The UCAC4, Tycho2, SAO, WDS, GCVS, and Bright star catalogs all list their stars with an apparent visual magnitude associated with each star. Over the years, this author has noted that these catalogs tend to disagree with one another about these magnitudes, despite the fact that most of them claim their magnitudes are good to around /100th of a magnitude.
The current study compiled all stars from these catalogs brighter than 13mv into a single list, sorted by right ascension.
The UCAC4 stars were also required to have an APASS mv.
Two runs were done, one with a half width of an arc second, and one with a half width of two arc seconds. Any star in this square of sky was added to one of 36 different lists, each representing a specific pair of catalogs.
For each star in the list (the index star), stars adjacent to it were scanned, and if they were within 1 or 2 arc seconds of the index star, their magnitude difference was calculated, counted, and stored.
From this information, the mean magnitude difference for each catalog pair was calculated, and from these means, the standard deviation was then calculated.
This information was then placed in tables. The top and left side rows of each table give the name of the catalog. The cell at any given intersection of these two rows consists of three numbers:
As one would expect, the numbers for each catalog pair are identical, irrespective of the catalog of the index star.
Some of these numbers, while showing a large deviation in magnitude, are not errors. The Tycho catalog, for instance, attempts to resolve binaries as close as 0.8", so a large magnitude disparity is to be expected.