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Astronomical Data Analysis Software and Systems XIII ASP Conference Series, Vol. 314, 2004 F. Ochsenbein, M. Al len, and D. Egret, eds.

A Comparison of Large All-Sky Catalogs
D. J. Mink, W. R. Brown, M. J. Kurtz Smithsonian Astrophysical Observatory, Cambridge, MA 02138 Abstract. Recent large catalogs have revolutionized our ability to do astrometry with CCD images. The recently published FITS World Coordinate System standard has provided a standard way of parameterizing that astrometry, and the WCSTools and SExtractor software packages allow the automation of the "plate-fitting" process. We have amassed 1728 15 в 30 CCD images of a portion of the northern sky. After matching image point sources to ob jects in each of the catalogs and fitting world coordinate systems to them using the IMWCS program, we find mean residuals between observed and catalog star positions of between 0. 09 and 0. 25 for the latest catalogs.

1.

Introduction

In the last few years, several motivations for acquiring sub-arcsecond astrometry of faint astronomical sources have arisen. Surveys and studies of specific ob jects at radio and X-ray wavelengths require exact optical or infrared positions to identify optical counterparts. Small aperture spectroscopes such as the 300-fiber MMT Hectospec require input positions better than a half arcsecond. The usual method of acquiring positions for faint, uncatalogued ob jects is to match the brighter stars in a CCD image to one of the deep catalogs which have been developed over the past 20 years by the Space Telescope Science Institute, the U.S. Naval Observatory, and the 2 Micron All Sky Survey. Table 1 shows the history of those large catalogs. At the same time, standards and software for associating image pixels with sky positions as world coordinate systems have been developed, culminating in two papers (Greisen & Calabretta 2002, Calabretta & Greisen 2002) and a software package which utilizes Calabretta's WCSLIB with real images (Mink 1997, Mink 1999, Mink 2002). As more and more optical images were matched to catalogs, the question of the accuracy of the positions of ob jects in the catalogs arose. We set out to compare how well various catalogs fit a large set of images. 2. Data and Analysis fA Century Survey o images of a portion as 216 exposures by et al. 1998) on the f galaxies (Geller et al. 1997), 1728 15 by 30 of the northern sky over the north galactic the 8-detector, one-degree-square MOSAIC KPNO 0.9 m telescope in 1998 December 141 c Copyright 2004 Astronomical Society of the Pacific. All rights reserved.

As part of the C arcminute CCD pole were taken camera (Muller


142 Table 1.
Year 1989 1996 1998 2001 2002 2003 2003

Mink, Brown, & Kurtz Growing Astronomical Catalogs
Numb er of Sources 25,541,952 488,006,860 526,280,881 998,402,801 1,036,366,767 470,992,970 48,366,996 Reference Lasker et al. 1990 Monet 1996 Monet 1998 McLean et al. 2000 Monet et al. 2003 Cutri et al. 2003 Zacharias et al. 2000

Catalog HST Guide Star Catalog (GSC I) USNO-A1.0 Catalog USNO-A2.0 Catalog GSC I I Catalog (2.2.01) USNO-B1.0 Catalog 2MASS Point Source Catalog USNO UCAC2 Catalog

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 ob jects 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 ob jects found in that image. The WCSTools imwcs program was then run on each image. The IMWCS program fits the same number of brightest catalog ob jects and brightest image ob jects limited by whichever there were fewer of; with these wide field images, the number of catalog ob jects in the field was usually the limit. The IMWCS program fit all eight parameters of the FITS WCS tangent plane pro jection 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 ob jects 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 ob jects in the image mapped to sky coordinates through the fit world coordinate system and the catalog position of the closest ob ject, which is almost always within one arcsecond. 3. Results

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 CatalogImage 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.


A Comparison of Large All-Sky Catalogs

143

?38h

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Figure 1.

Distribution of image-catalog radial offsets.

Table 2.
Catalog

Fits of Various Catalogs to 1728 Images.
Catalog Range 26-61 119-454 90-752 93-365 136-1125 136-957 136-654 109-409 40-72 Stars Mean 35.78 258.52 226.73 171.06 586.50 528.38 365.13 227.89 51.52 Match Range 25-64 78-353 87-378 86-331 51-661 62-653 116-555 98-375 40-69 es fit Mean 34.16 189.10 179.16 153.26 418.47 382.16 309.98 205.14 48.80 Image-Catalog (arcsec) Range Mean(Sigma) 0.168-0.654 0.321(0.085) 0.301-0.457 0.368(0.024) 0.123-0.343 0.196(0.043) 0.154-0.347 0.220(0.024) 0.182-0.523 0.267(0.030) 0.181-0.479 0.251(0.029) 0.155-0.430 0.223(0.023) 0.136-0.326 0.192(0.021) 0.054-0.159 0.091(0.015)

GSC-I USNO-A2.0 GSC-I I 2MASS PSC B1.0/id=2 B1.0/id=3 B1.0/id=4 B1.0/id=5 UCAC2

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 (HЬg, 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 ob ject was found. Thus the most recent catalogs all cluster around 0. 2 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 0. 1, 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


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Mink, Brown, & Kurtz

0. 1, at least for these CCDs, and that there is room for improvement in the astrometry of current deep all sky catalogs. Acknowledgments. This pro ject makes use of several publicly available catalogs. The 2MASS PSC is from the Two Micron All Sky Survey, a joint pro ject of the University of Massachusetts and IPAC at Caltech, funded by NASA and the NSF. The GSC-II is a joint pro ject 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, STECF, the International GEMINI pro ject, 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. References Bertin, E. & Arnouts, S. 1998, A&AS117, 393 Brown, W. R. et al. 2001, AJ, 122, 714 Calabretta, M. R. & Greisen, E. W. 2002, A&A, 395, 1077 Cutri, et al. 2003, VizieR On-line Data Catalog: II/246. Geller, M. J., et al. 1997, AJ, 114, 2205 Greisen, E. & Calabretta, M. 2002, A&A, 395, 1061 HЬg, E., et al. 2000, A&A, 355, L27 Lasker, B. M., Sturch, C. R., McLean, B. J., Russell, J. L., Jenkner, H., & Shara, M. M. 1990, AJ, 99, 2019 McLean, B. J., Greene, G. R., Lattanzi, M. G., Pirenne, B. 2000, in ASP Conf. Ser., Vol. 216, ADASS IX, ed. N. Manset, C. Veillet, & D. Crabtree (San Francisco: ASP), 145 Mink, D. J. 1997, in ASP Conf. Ser., Vol. 125, ADASS VI, ed. G. Hunt & H. E. Payne (San Francisco: ASP), 249 Mink, D. J. 1999, in ASP Conf. Ser., Vol. 172, ADASS VIII, ed. D. M. Mehringer, R. L. Plante, & D. A. Roberts (San Francisco: ASP), 498 Mink, D. J. 2002, in ASP Conf. Ser., Vol. 281, ADASS XI, ed. D. A. Bohlender, D. Durand, & T. H. Handley (San Francisco: ASP), 169 Monet, D. G. 1996, BAAS, 28, 905 (abstract) Monet, D. G. 1998, BAAS, 30, 1427 (abstract) Monet, D. G. et al. 2003, AJ, 125, 984 Muller, G. P., Reed, R., Armandroff, T., Boroson, T. A., & Jacoby, G. H. 1998, Proc. SPIE, 3355, 577 Zacharias, N., et al. 2000, AJ, 120, 2131