Calibration of EFOSC2 broadband linear imaging polarimetry

TL;DR

This paper details the calibration process for EFOSC2's broadband imaging polarimetry mode, achieving a calibration uncertainty of about 0.1%, which enhances future and archival polarimetric observations.

Contribution

It provides a comprehensive calibration method for EFOSC2's broadband polarimetry, with publicly available code, improving data quality and scientific potential.

Findings

Calibration uncertainty ~0.1% achieved

Calibration codes made publicly available

Enables improved and archival polarimetric data analysis

Abstract

EFOSC2 (the European Southern Observatory Faint Object Spectrograph and Camera v2) is one of the workhorse instruments on ESO's New Technology Telescope (NTT), and is one of the most popular instruments at La Silla observatory. It is mounted at a Nasmyth focus, and therefore exhibits strong, wavelength and pointing-direction dependent instrumental polarisation. In this document we describe our efforts to calibrate the broadband imaging polarimetry mode, and provide a calibration for broadband B, V, R filters to a level that satisfies most use cases (i.e. polarimetric calibration uncertainty ~0.1%). We make our calibration codes public. This calibration effort can be used to enhance the yield of future polarimetric programmes with EFOSC2, by allowing good calibration with a greatly reduced number of standard star observations. Similarly, our calibration model can be combined with archival calibration observations to post-process data taken in past years, to form a EFOSC2 legacy archive with substantial scientific potential.