The 10 CD-ROM set which comprises the US Naval Observatory sky catalogue (version 1.0) is now available at both Epping and Siding Spring. The catalogue contains the accurate positions (±0.25 arcsec) and magnitudes (±0.25-0.40 mag) for more than 4.8x109 sources over the whole sky. It was compiled by David Monet at the USNO using the Precision Measuring Machine.
The catalogue is based on scans of POSS-I O and E plates for declinations north of -35 degrees and on SERC-J survey and ESO-R plates at more southerly declinations. The catalogue reaches J~23, R~22, O~21 and E~20. The catalogue may be accessed by running the starnet program on the current Solaris machines; aaossz at Epping and aatssz at Siding Spring.
We have independently verified the accuracy of the positions by comparing the positions of bright stars (14<J<15), faint galaxies (J<20) and faint QSO candidates (18<J<21) selected from APM measurements of UKST J plates. The objects were selected from two strips in declination 10h<RA<15h at Dec=0 degrees and 22h<RA<4h at Dec=-30 degrees.
A histogram of the difference between USNO and APM positions is largely gaussian. For all three classes of object, the rms lies between 0.6 and 0.7 arcsec, larger than would be expected for the claimed error of 0.25 arcsec in each catalogue.
Systematic differences in astrometry are small and independent of object type. There is a shift in the objects' mean absolute position of 0.2 arcsec RA (USNO positions west of the APM positions) in the equatorial strip. In the -30 degree strip the shift is slightly greater, with the USNO positions 0.6 arcsec north of APM positions.
We have not addressed the questions of completeness or photometry and further work is still required to explain fully the differences in astrometry.
Robert Smith and Brian Boyle