[vsnet-chat 1521] New very deep sequences

[bas@lowell.Lowell.Edu (Brian Skiff)]

Folks,
     The December 1998 issue of the ApJ Supplements contains an interesting
paper (one of a series) summarizing data from a deep multicolor survey mainly
to find quasars and other extragalactic objects (Osmer et al. 1998 ApJ Suppl
119, 189).  The paper describes a catalogue of some 19,000 objects detected in
CCD frames covering six intermediate- to high-latitude fields.  Astrometry
and photometry are provided down to mag. 23 or so; the internal precision is
good down to about mag. 21.  The photometric data include Johnson UBV and 
three other redder non-standard passbands.
     For the purposes of wide-field survey and photographic-plate calibration,
I have extracted small subsets of the data.  I first omitted all objects with
nominal errors > 0.03 mag.  I also deleted nonstellar objects appearing in
Tables 1 and 2 of the paper, and pathological stars from Table 3.  I then
ran the remaining stars into the Strasbourg VizieR utility, looking at
USNO-A2.0 for objects within 15 arcsec of each object.  This allowed me to
obtain improved equinox 2000 coordinates for the objects (these were mostly
within 2" of A2.0), and to trim the list further to omit stars with near
neighbors.  The faintest stars are beyond the limit of A2.0, so I adopted
the precessed Osmer et al. positions.  Figure 3 of the paper suggests stars
brighter than about V=15.5 were saturated on the CCD, so I deleted these as
well.  For some of the well-populated fields, I trimmed the lists further to
include about 20 stars per magnitude interval.
     All this left about 820 stars.  If you're greedy, the complete source
files are available starting at:

http://vsnet.astronomy.ohio-state.edu/~posmer/DMS

Conveniently, all but one of the six fields lie well away from existing deep
sequences; those five are near the Equator---indeed reasonably near the
ecliptic, too, making them useful for asteroid survey work as well.  The one
northern field (near 17h15m/+50) happens to be about 1 degree from the BVRI
sequence by Majewski et al. (1994 PASP 106, 1258), which reaches about as
faint.  In any case, I found no overlaps.  The slightly dodgy photometric
calibration (see Paper I of the series:  ApJ Suppl 104, 185) suggests the
external calibration errors are probably in the 0.05-0.10 range, i.e. okay for
the purposes I have mind, but these are nothing like bona-fide standards.  But
there are lots of stars, and they go faint.  By the way, these data confirm 
again that USNO-A2.0 magnitudes are typically ~0.5 mag. too bright, and that
at the limit of the catalogue a scale error sets in, such that A2.0 magnitudes
are ~1.5 magnitudes too bright among the very faintest stars.
     I tried to determine whether the Osmer et al. "almost-R" data were readily
transformable to the standard R system, but this was not obvious from a simple
inspection of stars of ordinary color.  Thus I trimmed the file to include
only the BV data.  Note that for stars with B-V < 1.3, you can get quite good
R magnitudes from:  R = V - [0.508(B-V) + 0.04].  A sample of the file is
shown below; the complete version is copied out _temporarily_ to the Lowell
ftp area:

http://ftp.lowell.edu/pub/bas/starcats/dms.dat   (44Kb)

I will be fiddling with the file a bit more before larding it into my larger
file of faint photometric reference stars:

http://ftp.lowell.edu/pub/bas/starcats/loneos.stds   (1.3Mb)

...but it should be merged there in a few days.


\Brian

==========

#ID# fld      RA  (2000)  Dec          B        V
 560 01e   1 02 29.08   -0 40 08.5   16.450   15.652
 326 01e   1 02 50.07   -0 44 23.5   16.909   16.109
 485 01e   1 03 02.15   -0 41 32.5   16.699   16.135
 843 01e   1 02 56.56   -0 35 29.1   17.104   16.247
5022 01w   1 02 17.73   -0 50 05.2   17.270   16.356
5387 01w   1 02 17.97   -0 45 45.5   17.169   16.382
   6 01e   1 03 17.83   -0 50 27.2   17.135   16.479
 389 01e   1 03 10.30   -0 43 14.9   16.985   16.500
5466 01w   1 01 59.17   -0 44 52.0   17.172   16.544

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