Improving source positions in the OCARS catalog: A first approach

TL;DR

This paper presents a novel method to unify and improve the positional accuracy of radio sources in the OCARS catalog by aligning it with the ICRF3 reference frame, significantly reducing systematic errors.

Contribution

First attempt to create a unified, ICRF3-aligned set of radio source coordinates in the OCARS catalog, improving positional consistency.

Findings

Systematic errors in OCARS positions were almost completely eliminated.

New OCARS positions show high alignment with ICRF3.

Method effectively reduces inhomogeneity in source coordinates.

Abstract

OCARS (Optical Characteristics of Astrometric Radio Sources) is a compiled catalog of various additional data associated with astrometric radio sources whose coordinates have been determined from very long baseline interferometry (VLBI) observations. It contains source coordinates, object type, redshift, optical and near-infrared magnitudes. Until now, OCARS source coordinates were simply copied from input catalogs and, as a result, were systematically inhomogeneous. This work is the first attempt to obtain a unified set of radio source coordinates aligned to the International Celestial Reference Frame (ICRF), more specically to the third ICRF release, ICRF3. Comparison of the source coordinates in the old OCARS version as of December 2024 and the new OCARS version as of March 2025 with the ICRF3-SX catalog using the vector spherical harmonics (VSH) technique showed almost complete elimination of systematic errors in new OCARS positions relative to the ICRF3 frame.