UCAC3: Astrometric Reductions

TL;DR

This paper details the astrometric reduction process for UCAC3, correcting systematic errors using extensive CCD data and 2MASS, resulting in improved positional accuracy aligned with the ICRF.

Contribution

It introduces new reduction techniques and systematic error corrections for UCAC3, enhancing positional accuracy over previous catalogs.

Findings

Errors up to 200 mas corrected using look-up tables

Systematic errors reduced compared to previous catalogs

Final positions aligned with ICRF using Tycho-2 data

Abstract

Presented here are the details of the astrometric reductions from the x,y data to mean Right Ascension (RA), Declination (Dec) coordinates of the third U.S. Naval Observatory (USNO) CCD Astrograph Catalog (UCAC3). For these new reductions we used over 216,000 CCD exposures. The Two-Micron All-Sky Survey (2MASS) data are extensively used to probe for coordinate and coma-like systematic errors in UCAC data mainly caused by the poor charge transfer efficiency (CTE) of the 4K CCD. Errors up to about 200 mas have been corrected using complex look-up tables handling multiple dependencies derived from the residuals. Similarly, field distortions and sub-pixel phase errors have also been evaluated using the residuals with respect to 2MASS. The overall magnitude equation is derived from UCAC calibration field observations alone, independent of external catalogs. Systematic errors of positions at UCAC observing epoch as presented in UCAC3 are better corrected than in the previous catalogs for most stars. The Tycho-2 catalog is used to obtain final positions on the International Celestial Reference Frame (ICRF). Residuals of the Tycho-2 reference stars show a small magnitude equation (depending on declination zone) that might be inherent in the Tycho-2 catalog.