UCAC3 pixel processing

The third US Naval Observatory (USNO) CCD Astrograph Catalog, UCAC3 was released at the IAU General Assembly on 2009 August 10. It is a highly accurate, all-sky astrometric catalog of about 100 million stars in the R = 8 to 16 magnitude range. Recent epoch observations are based on over 270,000 CCD exposures, which have been re-processed for the UCAC3 release applying traditional and new techniques. Challenges in the data have been high dark current and asymmetric image profiles due to the poor charge transfer efficiency of the detector. Non-Gaussian image profile functions were explored and correlations are found for profile fit parameters with properties of the CCD frames. These were utilized to constrain the image profile fit models and adequately describe the observed point-spread function of stellar images with a minimum number of free parameters. Using an appropriate model function, blended images of double stars could be fit successfully. UCAC3 positions are derived from 2-dimensional image profile fits with a 5-parameter, symmetric Lorentz profile model. Internal precisions of about 5 mas per coordinate and single exposure are found, which are degraded by the atmosphere to about 10 mas. However, systematic errors exceeding 100 mas are present in the x,y-data which have been corrected in the astrometric reductions following the x,y-data reduction step described here.Comment: 16 pages, 10 figures, 1 table, accepted by A

Astrometric quality of the USNO CCD Astrograph (UCA)

The US Naval Observatory 8--inch astrograph has been equipped with a Kodak 1536x1024 pixel CCD since June 1995, operating in a 570--650 nm bandpass. With 3--minute exposures well exposed images are obtained in the magnitude range $R \approx 8.5 - 13.5^{m}$. An astrometric precision of 10 to 15 mas for those stars is estimated from frame--to--frame comparisons. External comparisons reveal an accuracy of about 15 mas for those stars in a 20' field of view. For fainter stars, the error budget is dominated by the S/N ratio, reaching $\approx 100$ mas at $R=16^{m}$ under good observing conditions.Comment: Astronomical Journal accepted (May 97), 19 pages, AASTeX Latex, tables and figures from anonymous ftp crux.usno.navy.mil /pub/nz

Photometric observations of selected, optically bright quasars for Space Interferometry Mission and other future celestial reference frames

Photometric observations of 235 extragalactic objects that are potential targets for the Space Interferometry Mission (SIM) are presented. Mean B, V, R, I magnitudes at the 5% level are obtained at 1 - 4 epochs between 2005 and 2007 using the 1-m telescopes at Cerro Tololo Inter-American Observatory and Naval Observatory Flagstaff Station. Of the 134 sources which have V magnitudes in the Veron & Veron-Cetty catalog a difference of over 1.0 mag is found for the observed-catalog magnitudes for about 36% of the common sources, and 10 sources show over 3 mag difference. Our first set of observations presented here form the basis of a long-term photometric variability study of the selected reference frame sources to assist in mission target selection and to support in general QSO multi-color photometric variability studies.Comment: 40 pages, 13 figures, 4 table