[vsnet-chat 1414] Photometric catalogue progress

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

     My efforts to produce a comprehensive photometric catalogue of faint stars
for calibration of wide-field survey images have proceeded apace.  The current
version posted at:

http://ftp.lowell.edu/pub/bas/starcats/loneos.stds

now contains over 14500 stars, and the file is a bit over a megabyte.  (N.B.
the "version" of the file is given by the date in the second line.)  About
half the stars lie between V mag. 10 and 13.5, and nearly all the remainder
between 13.5 and 18.5.  The distribution is unbiased with respect to location,
although I have generally avoided fields in the Magellanic Clouds and in the
densest parts of the Milky Way, where star selection needs to be rather more
careful because of crowding.
     The brighter stars are included to provide calibration on _any_ image for
those doing sky patrol work with small instruments (typically telephoto lenses
with either emulsion or CCD detectors) such as surveys by Takamizawa, Kaiser,
and others.  The fainter stars are included for use by on-going wide-field
surveys with somewhat larger instruments, such as those in search of asteroids,
and for calibration of digitized sky survey plates, such as the USNO series of
immense star catalogues.  These stars in effect more than double the number of
sequences available from the Guide Star Photometric Catalogue (GSPC), and reach
to similar or fainter magnitude limits.  No, we do not have all-sky coverage to
mag. 20, but this is what we can do _now_.
     Inevitably, most of the stars have only BV photometry, but I have included
essentially all the faint VRI photometry appearing in the literature.  For
little-reddened stars, the relationships between B-V and V-R (or even V-I) are
very tight---better than much of the photometry included here.  Thus those
needing V-R colors (or R magnitudes) can reliably derive them from the BV data
given in the file.  One conversion, derived from 260 Landolt standards with
B-V < 1.3 is:

V-R = 0.508(B-V) + 0.040   (mean residual 0.017 mag.)

but see also Reid & Gilmore 1982 (MNRAS 201,73) for another transformation
(plus one for B-V ---> V-I) based on the Cousins E-region standard-star data.
For most survey applications, these are entirely satisfactory.

     Recent additions include the following:  selected stars in the fields of
a few hundred open and globular clusters, chosen as far as possible to be
uncrowded (no bright companions closer than ~15") and well away from cluster
centers; more of the Demers et al. southern sequences (A&AS 99,437 & 461),
which provide groups of several stars down to V ~16 in several hundred fields;
additional sequences around active galaxies/quasars/blazars.  In many cases
these various sequences were among those adopted from the literature in the
GSPC, and I have thus been able to revise and extend the GSPC sequences with
fainter stars and/or better data and VRI colors.  About 275 (among some 1100)
relatively bright stars observed by Andruk et al. in 50 northern fields have
been added.  In all cases I have vetted the published data to look for
inconsistent colors and so on.  Many papers were rejected entirely as a result
of this cursory analysis.  The aim is to be able to rely on the data at the
0.05 to 0.10 mag. level.  Data from even nominally reliable sources can have
systematic errors of similar size, so only a small percentage of the stars
here can be considered to be even "secondary standards"; the bulk are merely
stars that have been observed by someone whose published results seem to be
generally reliable (the GSPC stars are good examples).
     By far the largest source of "new" BVRI data are files recently rereduced
and made available by Arne Henden (USRI/USNO-Flagstaff).  These contain results
from several dozen fields along the Equator and around variable stars typically
reaching to mag. 18 with both good precision and accuracy.  From his several
megabytes of data I have selected about a dozen stars in each field.  Before
being added to the file, his data have been rounded to two decimals and the V-I
color calculated from V-R and R-I in the original.  This work is an extremely
valuable resource for the astronomical community, useful for all kinds of work
beyond their original application.  The source files can be found starting at:

http://ftp.nofs.navy.mil/pub/outgoing/aah

     I'll mention again that Michael Richmond (mwrsps@rit.edu) has provided
some useful products based on the list.  All-sky plots of the distribution of
the stars in both equal-area and rectangular projections can be found at:

http://a188-L009.rit.edu/tass/catalogs/catalogs.html#loneos

http://a188-L009.rit.edu/tass/catalogs/loneos_radec.gif

In addition, Michael has produced a version of the list suitable for many types
of automated analysis.  This file includes reformatted names, RA/Dec converted
to decimal degrees, and gives the BVRI magnitudes explcitly rather than as V
magnitudes and colors.  This modified file is available at:

http://a188-L009.rit.edu/tass/catalogs/loneos.cat

     I've done a fairly complete literature survey for Cousins VRI photometry,
and I think I have added just about all of it that's any good and that includes
faint stars outside of star clusters.  I am adding BV data as I come across
them, but the collection is not nearly so complete as for VRI.  If you know of
faint sequences that are missing from the file, particularly those that include
VRI colors (even V-R only is helpful), I'll be glad to add them in---just send
me the citation.  Getting coordinates for stars identified only on charts is
not a problem.

\Brian

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