Investigating the pupil dependence of the polarimetric sensitivity on SALT

Danièl Groenewald; Kenneth Nordsieck

doi:10.1117/12.3017792

18 July 2024 Investigating the pupil dependence of the polarimetric sensitivity on SALT

Proceedings Volume 13096, Ground-based and Airborne Instrumentation for Astronomy X; 130962Q (2024) https://doi.org/10.1117/12.3017792
Event: SPIE Astronomical Telescopes + Instrumentation, 2024, Yokohama, Japan

Conference Poster

Abstract

The Robert Stobie Spectrograph (RSS) is an imaging spectrograph designed to occupy the Prime Focus Instrument Package of the Southern African Large Telescope (SALT). Grating spectropolarimetry, 3200 - 9000 ˚A, with resolution R up to 10 000, is always available, using rotating half and quarter waveplates near the telescope focal plane and a calcite mosaic beamsplitter in the collimated space. This paper describes improvements made to the polarimetric calibration to correct for an unexpected time-dependent position angle ‘ripple’ vs wavelength which has been seen in high-precision RSS observations, and which appears to be due to variations in illumination of the pupil during an observation. A new calibration technique, which explicitly models the pupil dependence of the polarimetric sensitivity, has been developed. The net spectral polarization sensitivity vs field of view position X, Y , and wavelength λ is computed as the polarization pupil dependence, weighted by the pupil intensity illumination of the observation. We find that the polarization pupil dependence, degree P and position angle θ are adequately represented by a polynomial function of position in the pupil. A pupil intensity illumination model for SALT has been devised, calibrated by twilight observations through a pinhole slitmask grid, observed out of focus. The polarization pupil dependence model has been calibrated using observations of the full Moon through a similar mask, plus a polaroid. The final calibration has been checked using repeated observations of known highly polarized standard stars.

(2024) Published by SPIE. Downloading of the abstract is permitted for personal use only.

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Danièl Groenewald and Kenneth Nordsieck "Investigating the pupil dependence of the polarimetric sensitivity on SALT", Proc. SPIE 13096, Ground-based and Airborne Instrumentation for Astronomy X, 130962Q (18 July 2024); https://doi.org/10.1117/12.3017792

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