and bled out the CCD ones 
that were flagged as having BOTH B and V via CCD (a smaller subset, somewhat 
more objects can be had by only looking for CCD based V magnitudes).  The full 
total of all objects was about 7 million stars, whilst the CCD in both V and B 
was only slightly more than 10% of that, going on for 800,000, down to magnitude 
18 or so at times.

I then matched that to the LONEOS file of Brian Skiffs.

Very few crossmatches were made, only a couple of hundred, which isn't a goodly 
amount for checking, but on the whole it looks like objects with V flag set to 3 
in SPM4, ie the V magnitude comes from the CCD part of the survey, is good to 
about 0.1 magnitudes, with an offset mean and median both about 0.04 magnitudes.

Well, this is far better than any other general survey sourced V magnitudes in 
these parts of the sky.  It will not be as good as eventual southern surveys, 
including APASS (when latter is used critically instead of blindly), but it's 
here and now.

(The catalogue of course is also a quality positional reference as it carries 
proper motion information.)

The problem is using it.  It's a big download, and although a simple TDF can be 
built for fields quite easily for each one degree strip for Guide users, with 
the CCD V objects notable via conditional testing in help dialogues for each 
object, it's general use is likey going to need VizieR inclusion.  Such a thing 
would require numbers of people requesting of people at CDS to include the data, 
so if it'd be useful, people need to ask, because SPM3.3 never got to VizieR, 
despite being pending on their list for inclusion for some time via their own 
decision (last VizieR included SPM is SPM2).

The SPM V magnitudes from photographic plates are not so good.  They're better 
than USNO A2.0 or GSC-ACT derived V magnitudes, but that's about it.  For SPM 
3.3 I estimated V photographic was only good to +/- 0.25 when checked against 
loneos.phot for as allsky as possible, which is better than the USNO x.x or GSC 
+/- 0.5, but isn't great.

Those used to downloading and handling large datasets, with a bit of MySQL about 
them, could source SPM4 from above and bleed out the CCD flagged V quite easily 
to make a subset which likely will be much smaller than the whole, but still 
likely several hundred megabytes uncompressed.

John



      
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