Bright Star Astrometry with URAT

TL;DR

URAT is conducting a comprehensive astrometric survey of the northern sky since 2012, achieving milliarcsecond precision and revealing discrepancies in Hipparcos star positions, thus enhancing stellar position data.

Contribution

This paper presents the URAT survey's methodology, data, and its findings on star position accuracy, providing updated astrometric data and identifying significant discrepancies with Hipparcos.

Findings

Positions of 66,202 Hipparcos stars measured with 8-20 mas accuracy.

Approximately 20% of Hipparcos stars show position discrepancies of 75 mas or more.

Some stars are observed with offsets up to an arcsecond from Hipparcos predictions.

Abstract

The U.S. Naval Observatory Robotic Astrometric Telescope (URAT) is observing the northern sky since April 2012 for an astrometric survey. Multiple overlaps per year are performed in a single bandpass (680$-$750 nm) using the "redlens" 20 cm aperture astrograph and a mosaic of large CCDs. Besides the regular, deep survey to magnitude 18.5, short exposures with an objective grating are taken to access stars as bright as 3rd magnitude. A brief overview of the program, observing and reductions is given. Positions on the 8 to 20 mas level are obtained of 66,202 Hipparcos stars at current epochs. These are compared to the Hipparcos Catalog to investigate its accuracy. About 20\% of the observed Hipparcos stars are found to have inconsitent positions with the Hipparcos Catalog prediction on the 3 sigma level or over (about 75 mas or more discrepant position offsets). Some stars are now seen at an arcsec (or 25 sigma) off their Hipparcos Catalog predicted position.