  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.
