Ultraviolet astronomical polarimetry: Some results and prospects

The new territory of ultraviolet imaging polarimetry is being explored at the University of Wisconsin Space Astronomy Lab by a sounding rocket payload, the Wide-Field Imaging Survey Polarimeter (`WISP'). WISP uses an 8-inch F/1.9 all-reflective Schmidt telescope, a large rotatable stressed CaF<sub>2</sub> waveplate, and a fixed polarizing mirror coated with an opaque monolayer of ZrO<sub>2</sub>, illuminated at the Brewster angle. The payload has flown successfully four times, targeting the Pleiades Reflection Nebula, the Large Magellanic Cloud, comet Hale-Bopp, and the diffuse light of our Galaxy. An improved payload in the concept stage is Cosmic Ultraviolet Polarimetric Imaging Device (`CUPID'), which would use a 20-inch Gregorian Paul-Baker telescope, WISP-style polarimetric optics, and multilayer reflective coatings to achieve wide-field imaging polarimetry with more than 15 times the sensitivity of WISP. It is to be used to separate the Galactic and Cosmological components of the ultraviolet diffuse background using their polarimetric properties. High resolution stellar spectropolarimetry in the far ultraviolet is to be pioneered by a sounding rocket payload in the design stage, the Far-Ultraviolet Spectropolarimeter (`FUSP'). This instrument is to have a spectral resolution of 0.07 nm and a spectral coverage from 105 to 145 nm. It uses a 20-inch telescope with polarimetric optics at the prime focus, and a far ultraviolet spectrometer using an aberration-corrected holographic grating. The polarimetric analyzer will be a thin LiF stressed waveplate, followed by a Brewster-angle polarizer of natural diamond. Scientific goals include diagnosis of the geometry and magnetic fields in stellar envelopes via resonance scattering and the Hanle Effect.