RoboPol: a four-channel optical imaging polarimeter

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 ′ × 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 • in polarization angle.

Eliminating artefacts in polarimetric images using deep learning

“…In the RoboPol data-set, we had majority of artefacts due to scattering of light from off-axis stars at the interface of the Wollaston prisms (Ramaprakash et al 2019). Our method demonstrates that a binary classifier trained on images of stars and artefacts can successfully differentiate between them.…”

“…Each star appears at four locations due to splitting of light from a single source within the instrument, with the locations lying at the vertices of a diamond. A detailed design implementation is available in Ramaprakash et al (2019).…”

“…Unlike typical astronomy images, RoboPol images contain a mask ( Figure 1). More details about the mask can be found in Ramaprakash et al (2019). The code repository and documentation are available at https://github.com/deltapapa/Robopol-artifacts…”

“…RoboPol images contain reflections of bright stars due to the interface between the 2 Wollaston Prisms used in the instrument and it is this dominant class of artefacts that we target here. The interface between the Wollaston Prisms is shown in the diagram of the optical instrument design described in Ramaprakash et al (2019), and an example of the reflection artefact in Figure 1. The green box in the upper left quadrant shows two horizontally extended artefacts separated vertically.…”

The RoboPol Program: Optical Polarimetric Monitoring of Blazars

“…A more detailed description of the polarimeter can be found in Ramaprakash et al (2019) [34], while the pipeline and the control system are presented in detail in King et al (2014) [35].…”