WALOP-South: A Four Camera One Shot Imaging Polarimeter for PASIPHAE Survey. Paper I -- Optical Design

TL;DR

WALOP-South is an optical polarimeter designed for the PASIPHAE survey, capable of capturing polarization data in a single exposure over a wide field, with innovative optical design to correct aberrations.

Contribution

The paper presents the optical design of WALOP-South, addressing challenges in correcting dispersion and aberrations to achieve high-quality imaging for polarization measurements.

Findings

Achieves 0.05% polarimetric sensitivity across 35x35 arcmin field

Capable of measuring Stokes I, q, u in a single exposure

Delivers near seeing-limited PSFs for the entire field

Abstract

The WALOP-South instrument will be mounted on the 1 m SAAO telescope in South Africa as part of the PASIPHAE program to carry out a linear imaging polarization survey of the Galactic polar regions in the optical band. Designed to achieve polarimetric sensitivity of $0.05~\%$ across a $35\times35$ arcminute field of view, it will be capable of measuring the Stokes parameters I, q and u in a single exposure in the SDSS-r broadband and narrowband filters between $0.5~{\mu}m - 0.7~{\mu}m$. For each measurement, four images of the full field corresponding to linear polarization angles of 0 deg, 45 deg, 90 deg and 135 deg in the instrument coordinate system will be created on four detectors from which the Stokes parameters can be found using differential photometry. In designing the optical system, major challenges included correcting for the dispersion introduced by large split angle Wollaston Prisms used as analysers as well as other aberrations from the entire field to obtain imaging quality PSF at the detector. We present the optical design of the WALOP-South instrument which overcomes these challenges and delivers near seeing limited PSFs for the entire field of view.