GASP - Galway Astronomical Stokes Polarimeter

The Galway Astronomical Stokes Polarimeter (GASP) is an ultra-high-speed, full Stokes, astronomical imaging polarimeter based upon a Division of Amplitude Polarimeter. It has been developed to resolve extremely rapid stochastic (∼ms) variations in objects such as optical pulsars, RRATs and magnetic cataclysmic variables. GASP has no moving parts or modulated components, so the complete Stokes vector can be measured from just one exposure making it unique to astronomy. Furthermore the time required for the determination of the full Stokes vector is limited only by the time resolution of the detectors used and the incident photon fluxes. GASP utilizes a modified Fresnel rhomb, which acts as a highly achromatic quarter wave plate and a beamsplitter (referred to as an RBS). Here we present a description of how the DOAP works, some of the optical designs for the polarimeter, and give some preliminary results. Calibration is an important, and difficult issue with all polarimeters, but particularly in astronomical polarimeters. We give a description of calibration techniques appropriate to this type of polarimeter, particularly the Eigenvalue Calibration Method of Compain & Drevillon