Dual-Beam Optical Linear Polarimetry from Southern Skies. Characterisation of CasPol for high precision polarimetry

Abstract: We present a characterization of CasPol, a dual-beam polarimeter mounted at the 2.15 meter Jorge Sahade Telescope, located at the Complejo Astron\'omico El Leoncito, Argentina. The telescope is one of the few available meter-sized optical telescopes located in the Southern Hemisphere hosting a polarimeter. To carry out this work we collected photo-polarimetric data along five observing campaigns, the first one during January, 2014, and the remaining ones spread between August, 2017 and March, 2018. The data were taken through the Johnson-Cousins V, R and I filters. Along the campaigns, we observed eight unpolarized and four polarized standard stars. Our analysis begun characterizing the impact of seeing and aperture into the polarimetric measurements, defining an optimum aperture extraction and setting a clear limit for seeing conditions. Then, we used the unpolarized standard stars to characterize the level of instrumental polarization, and to assess the presence of polarization dependent on the position across the charge coupled-device. Polarized standard stars were investigated to quantify the stability of the instrument with wavelength. Specifically, we find that the overall instrumental polarization of CasPol is $\sim 0.2 \%$ in the V, R and I bands, with a negligible polarization dependence on the position of the stars on the detector. The stability of the half-wave plate retarder is about 0.35 degrees, making CasPol comparable to already existing instruments. We also provide new measurements in the three photometric bands for both the unpolarized and polarized standard stars. Finally, we show scientific results, illustrating the capabilities of CasPol for precision polarimetry of relatively faint objects.