A Global Astrometric Solution for Pan-STARRS referenced to ICRF2

TL;DR

This paper presents a new global astrometric solution for Pan-STARRS1 data that reduces catalog-to-catalog errors by referencing to ICRF2, improving absolute astrometry using a least squares adjustment with extragalactic objects.

Contribution

The paper introduces a novel global astrometric method that directly ties Pan-STARRS1 data to ICRF2, avoiding propagation of catalog errors and providing more accurate absolute positions.

Findings

Systematic errors can be effectively corrected.

Residual position errors are around 60 mas, larger than expected.

Small-scale residual errors are not fully corrected, with explanations discussed.

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-to-catalog 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.