A Global Astrometric Solution for Pan-Starrs Referenced to Icrf2

Abstract

We describe development and application of a Global Astrometric Solution (GAS) to the problem of Pan-STARRS1 (PS1) astrometry. Current PS1 astrometry is based on differential astrometric measurements using 2MASS reference stars, thus PS1 astrometry inherits the errors of the 2MASS catalog. The GAS, based on a single, least squares adjustment to approximately 750k "grid stars" using over 3000 extragalactic objects as reference objects, avoids this catalog-tocatalog propagation of errors to a great extent. The GAS uses a relatively small number of Quasi-Stellar Objects (QSOs, or distant AGN) with very accurate (<1 mas) radio positions, referenced to the ICRF2. These QSOs provide a hard constraint in the global least squares adjustment. Solving such a system provides absolute astrometry for all the stars simultaneously. The concept is much cleaner than conventional astrometry but is not easy to perform for large catalogs. In this paper we describe our method and its application to Pan-STARRS1 data. We show that large-scale systematic errors are easily corrected but our solution residuals for position (∼60 mas) are still larger than expected based on simulations (∼10 mas). We provide a likely explanation for the reason the small-scale residual errors are not corrected in our solution as would be expected.

The Precious Set of Radio-optical Reference Frame Objects in the Light of Gaia DR2 Data

“…Up until the advent of Gaia data, the bulk of optical counterparts of RORF objects were considered to provide high-quality absolute reference. Berghea et al (2016) used the VLBI-measured radio positions from the Optical Characteristics of Astrometric Radio Sources (OCARS) compilation (Malkin 2013;Titov & Malkin 2009;Malkin 2016) as hard constraints for a global astrometric adjustment of the Pan-STARRS catalog and investigated the deviant cases in the process. They determined that a non-negligible fraction of the VLBI sources (∼ 10%) have mismatching optical positions in Pan-STARRS beyond the statistical expectation.…”

Astrometric Evidence for a Population of Dislodged AGNs

“…The ultimate source of this very substantial additional noise in up-to-date astrometric catalogs is probably the Astrographic Catalog 2000 (Urban et al 1998), which is burdened with small-scale zonal errors because of the insufficient number of Hipparcos reference stars. More recently, Berghea et al (2016) used the VLBI-measured radio positions of a larger sample of sources from the OCARS compilation (Malkin 2013;Titov & Malkin 2009) as hard constraints for a global astrometric adjustment of the Pan-STARRS catalog and investigated the deviant cases in the process. They determined that a non-negligible fraction of the VLBI sources (∼ 10%) have mismatching optical positions in Pan-STARRS beyond the statistical expectation.…”

Connecting VLBI and Gaia Celestial Reference Frames

“…Moreover, the structure effects are often variable. Although many studies are devoted to this factor, see, Fey and Charlot (1997);da Silva Neto et al (2002); Moór et al (2011); Bouffet et al (2013); Zacharias and Zacharias (2014); Berghea et al (2016) and papers cited therein, there are insufficient data to quantify the impact of source structure on the accuracy of the orientation angles between optical and radio frames. Evidently, the most complete study is provided by Zacharias and Zacharias (2014) used in the current work.…”