WALOP-South: A wide-field one-shot linear optical polarimeter for PASIPHAE survey

WALOP (Wide-Area Linear Optical Polarimeter)-South, to be mounted on the 1m SAAO telescope in South Africa, is first of the two WALOP instruments currently under development for carrying out the PASIPHAE survey. Scheduled for commissioning in the year 2021, the WALOP instruments will be used to measure the linear polarization of around $10^{6}$ stars in the SDSS-r broadband with $0.1~\%$ polarimetric accuracy, covering 4000 square degrees in the Galactic polar regions. The combined capabilities of one-shot linear polarimetry, high polarimetric accuracy ($< 0.1~\%$) and polarimetric sensitivity ($< 0.05~\%$), and a large field of view (FOV) of $35\times35~arcminutes$ make WALOP-South a unique astronomical instrument. In a single exposure, it is designed to measure the Stokes parameters $I$, $q$ and $u$ in the SDSS-r broadband and narrowband filters between $500-700~nm$. During each measurement, four images of the full field corresponding to the polarization angles of $0^{\circ}$, $45^{\circ}$, $90^{\circ}$ and $135^{\circ}$ will be imaged on four detectors and carrying out differential photometry on these images will yield the Stokes parameters. Major challenges in designing WALOP-South instrument include- (a) in the optical design, correcting for the spectral dispersion introduced by large split angle Wollaston Prisms used as polarization analyzers as well as aberrations from the wide field, and (b) making an optomechanical design adherent to the tolerances required to obtain good imaging and polarimetric performance under all temperature conditions as well as telescope pointing positions. We present the optical and optomechanical design for WALOP-South which overcomes these challenges.