# RoboPol: A four-channel optical imaging polarimeter

## TL;DR

RoboPol is a four-channel optical polarimeter designed for multi-wavelength imaging polarimetry, capable of measuring linear polarization of point sources with high accuracy and stability over five years of operation.

## Contribution

The paper introduces RoboPol's design, performance, and stability, highlighting its ability to measure polarization in multiple bands simultaneously with minimal systematic errors.

## Key findings

- Achieves systematic uncertainty below 0.1% in fractional polarization
- Maintains instrumental polarization variation within 0.1% in p and 1° in angle
- Operates effectively over five years with consistent performance

## Abstract

We present the design and performance of RoboPol, a four-channel optical polarimeter operating at the Skinakas Observatory in Crete, Greece. RoboPol is capable of measuring both relative linear Stokes parameters $q$ and $u$ (and the total intensity $I$) in one sky exposure. Though primarily used to measure the polarization of point sources in the R-band, the instrument features additional filters (B, V and I), enabling multi-wavelength imaging polarimetry over a large field of view (13.6' $\times$ 13.6'). We demonstrate the accuracy and stability of the instrument throughout its five years of operation. Best performance is achieved within the central region of the field of view and in the R band. For such measurements the systematic uncertainty is below 0.1% in fractional linear polarization, $p$ (0.05% maximum likelihood). Throughout all observing seasons the instrumental polarization varies within 0.1% in $p$ and within 1$^\circ$ in polarization angle.