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, millisecond 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
