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[vsnet-chat 5674] USNO-B1.0 astrometry test

     I made a quick test of the USNO-B1.0 astrometry using three samples of
objects from the ICRF.  These are distant radio galaxies used to define the
reference frame for RA/Dec.  Redshifts are typically z ~2, so proper motion
is effectively zero to several decimal places.  The large redshifts also mean
the objects are essentially stellar at the image scale of the Schmidt plates.
Errors in the positions of these objects are a milliarcsec or less, thus any
differences from these are essentially entirely with the comparison catalogue.
     I chose ICRF objects in three gores two hours of RA wide (0h-2h, 9h-11h,
18h-20h), which yielded about 125 objects appearing in both catalogues, a large
enough sample to see what's going on.  The Dec range is from -85 to +80.
Since they are chosen randomly the ICRF objects will be scattered randomly over
the Schmidt plates scanned for B1.0, so there shouldn't be any bias as to
plate-location, Declination, etc.
     The list below is sorted in order of increasing offset.  There were
several additional objects in the sample with offsets > 1".  I examined each
of these on DSS scans and found the positions were poor for good reason,
usually due to crowding at low galactic latitude.  The remaining 120 objects
have a median offset of 0".25.
     This is identical to the median error for a similar batch of ICRF objects
matched against USNO-A2.0.  The implication is that, in the absence of proper
motion, the intrinsic quality in the B1.0 astrometry is no better than for
A2.0.  The advantage for most uses, however, is that the positions are for a
much more recent epoch than A2.0, so where there _is_ proper motion (such as
for typical stars at high latitude), the catalogue will appear to be superior
to A2.0 (particularly north of -17 Dec, where A2.0 draws only from POSS-I). 
External errors for A2.0 among faint stars (V >~ 15) at present are about
0".5 - 0".6, due largely to the ~50-year interval of proper motions for faint
stars from POSS-I plates.
     About one-third of the "zero-motion" ICRF objects have B1.0 proper motions
greater than 6 mas/year (the catalogue lower limit).  The B1.0 motions are
in plate-wise xi-eta space, so aren't really quite correct, so it would be
unfair to say anything about the real external errors in the motions until
B2.0 comes along.
     Some tests of the B1.0 photometry will be forthcoming.

