The advent of large catalogs with more accurate positions enables the WCSTools
IMWCS program to fit more accurate world coordinate systems to images. There
has always been a question as to how the error in matching images to source
catalogs was split between the uncertainties of the catalog positions, the
uncertainties of the image centroids, nonlinearities in detector and telescope
optics, and nonlinearities in the actual detector chip.
Figure 1 shows an image of a field centered on a random galaxy with sources from the GSC II Catalog plotted over it based on a fit WCS.
Figure 2 compares the results of world coordinate system fits to the field based on sources from the USNO-A2.0, GSC II, 2MASS Point Source, USNO-SA2.0, UJ, GSC-ACT, and HST GSC Catalogs. For each catalog, the first column indicates the number of stars matched between the catalog and the image, the second column shows how many catalog stars were found in the image (with an upper limit of 300), and the third column indicates the mean separation between the image and catalog positions for the matched stars after the WCS fit in units of arcseconds times 1000 so that the bars all fit on the same scale.
Note that the recent epoch catalogs fit better than the USNO-A2.0 catalog, which is based on the 1950's Palomar Sky Survey. Further tests are being undertaken to discover exactly what the position errors really are, but this simple test shows that the better the astrometry in a source catalog, the better it will match an image.
Figure 1 (Click for bigger graph)
Figure 2 (Click for bigger graph)