  Astrometric positions of moving objects in the Solar System have been
measured using a variety of star catalogs in the past. Previous work has shown
that systematic errors in star catalogs can affect the accuracy of astrometric
observations. That, in turn, can influence the resulting orbit fits for minor
planets. In order to quantify these systematic errors, we compare the positions
and proper motion of stellar sources in the most utilized star catalogs to the
second release of the Gaia star catalog. The accuracy of Gaia astrometry allows
us to unambiguously identify local biases and derive a scheme that can be used
to correct past astrometric observations of solar system objects. Here we
provide a substantially improved debiasing scheme for 26 astrometric catalogs
that were extensively used in minor planet astrometry. Revised corrections near
the galactic center eliminate artifacts that could be traced back to reference
catalogs used in previous debiasing schemes. Median differences in stellar
positions between catalogs now tend to be on the order of several tens of
milliarcseconds (mas) but can be as large as 175 mas. Median stellar proper
motion corrections scatter around 0.3 mas/yr and range from 1 to 4 mas/yr for
star catalogs with and without proper motion, respectively. The tables in this
work are meant to be applied to existing optical observations. They are not
intended to correct new astrometric measurments as those should make use of the
Gaia astrometric catalog. Since previous debiasing schemes already reduced
systematics in past observations to a large extent, corrections beyond the
current work may not be needed in the foreseeable future.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics