The photometric accuracy of various electronic catalogues which can be used with software such as Astrometrica or AIP4WIN is somewhat is variable. In order of decreasing accuracy they are; USNO A2.0, USNO B1.0, UCAC 2 and the GSC.
2.0 USNO-B1.0 catalogue
The USNO-B catalogue is the successor of the USNO-A2.0 catalogue and includes positions, proper motions and magnitudes for more than a billion objects.
2.2 Accessing the USNO-B1.0 Star Catalogue
Due to its size, 80 Gigabytes, the catalogue is not generally available on CD or DVD, but may be accessed at the USNO website. On accessing the catalogue one is presented with a form to complete.
To obtain a star chart and star list complete the following boxes; RA, Dec, Size of rectangle (make this a little larger than your CCD FOV). Select ‘USNO B1.0’ in ‘Catalogue(s) to Extract Data From’ and ‘All Surveys’ in ‘Image Data’ boxes. Tick ‘Catalogue Lists’ and ‘Finder Charts’. Select the appropriate magnitude to match your image (too many stars can be confusing) in the ‘Min’ and ‘Max’ ‘Magnitude’ boxes. Finally click on ‘Retrieve Data’ and the list and chart should be returned in under a minute. The chart is not a necessity but is recommended so that it can be compared with the CCD image(s) as a check that the correct data has been obtained.
The results appear in a window at the top of the screen. To view the list click on ‘USNO B1.0 Star List’, to view the chart click on ‘Star Chart’.
2.3 Astrometrica settings
In ‘File/Settings’ select the path to the Star List in ‘Directories/Star Catalogue’ in ‘Environment’ and ‘USNO B 1.0’ in the ‘Star Catalogue’ box in ‘Program’. Astrometrica (from version 18.104.22.1684) will then connect automatically to the internet when astrometry is performed in the usual way. An example of an image with overlays and a line from the MPC report generated are shown below.
00403 C2000 10 16.82836 22 47 03.71 +05 00 04.1 13.5 V 940
3.0 USNO UCAC2 catalogue
This catalogue, which can be used with the latest versions of Astrometrica, is now available from the United States Naval Observatory on three CD-ROM’s.
In the September 2003 edition of ‘The Astronomer’ Peter Birtwhistle pointed out that his magnitude estimates using the UCAC catalogue were approximately 0.5 mag fainter than estimates using either the USNO SA2.0 or B1.0 catalogues.
In the same issue