As more and more optical images were matched to catalogs, the question of the accuracy of the positions of objects in the catalogs arose. We set out to compare how well various catalogs fit a large set of images.
|Year||Catalog||Number of Sources||Reference|
|1989||HST Guide Star Catalog (GSC I)||25,541,952||Lasker et al. 1990|
|1996||USNO-A1.0 Catalog||488,006,860||Monet 1996|
|1998||USNO-A2.0 Catalog||526,280,881||Monet 1998|
|2001||GSC II Catalog (2.2.01)||998,402,801||McLean et al. 2000|
|2002||USNO-B1.0 Catalog||1,036,366,767||Monet et al. 2003|
|2003||2MASS Point Source Catalog||470,992,970||Cutri et al. 2003|
|2003||USNO UCAC2 Catalog||48,366,996||Zacharias et al. 2000|
As part of the CfA Century Survey of galaxies (Geller et al. 1997), 1728 15 by 30 arcminute CCD images of a portion of the northern sky over the north galactic pole were taken as 216 exposures by the 8-detector, one-degree-square MOSAIC camera (Muller et al. 1998) on the KPNO 0.9 m telescope in 1998 December and 1999 January and processed as described in Brown et al. 2001. A correction was made for distortion across the wide field and a world coordinate system was fit to objects in the images found by SExtractor (Bertin & Arnouts 1998) using WCSTools and the GSC-I catalog. The resulting image catalogs, with image coordinates and approximate right ascension and declination, became the raw data for our study.
The uniformity of the images and the fact that they cover a portion of the sky well away from the dense star fields of the galactic plane made them ideal for automatic star matching. Unix shell scripts written for each catalog set up an initial FITS header for each of the 1728 images with the center being the mean position of the objects found in that image.
The WCSTools imwcs program was then run on each image. The IMWCS program fits the same number of brightest catalog objects and brightest image objects limited by whichever there were fewer of; with these wide field images, the number of catalog objects in the field was usually the limit. The IMWCS program fit all eight parameters of the FITS WCS tangent plane projection to all of the catalog-image matches in the field. The program made three additional iterations per image following an intial fit. The second fit used the refined parameters which might have changed the position and size of the catalog section to be matched. In the two final passes, the tolerance in the catalog-image match was reduced by half each time to eliminate both bad matches and objects whose catalog positions did not match their actual positions. The goodness of a fit for an image is judged by the mean radial offset between the position of the objects in the image mapped to sky coordinates through the fit world coordinate system and the catalog position of the closest object, which is almost always within one arcsecond.
The means of the individual image offsets were used to compare how well each catalog matched the sky as captured by our 1728 CCD images. The GSC-I was used as a baseline, despite the fact that it matched 25 or more stars in only 353 of the 1728 images. Figure 1 shows the distribution of mean Catalog-Image positions in arcseconds. Table 2 shows how many catalog stars were found in each images as a range and an average, how many catalog stars were fit to image stars, as a range and an average for each catalog and the range of mean (Observed-Catalog) radial offsets per image, and the mean and standard deviation of that mean for the entire data set.
|Catalog||Catalog Stars||Matches fit||Image-Catalog (arcsec)|
The GSC-I based on plates from the 1980's does better than the USNO-A2.0 which is based on plates from the 1950's, probably due to the motions of stars in the intervening years, though the shorter exposures of the GSC-I may also have given better centers. The more recent GSC-II, 2MASS PSC, and USNO-B1.0 catalogs all are based on the Tycho-2 astrometric reference catalog (Høg, et al. 2000), and give similar results. When the USNO-B1.0 gave worse results than expected, it was filtered by the number of plates (POSS I red and blue, POSS II red and blue, and N) on which the object was found. Thus the most recent catalogs all cluster around 02 mean offset. Only 303 images were fit to the recently-released UCAC2 catalog which covers our field, but it is incomplete, so the automatic matching algorithm does not work perfectly. The mean offset was 01, tightly clustered as the standard deviation and Figure 1 show, half that of the other catalogs. This shows that detector nonlinearity is not an issue above 01, at least for these CCDs, and that there is room for improvement in the astrometry of current deep all sky catalogs.
This project makes use of several publicly available catalogs. The 2MASS PSC is from the Two Micron All Sky Survey, a joint project of the University of Massachusetts and IPAC at Caltech, funded by NASA and the NSF. The GSC-II is a joint project of the Space Telescope Science Institute, operated by AURA for NASA under contract NAS5-26555 and the Osservatorio Astronomico di Torino, which is supported by the Italian Council for Research in Astronomy, with additional support is provided by ESO, ST-ECF, the International GEMINI project, and ESA's Astrophysics Division. The GSC-I was produced at the Space Telescope Science Institute under U.S. Government grants based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The USNO A and B-1.0 catalogs were provided by Dave Monet and Steve Levine of the U.S. Naval Observatory at Flagstaff on 21 CDROMs and one 128-gigabyte disk drive, respectively. A beta version of the UCAC2 and a good discussion of limiting factors in CCD astrometry were kindly provided by Norbert Zacharias of the U.S. Naval Observatory at Flagstaff.
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