The optical development and calibration of the Galway Astronomical Stokes Polarimeter (GASP) as a multi-detector system for the polarimetric observations of variable optical sources

Abstract

Polarised light from astronomical targets can yield a wealth of information about their source radiation mechanisms, and about the geometry of the scattered light regions. To date, stellar observations have focused, for the most part, on the linearly polarised component of the optical emission. Observations have been restricted due to inadequate instrumentation, and particularly the need for suitable observing conditions and the availability of luminous targets. The motivation in developing the Galway Astronomical Stokes Polarimeter (GASP) makes possible the ability to observe beyond these sources. GASP is based upon division of amplitude polarimeter (DOAP) (Compain and Drevillon, 1998), which measures the four components of the Stokes vector (I, Q, U and V) simultaneously. This work establishes a suitable optical design that develops GASP as an imaging polarimeter, enabling an acceptable FOV and pixelscale for a 5-m class telescope. It also focuses on two important aspects of using the GASP polarimeter - the calibration of GASP as an imaging polarimeter, and the application of GASP to astronomical targets. The Eigenvalue Calibration Method (ECM) is used to calibrate GASP by reducing systematic errors as a result of calibration optics. It includes all the optical elements of the polarimeter in the output, including the characteristics of the reference ECM samples. The results of the ECM using GASP as an 8-detector polarimeter give degree of linear polarisation to accuracies of 0.2%, and the polarisation angle is measured to within 0.1(o). The ECM was also implemented for GASP using imaging detectors. A number of data analysis methods were explored using image registration techniques; the calibration was performed on a pixel-by-pixel basis. Comparable polarimetric errors, when imaging, were found to those using an 8 detector system, with results of spatial and temporal calibration stability also presented.

In November 2012, GASP was allocated observing time on the Hale 200 inch telescope on Palomar Mountain to test the instrument's limitations, and make observations of the Crab pulsar whose linearly polarised emission has been studied extensively. The results of the calibrated data are presented; which were then applied to observational data, performing Stokes reduction and reconstruction of polarimetric information. The results of this observing campaign find the ECM to be a very powerful technique, where GASP was shown to measure observational polarisation accurately, in particular 0.1(o) on the PA. The findings of the calibration also show that, when imaging, GASP is sensitive to field position; evidence of this is found in observational results of Zenith flat-field when applying a pixel-by-pixel analysis technique. Science results of the Crab Pulsar and Nebula are presented. The first set of results produce polarimetric information for the pulsar and Trimble 28 using a set of integrated full frame data. These results are compared to what has been found in the literature. GASP also observed the pulsar at a frame rate > 1000 frames per second in order to perform phase resolved polarimetry. The findings of these results are presented in Chapter 7.