  The POLISH2 optical polarimeter has been in operation at the Lick Observatory
3-m Shane telescope since 2011, and it was commissioned at the Gemini North 8-m
in 2016. This instrument primarily targets exoplanets, asteroids, and the Crab
pulsar, but it has also been used for a wide variety of planetary, galactic,
and supernova science. POLISH2's photoelastic modulators, employed instead of
rotating waveplates or ferro-electric liquid crystal modulators, offer the
unprecedented ability to achieve sensitivity and accuracy of order 1 ppm
(0.0001%), which are difficult to obtain with conventional polarimeters.
Additionally, POLISH2 simultaneously measures intensity (Stokes I), linear
polarization (Stokes Q and U), and circular polarization (Stokes V), which
fully describe the polarization state of incident light. We document our
laboratory and on-sky calibration methodology, our archival on-sky database,
and we demonstrate conclusive detection of circular polarization of certain
objects.
