HIPPI-2: A versatile high-precision polarimeter

Abstract

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.

The rotation of α Oph investigated using polarimetry

“…Between September 2016 and April 2019 we also made 29 new high-precision polarimetric observations of α Oph in seven photometric bands with a variety of instrument and telescope configurations. The bulk of the observations were made using HIPPI (HIgh Precision Polarimetric Instrument, Bailey et al 2015) and its successor HIPPI-2 (Bailey et al 2020) at the 3.9-m Anglo-Australian Telescope (AAT) at Siding Spring Observatory. Two observations were made at the Gemini North telescope on Mauna Kea with HIPPI-2.…”

“…A single observation was made at UNSW Observatory in Syd- ney with a 35-cm Celestron C14 telescope using Mini-HIPPI (Bailey et al 2017). The standard operating procedures for each instrument were followed, with the reduction procedures described for HIPPI-2 by Bailey et al (2020) used to process all of the data.…”

“…For the HIPPI-2 observations our standard filter set, described in Bailey et al (2020), was used; briefly, this includes 425-and 500-nm short-pass filters (425SP, 500SP), SDSS g , Johnson V, SDSS r and a 650 nm long-pass (650LP) filter. With HIPPI, Omega Optics versions of the SDSS g and r Notes: * Indicates use of a 2× negative achromatic lens, effectively making the foci f/16.…”

“…With HIPPI, Omega Optics versions of the SDSS g and r Notes: * Indicates use of a 2× negative achromatic lens, effectively making the foci f/16. a A full description along with transmission curves for all the components and modulation characterisation of each modulator in the specified performance era can be found in Bailey et al (2020).…”

“…Exact values are given in Table 2 for each observation. c The observations used to determine the telescope polarization (TP) for the Mini-HIPPI run are described in Bailey et al (2017), those for HIPPI-2 up until 2019MAR are described in Bailey et al (2020); TPs for the 2019APR run are made up of, V: 3× HD 49815, 2× HD 102647; r : 3× HD 49815, 1× HD 102647; and for the 2017AUG HIPPI run, 425SP: 2× HD 2151, 3× HD 48915, 2× HD 102647, 2× HD 102870; 500SP: 2× HD 2151, 3× HD 48915, 2× HD 102647, 2× HD 102870; g : 2× HD 2151, 2× HD 48915, 2× HD 102647, 2× HD 102870; r : 3× HD 2151, 3× HD 48915, 2× HD 102647, 2× HD 102870; 650LP: 3× HD 2151, 3× HD 48915, 1× HD 102870. d The high-polarization standards observed to calibrate position angle, and the values of σ P A for the HIPPI-2 runs up until 2019MAR are given in Bailey et al (2020), and those for the Mini-HIPPI run are given in Bailey et al (2017); for the 2019APR run the standards observed in g were: HD 80558, 2× HD 147084, HD 111613, HD 187929 giving σ P A = 0.27 • , based on a smaller number of observations (one each of HD 147084 and HD 187929 in each band), corrections were made to different bands as follows: V -2.145 • , r 1.575 • ; for the 2017AUG HIPPI run the g observations were: HD 147084, HD 154445, HD 187929 giving σ P A = 0.53 • . e Dates given are for observations of α Oph and/or control stars.…”