Catalog Comparisons

Not all catalogs are created equal when it comes to photometry. So far the best ones are Tycho and UCAC2 (a 3 CD set, free from USNO), as the following graph illustrates.

 Figure x. Comparison of V-mag for 3 catalogs available in planetarium programs. Landolt magnitudes are the standard (truth).

This graph was constructed from a few stars chosen at 5 random locations (well, 5 Landolt area locations). Given that the Landolt magnitudes are a primary standard, biases and scatter are quite different for these three catalogs. Maximum errors of ~1.1 magnitude are possible for the much-maligned GSC, which was created as a guide star catalog for Hubble Space Telescope and doesn't claim to have good photometry. Now that the UCAC2 catalog is available (for free) on a 3-CD set, and considering that planetarium programs can be easily confgured to support this catalog, there's really no excuse for anyone serious about photometry to not upgrade for use of this catalog. This brief analysis suggests that an additional improvement can be had by adding ~0.28 mag to UCAC2 magnitudes for stars fainter than ~10.5. When this is done we can expect the following performance. The Tycho catalog is somewhat better, but it doesn't go any deeper than ~11.3 V-mag, where most stars are saturated for asteroid searches.

 Figure x. UCAC2 V-magnitudes versus the Landolt starndard V-magnitudes, after adding 0.28 mag to the UCAC2 stars fainter than 10.5.
 
The UCAC2 catalog is complete for declinations south of +40 degrees (some stars go as far north as +55 degrees). The USNO web site http://ad.usno.navy.mil/ucac/ modestly states that the photometry is poor, being 0.1 to 0.3 mag. For astrometrists, that's good photometry. To order the 3-CD set containing 48 million UCAC2 stars from USNO:  send a short e-mail message to nz@usno.navy.mil with "request UCAC2" in the subject line and give your mailing address in the main part of the message. Please use upper case letters for your name and address in a format directly usable for mailing lables.

To install it on your hard disk, and configure TheSky 6.0 to support it, follow the Help instructions for TheSky. If you have trouble, you may e-mail me.

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This site opened:  January 1, 2005.  Last Update:  January 2, 2005