Astrometric Redshifts for Quasars
  The wavelength dependence of atmospheric refraction causes differential
chromatic refraction (DCR), whereby objects imaged at different optical/UV
wavelengths are observed at slightly different positions in the plane of the
detector. Strong spectral features induce changes in the effective wavelengths
of broad-band filters that are capable of producing significant positional
offsets with respect to standard DCR corrections. We examine such offsets for
broad-emission-line (type 1) quasars from the Sloan Digital Sky Survey (SDSS)
spanning 0<z<5 and an airmass range of 1.0 to 1.8. These offsets are in good
agreement with those predicted by convolving a composite quasar spectrum with
the SDSS bandpasses as a function of redshift and airmass. This astrometric
information can be used to break degeneracies in photometric redshifts of
quasars (or other emission-line sources) and, for extreme cases, may be
suitable for determining "astrometric redshifts". On the SDSS's southern
equatorial stripe, where it is possible to average many multi-epoch
measurements, more than 60% of quasars have emission-line-induced astrometric
offsets larger than the SDSS's relative astrometric errors of 25-35 mas.
Folding these astrometric offsets into photometric redshift estimates yields an
improvement of 9% within Delta z+/-0.1. Future multi-epoch synoptic surveys
such as LSST and Pan-STARRS could benefit from intentionally making ~10
observations at relatively high airmass (AM~1.4) in order to improve their
photometric redshifts for quasars.
