Variable Star Polarimetry with PICSARR-2

Abstract

We describe the upgraded Polarimeter using Imaging CMOS Sensor and Rotating Retarder 2 (PICSARR-2), describe its applications, and characterize its performance for stellar polarimetry on a 36-inch and 14-inch telescope. On the larger telescope in the SDSS $g^{\prime }$, $r^{\prime }$ and $i^{\prime }$ filters a precision of ${\sigma }_p=$ 5.7 ppm on bright stars is recorded using a fast modulation rate corresponding to frame exposures of 12 ms; accounting for the internal errors in the individual observations gives a limiting precision of $e_p=$ 1.3 ppm. Longer frame rates are required for stars with $m>5$, but the recorded errors only underperform a photon shot noise derived extrapolation for $m>8$, when even longer frame exposures are required. Stars as faint as $m=11$ were observed. The position angle precision is measured as 0.0845 degrees, and there is very good agreement between observations made by both the PICSARR-2 and HIPPI-2 polarimeters. On the smaller telescope the instrument's performance approaches similar levels, and there is good cross-platform stability in the observation of standard stars. PICSARR-2 is therefore an excellent instrument to explore stellar variability due to a range of phenomena; examples from our ongoing pulsating star campaign and other variable star programs are presented.

27 pages, 19 figures, accepted to JAAVSO