Characterization of Instrumental Polarization in the MOPTOP Instrument

Abstract

MOPTOP is a polarimeter currently installed on the fully robotic 2 m Liverpool Telescope, designed to allow accurate measurements of the variability of the polarization of astronomical objects like gamma-ray bursts and blazars. In this work, we present the characterization of the field position dependent instrumental polarization added by MOPTOP to the acquired images. This characterization is done by the acquisition of images of low polarized standard stars, for different positions over the CCD frame, for the filters BVRIL. These images were reduced using the MOPTOP pipeline, providing us with the q and u Stokes parameters. These values, in turn, were subtracted from the q and u values of the respective star, found in literature, resulting in the spatial distribution of the residual polarization added by the instrument. A plane was fitted to the obtained result, allowing us to calculate the q and u values added by MOPTOP.