Note: this software is currently very much alpha software!
Polaris is a prototype for the polarization reduction step for VSTPol.
VSTPOL is the polarization variant of the VLT (Very Large Telescope) Survey Telescope, a 2.6 meter survey telescope located at the Paranal VLT site in Chile. Attached to it is the 1-square degree OmegaCAM instrument, consistent of 32 CCDs with a total of 268 million (16K times 16K) pixels. A rotatable polarization filter is placed in a spare slot in the light pathway, and the combination of different rotations allows for measuring the various I, Q, U and V Stokes parameters of the polarization of imaged sources.
The pipeline handles the input images for different rotations, combining them such that the Stokes parameters are retrieved, through demodulation of the input frames. Since these frames are not obtained simultaneously, but in sequence after each other, corrections are then applied, to handle shifts and differences in the point spread functions between the frames. This is largely done through use of stars in the field, which for a large fraction are taken to be unpolarized. Selection of these stars, from the GAIA catalog, can strengthen this assumption, for example by using stars relatively in the Solar neighbourhood, which avoids polarization due to gas and dust; and by selecting stellar types that are known to be little to un-polarized.
Installation is straightforward. Create a Python virtual environment and activate it. Then, from the downloaded or cloned project, run
pip install .
or install it from its GitHub repository directly
pip install git+https://github.com/evertrol/polaris.git
to install Polaris in that virtual environment.
To install a development environment, use
pip install -e '.[dev]' .
This will install some of the packages used for development (formatters, linters etc).
Documentation can be find inside the docs directory.
Polaris is copyright 2025 University of Amsterdam, and licensed under the MIT license. See LICENSE for the details.