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[vsnet-chat 1791] Cousins E-region standards

     For some time I have recommended to southern-hemisphere amateur observers
doing either single-channel or CCD photometry to use as calibration the primary
standards established by Alan Cousins and his later collaborators.  These have
been extensively observed at the South African Astronomical Observatory for
several decades, and lie mostly in the Harvard "E regions" near -45 Dec.  As
such, the stars _define_ the standard UBVRI photometric system in the southern
sky.  The most recent revision of this list was published in 1989 by Menzies
et al., in SAAO Circular no. 13, page 1.
     Despite the recommendation, I was a little surprised recently to find that
the data are not available on-line from either the data centers, Mermilliod's
photometric database, nor from the SAAO itself.  The SAAO Circulars are not
not widely circulated, unknown even to many professionals.  I have thus
keyed-in the most recent revision of the standards, and provide IDs, improved
coordinates, and notes for the stars.  The complete file (about 63Kb) is copied
out to the Lowell ftp area, and can be obtained using an ordinary Web browser at:

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

In order to fit the data onto a single line without cramming things together,
the file is 83 columns wide.  A few sample lines are shown below.
     Although most of the stars are relatively bright---too bright for
those using CCDs on 1-m class telescopes---the majority lying between mag. 6
and 9 are perfect for telescopes in the 20cm-40cm aperture range most commonly
used by amateurs, where a 10-second exposure should give high signal-to-noise
without saturating.  Batches of them are always up covering a range of airmass,
and additional sets are given in the F regions at -75 Dec and near each of the
Clouds, totalling some 670 stars.  Per-star errors are typically 5 mmag or
less, and the global systematic errors are no more than 1 or 2 mmag.
     There has been some discussion in the professional literature about the
real but small systematic differences between the Landolt standards along the
Equator and these stars at -45 Dec.  Although readily observable by careful
workers using single-channel photometers, I know of no paper where results
using CCDs have shown a discernable difference when both sets of standards were
mixed.  The zero points are within a few millimagnitudes of each other, and
neither series has errors as a function of Right Ascension when compared to a
third high-accuracy dataset.  They define two slightly different photometric
systems, but their internal consistency is impeccable.  The two best papers
describing the differences and limitations of both the Landolt and Cousins
standards are:

1995PASP..107..672B
  BESSELL M.S.
  Publ. Astron. Soc. Pac., 107, 672-682 (1995)
  UBVRI systems: resolving different versions.

1996AJ....111.1338T
  TAYLOR B.J., JONER M.D.
  Astron. J., 111, 1338-1346 (1996)
  The Cousins VRI system:  a consistency test of equatorial and southern
    standard stars.

The hueristic value of these two papers is very high for any photometrist.
Although neither set of standards is clearly preferable, for high-precision
work it is best to use either one set or the other, and not mix them.  For
most of us, however, it hardly matters.
     The star list includes a substantial number of wide double stars and
known variables, so it is adviseable to check the notes for these when
selecting stars to observe.  A few sentences directly from the Menzies et al.
paper are worth quoting on these points:  "While variable stars cannot be used
for fixing zero points, they may help in the determination of colour equations;
suspected variables should not be rejected per se, and it would be worthwhile
attempting to confirm or deny variability in those cases.  When stars with
companions are observed, care should be taken to include or exclude the
neighbouring stars as indicated in the notes.  Where no such advice is given,
companions are too faint to affect photometry at the 0.01 mag. level."  In
preparing the notes, I found additional cases where very close pairs are
included, but these should not affect their use for calibration.
     My version of the table includes the original E-region (or other) name in
the first column.  The second column shows either the HD number or Cordoba
Durchmusterung designation.  The many bright stars have Bayer letters and HR
numbers given in the notes.  The coordinates are for both equinox and epoch
2000, extracted from the Hipparcos/Tycho catalogues or the ACT.  The many
large-proper-motion stars (> ~0".2 per year) have the annual motion specified
in the notes.  The photometric data are copied verbatim from the SAAO paper.
Missing data were either not determined by the authors or apply to variable
stars.  The data are given to three decimals, excepting a few stars where U-B
only is given to just two.  For the variables, a notional value of V given to
0.1 mag. is listed (usually copied from the notes in the source table).

\Brian

=============================================

source:  1989SAAOC..13....1M
         MENZIES J.W., COUSINS A.W.J., BANFIELD R.M., and LAING J.D.
         South African Astron. Obs. Circ., 13, 1-13 (1989)
         UBV(RI)c standard stars in the E- and F-regions and in the
           Magellanic Clouds - a revised catalogue.



[CS62]   Name              RA  (2000)  Dec         V     B-V    U-B    V-R    V-I
E1-1     HD 8977         1 27 35.3  -46 09 06    7.704  0.087  0.091  0.038  0.084
E1-2     HD 8382         1 22 10.4  -46 13 18    8.444  0.206  0.112  0.126  0.274
E1-3     HD 8147         1 20 00.8  -45 27 32    9.657  0.086  0.098  0.037  0.085
E1-4     HD 8305         1 21 32.2  -45 08 15    7.455  0.362  0.001  0.213  0.427
E1-5     HD 7040         1 10 04.8  -44 29 44    9.451  0.365 -0.038  0.225  0.453
E1-6     HD 9404         1 31 29.6  -44 39 22    7.864  0.396 -0.026  0.235  0.472
E1-7     HD 7972         1 18 22.6  -44 11 49    8.613  0.440 -0.038  0.253  0.516
E1-8     HD 7533         1 14 40.5  -45 16 04    8.902  0.428 -0.003  0.248  0.502
E1-9     HD 9403         1 31 32.7  -43 50 48    8.187  0.518  0.035  0.291  0.574
E1-10    HD 7581         1 15 10.4  -44 37 17    8.369  0.492  0.060  0.276  0.543
E1-12*   HD 10190        1 38 42.7  -44 35 39    8.125  0.480 -0.018  0.278  0.554
E1-16    HD 8340         1 21 52.7  -44 39 37    9.756  0.596  0.111  0.333  0.641
E1-17    HD 8962         1 27 30.6  -43 56 03    9.855  0.579  0.053  0.327  0.648
E1-19    HD 8843         1 26 16.7  -44 41 30    9.638  0.824  0.375  0.452  0.868
E1-20    HD 8501         1 23 16.7  -44 40 18    9.852  0.745  0.282  0.408  0.781

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