The WALOP-North Instrument I: Optical Design, Filter Design, Calibration

TL;DR

The WALOP-North instrument is a high-precision, wide-field optical polarimeter designed for the PASIPHAE survey, featuring a novel filter design, four-channel CCD detection, and calibration methods to measure stellar polarization accurately.

Contribution

This paper introduces the optical and filter design of WALOP-North, a pioneering wide-field polarimeter with high accuracy and novel calibration techniques for astronomical surveys.

Findings

Achieves 0.1% polarization accuracy and 0.05% sensitivity.

Features a large 30x30 arcmin^2 field-of-view with four CCD channels.

Presents a novel filter design suitable for high-precision polarimetry.

Abstract

The Wide Area Linear Optical Polarimeter North (WALOP-North) is an optical polarimeter designed for the needs of the PASIPHAE survey. It will be installed on the 1.3m telescope at the Skinakas Observatory in Crete, Greece. After commissioning, it will measure the polarization of millions of stars at high Galactic latitude, aiming to measure hundreds of stars per $deg^2$. The astronomical filter used in the instrument is a modified, polarimetrically-neutral broadband SDSS-r. This instrument will be pioneering one due to its large field-of-view (FoV) of $30\times 30$ $arcmin^2$ and high accuracy polarimetry measurements. The accuracy and sensitivity of the instrument in polarization fraction will be at the 0.1\% and 0.05\% level, respectively. Four separate 4k$\times$4k CCDs will be used as the instrument detectors, each imaging one of the $0\deg{}, 45\deg{}, 90\deg{}$ and $135\deg{}$ polarized FoV separately, therefore making the instrument a four-channel, one-shot polarimeter. Here, we present the overall optical design of the instrument, emphasizing on the aspects of the instrument that are different from WALOP-South. We also present a novel design of filters appropriate for polarimetry along with details on the management of the instrument size and its polarimetric calibration.