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HIPPI-2: A Versatile High Precision Polarimeter

We describe the High-Precision Polarimetric Instrument-2 (HIPPI-2) a highly versatile stellar polarimeter developed at the University of New South Wales (UNSW). Two copies of HIPPI-2 have been built and used on the 60-cm telescope at Western Sydney University's (WSU) Penrith Observatory, the 8.1-m Gemini North Telescope at Mauna Kea and extensively on the 3.9-m Anglo-Australian Telescope (AAT). The precision of polarimetry, measured from repeat observations of bright stars in the SDSS g' band, is better than 3.5 ppm (parts per million) on the 3.9-m AAT and better than 11 ppm on the 60-cm WSU telescope. The precision is better at redder wavelengths and poorer in the blue. On the Gemini North 8-m telescope the performance is limited by a very large and strongly wavelength dependent telescope polarization that reached 1000's of ppm at blue wavelengths and is much larger than we have seen on any other telescope.