\Brian

-------------------------------------------------------------------------------

    RA      (J2000)     Dec         offset (")
10  7 41.498082   +13 56 29.60073   0.022
19 12 40.019176  -80 10  5.94627    0.032
19 39 57.256570  -10  2 41.52067    0.033
10 41 48.897638   +52 33 55.60790   0.049
19 24 51.055959  -29 14 30.12084    0.055
 1  8 38.771070   + 1 35  0.31714   0.058
18 32 50.185631   +28 33 35.95530   0.063
 0 50 41.317392  -09 29  5.21022    0.070
 1 33  5.762585  -52  0  3.94693    0.073
 9 15 52.401620   +29 33 24.04274   0.073
19 11  9.652867  -20  6 55.10860    0.076
10 51 48.789073   +21 19 52.31411   0.079
10 58 29.605209   + 1 33 58.82372   0.087
 1  6 45.107969  -40 34 19.96036    0.088
19 57 59.819271  -38 45  6.35626    0.092
 9 54 56.823626   +17 43 31.22242   0.096
 1 13 27.006813   +49 48 24.04351   0.099
18 23 14.108739   +79 38 49.00270   0.100
 1 13 43.144954   + 2 22 17.31631   0.102
19 37 16.217368  -39 58  1.55290    0.104
 1 54 56.289917   +47 43 26.53907   0.113
 9 58 20.949621   +32 24  2.20929   0.116
10 58 43.309786  -80  3 54.15949    0.123
19 39 26.657726  -15 25 43.05792    0.126
 9 21 29.353871  -26 18 43.38615    0.129
18  0 45.683911   +78 28  4.01854   0.134
10 41 46.781639   +52 33 28.23127   0.146
10 42 44.605212   +12  3 31.26407   0.151
10 35  2.155274  -20 11 34.35975    0.152
19 41 21.768473  -62 11 21.05558    0.152
 1 15 17.099966  -01 27  4.57725    0.157
 1 16 12.521959  -11 36 15.43371    0.162
18 24  7.068372   +56 51  1.49088   0.163
 0 42  4.545183   +23 20  1.06129   0.165
10 48 27.619892   +71 43 35.93846   0.165
 1 51 27.146149   +27 44 41.79365   0.171
18  8 21.885910   +45 42 20.86644   0.173
19 55 42.738273   +51 31 48.54623   0.175
10 56 53.617499   +70 11 45.91587   0.179
18 24  2.855269   +10 44 23.77392   0.180
 0  5 57.175409   +38 20 15.14857   0.190
 9 20 58.458480   +44 41 53.98502   0.190
 9 27  3.013906   +39  2 20.85196   0.194
 0 24 42.989850  -42  2  3.94978    0.200
18 42  8.989953   +79 46 17.12801   0.201
 9 58 47.245101   +65 33 54.81806   0.202
 9 14 37.913439   + 2 45 59.24629   0.213
10 48  6.620574  -19  9 35.72656    0.214
10 24 44.809595   +19 12 20.41526   0.215
 1 37 41.299454   +33  9 35.13378   0.217
 0 10 33.990619   +17 24 18.76135   0.220
 0 11  1.246752  -26 12 33.37686    0.229
 0 16 11.088555  -00 15 12.44534    0.231
 1 52 18.059047   +22  7  7.70004   0.232
 1 26 42.792631   +25 59  1.30079   0.233
 9  9 10.091601   + 1 21 35.61770   0.237
 9  2 16.830898  -14 15 30.87530    0.239
18  6 50.680653   +69 49 28.10859   0.240
10 13 53.428730   +24 49 16.44127   0.241
 9 27 51.824304  -20 34 51.23227    0.246
19 40 25.528136  -69  7 56.97209    0.246
 1 20 31.663311  -27  1 24.65133    0.247
10 17 25.887530   +61 16 27.49687   0.252
 0 17  8.474953   +81 35  8.13633   0.259
 9 58 19.671648   +47 25  7.84250   0.264
 1 53 10.121676  -33 10 25.86226    0.265
 0 58 46.581252  -56 59 11.47054    0.270
 0  4 35.655596  -47 36 19.60356    0.273
 1  2 45.762387   +58 24 11.13669   0.276
 0 22 32.441263   + 6  8  4.26943   0.277
10 41 17.162504   + 6 10 16.92378   0.277
 9  3  3.990103   +46 51  4.13753   0.278
10 22 32.798030  -10 37 44.38842    0.289
18 54 57.299946   +73 51 19.90747   0.289
 1 50  2.697551  -07 25 48.49067    0.290
18 49 16.072300   +67  5 41.67993   0.292
 9 48 55.338145   +40 39 44.58719   0.308
 0 59  5.514949   + 0  6 51.62203   0.310
10 44 23.062554   +80 54 39.44303   0.310
 9 21 36.231054   +62 15 52.18035   0.315
18  0 24.765360   +38 48 30.69771   0.324
19 30  6.160015  -60 56  9.18402    0.332
11  3 31.526414  -32 51 16.69195    0.333
 1 36 58.594810   +47 51 29.10006   0.336
 1 19 35.000523   +32 10 50.05410   0.344
 1 12  5.824718   +22 44 38.78619   0.351
 1 41 25.832025  -09 28 43.67381    0.352
 1 37 38.346378  -24 30 53.88526    0.354
 0 49 59.473091  -57 38 27.33992    0.369
 0 29 14.242502   +34 56 32.24702   0.391
 0 13 31.130213   +40 51 37.14407   0.396
10 37 16.079728  -29 34  2.81318    0.401
 1 21 56.861698   + 4 22 24.73438   0.404
10 58 11.535365   +81 14 32.67521   0.411
10 24 29.586611  -00 52 55.49786    0.415
10 14 47.065445   +23  1 16.57091   0.431
18 42 33.641636   +68  9 25.22788   0.432
 0 19 45.786433   +73 27 30.01751   0.441
 0  6 13.892887  -06 23 35.33485    0.449
18 37 28.714952  -71  8 43.55453    0.455
 9 56 49.875356   +25 15 16.04963   0.460
10 44 55.911265   + 6 55 38.26250   0.481
10 23 11.565623   +39 48 15.38539   0.496
10 35  7.040181   +56 28 46.79673   0.509
 0 38 24.843613   +41 37  6.00069   0.529
 9 22 46.418275  -39 59 35.06753    0.539
 1 49 22.370918   + 5 55 53.56852   0.540
10 33  3.707841   +41 16  6.23297   0.545
19 27 48.495214   +73 58  1.56997   0.634
 1 57 34.964908   +74 42 43.22998   0.695
18  9 57.871831  -45 52 41.01367    0.769
18  1 32.314854   +44  4 21.90031   0.771
 0 10 31.005888   +10 58 29.50412   0.781
18 19 35.002525  -63 45 48.19427    0.833
 1 21 41.595041   +11 49 50.41319   0.878
 9 30 32.571196  -85 33 59.69200    1.016
19 39 25.026661  -63 42 45.62554    1.100
19 25 59.605374   +21  6 26.16209   1.679
10  6 14.009559  -50 18 13.47089    2.211
10 51  4.777555  -31 38 14.30773    2.270

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