[vsnet-chat 2544] USNO-A magnitude errors (again)

[Brian Skiff <bas@lowell.edu>]

     In a recent batch of papers at the Los Alamos preprint server is a
report by Morgan et al. about a new southern double quasar.  Included is a
BRI photometric sequence around the object.  There is a chart and offsets,
but no coordinates for the stars.  These are listed below from USNO-A2.0, along
with the photometry (rounded in the cases of some poor measurements).  The
quasar-pair itself, called CTQ 839, is also in the USNO catalogue, and is shown
as the first entry.  I've sorted the stars in order of decreasing brightness.
     The columns toward the right show magnitudes from USNO-A2.0 and their
offsets from standard B and R.  The final column shows the offset for the
red magnitudes from USNO-A1.0.  These offsets indicate the following for this
field (only):

1)  USNO-A2.0 blue magnitudes are too bright by around 1 magnitude from B
mag. 18 on down.

2)  USNO-A2.0 red magnitudes brighter than R ~18.5 are too bright by several
tenths of a magnitude.

3)  Visual magnitudes derived from some value intermediate between mb and mr
will be greatly in error.  

4)  As is common elsewhere in the southern sky, USNO-A2.0 red magnitudes
have a serious scale error, such that for R ~18.5 and fainter, everything is
given as mr 17.8 to 18.0.  From other deeper sequences I know that stars are
called 18.0 even if they are 20th or 21st magnitude in R.

5)  USNO-A1.0 red magnitudes in this field are too faint by ~1.3 magnitudes
for brighter stars, becoming even fainter below R ~18.5.  Visual magnitudes
derived from these will be even more in error (too faint).

6)  The _internal_ scatter of the USNO-A series is quite good, bearing in mind
the systematic errors just noted.

\Brian


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


source:  http://xxx.lanl.gov/abs/astro-ph/0001141


      RA  (2000)  Dec        R    B-R   R-I     mb    mr   del-b  del-r  del-r
QSO 2 52 57.9  -32 49 09  ~18                  17.4  17.1
5   2 52 59.4  -32 48 45   15.92  2.53  0.90   17.4  15.6   -1.1   -0.3   +1.2
7   2 53 08.6  -32 49 25   16.34  1.61  0.53   17.3  16.2   -0.7   -0.1   +1.3
8   2 53 10.1  -32 49 32   17.03  2.33  0.73   18.3  16.8   -1.1   -0.2   +1.3
3   2 52 52.1  -32 48 27   18.04  1.71  0.57   18.0  17.4   -1.8   -0.6   +1.3
1   2 52 44.3  -32 46 44   18.35  2.2   1.40   19.9  17.8   -0.7   -0.5   +1.9
2   2 52 50.6  -32 46 56   18.59  3.0   1.51   20.2  17.9   -1.4   -0.7   +2.0
6   2 53 00.5  -32 47 33   19.0   1.7   0.5    19.7  18.0   -1.0   -1.0   +2.1
4   2 52 55.6  -32 47 53   19.02  1.50  0.48   19.2  17.9   -1.3   -1.1   +1.7

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