  In a previous paper, we applied the Gaia DR2 catalog to improve the
astrometric accuracy of about 1.7 billion objects in Pan-STARRS1 Data Release 2
(PS1 DR2). We report here on further improvements made by utilizing Gaia EDR3
and correcting for the effects of differential chromatic refraction (DCR) in
declination. We extend the correction algorithm in Paper 1 by iteratively
subtracting color- and declination-dependent PS1/Gaia EDR3 declination
residuals. We determine the astrometric improvement for ~440 million reference
objects that are point-like and cross-match to Gaia EDR3. For this set of
objects, Gaia EDR3 provides a ~3% improvement in PS1 astrometry over Gaia DR2,
and DCR corrections provide an additional ~5% improvement. DCR corrections
increase substantially for objects observed away from the zenith. DCR
corrections lead to an astrometric improvement of ~30% for blue objects
(0<g-i<1) that are 50{\deg} away from the zenith. The amplitude of systematic
astrometric errors from these effects is substantially reduced to less than 1
mas for objects with PS1 colors in the range 0 < g-i < 4.5, which makes this a
useful astrometric reference catalog in fields where there are few Gaia stars.
The improved astrometric data will be available through the Mikulski Archive
for Space Telescopes PS1 catalog interfaces.
