Design, Construction and Calibration Of The Galway Astronomical Stokes Polarimeter (GASP)

Abstract

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 (~microsecond) variations in objects such as optical pulsars and magnetic CVs. The polarimeter has no moving parts or modulated components, so the complete Stokes vector can be measured from just one exposure -making it unique to astronomy -and the time required for the complete determination of the Stokes vector is limited only by detectors and photon fluxes. The polarimeter utilizes a modified Fresnel rhomb, which acts as a highly achromatic quarter wave plate and a beamsplitter (referred to as an RBS). We present a detailed description of the design process for the RBS and other optics of the polarimeter, and give a theoretical analysis of its expected performance. Calibration is an important, and difficult, issue with all polarimeters, but particularly in astronomical polarimeters. We give a detailed description of calibration techniques appropriate to this type of polarimeter -particularly a version of the Eigenvalue Calibration Method of Compain & Drevillon. ASTRONOMICAL IMAGING POLARIMETRYPolarimetry is a powerful diagnostic tool in astrophysics enabling scattering, source asymmetries, magnetic field configurations, and longitudinal magnetic field strengths, for example, to be investigated. High precision and high speed in astronomical polarimetry is difficult to achieve but such polarimetry would be beneficial for the examination of optical pulsars and magnetic CVs in timescales shorter than they have been observed to date.Currently, the commonest technique used in astronomical imaging polarimeters employs a half waveplate (HWP), then a polarizing beamsplitter, to produce two beams, the intensities of which are measured [1]. The HWP is then rotated in steps of 22.5° and the intensities re-measured. A data reduction process can then give I, Q and U of the Stokes vector (as in Eq.1, where I n is the intensity of the polarization state and T denotes the transpose). If the HWP is replaced by a quarter waveplate (QWP), V can be measured.

Characterization of beam splitters in the calibration of a six-channel Stokes polarimeter

“…With the advantages of high temporal resolution, high accuracy and good stability, the Stokes polarimeter (SP) has been widely used in many areas, such as astronomy [10][11][12], remote sensing [13,14] and material characterization [15,16]. Among the existing techniques, the SP based on the spatial division of amplitude is a typical configuration, which employs multiple NPBSs and PBSs in the optical paths [17].…”

Fast and optimal broad-band Stokes/Mueller polarimeter design by the use of a genetic algorithm

“…Polarimeters are applied in a wide range of fields, from astronomy [1,2,3], remote sensing [4] and medical diagnostics [5,6] to applications in ellipsometry such as characterizing gratings [7], nanostructures [8] and rough surfaces [9,10,11]. As all polarimeters are based on inverting so-called system matrices, it is well known that the measurement error from independent Gaussian noise is minimized when the condition number (κ) of these system matrices is minimized [12,13].…”