paper.md

title	tags	authors	affiliations	date	bibliography
Katsu: A Python package for Mueller and Stokes simulation and polarimetry	Python astronomy polarimetry laboratory mueller stokes	name orcid corresponding affiliation Jaren N. Ashcraft 0000-0001-5082-7442 true 1, 2, 3 name orcid affiliation Ewan S. Douglas 0000-0002-0813-4308 2 name orcid affiliation William Melby 0000-0001-5178-689X 3 name orcid affiliation Manxuan Zhang 0000-0003-3567-6839 3 name affiliation Kenji Mulhall 4 name orcid affiliation Ramya M. Anche 0000-0002-4989-6253 2 name affiliation orcid Emory Jenkins 1, 2 0000-0001-6829-7910 name orcid affiliation Maxwell A. Millar-Blanchaer 0000-0001-6205-9233 3	name index Wyant College of Optical Sciences, University of Arizona, USA 1 name index Steward Observatory, University of Arizona, USA 2 name index Department of Physics, University of California, Santa Barbara, USA 3 name index Independent contributor 4	16 August 2024	paper.bib

Summary

Katsu is an open-source Python package to address the need for polarimetric characterization of astronomical systems for the next generation of astronomy. It contains simple routines for the simulation of Mueller matrices and Stokes vectors to model how polarization is transformed by optical systems. One area where Katsu is distinct from other excellent packages capable of Mueller calculus simulation (e.g., Pypol [@pypol]) is its emphasis on broadcasted matrix calculations. All Mueller matrices available in katsu.mueller take a shape keword that append an arbitrary number of axes to the front of the initialized array, with the final two axes containing the Mueller matrix. This functionality is critical for accelerated computating on spatial data, which enables the direct measurement of polarization aberrations in the lab [@ashcraft2024]. Katsu also features a polarimetry module containing the data reduction routines for single-rotating retarder (SRR) Stokes polarimeters and dual-rotating retarder (DRR) Mueller polarimeters. In the pursuit of open-source instrumentation in the laboratory, Katsu supports an interface to the Newport Agilis series rotation mounts in the motion module to assist with performing polarimetry with rotating retarders.

To interpret the measurements made in the lab, Katsu features the polar decomposition of Mueller matrices published by Lu and Chipman [@Lu:96]. This decomposes a Mueller matrix $\mathbf{M}$ into its constituent depolarizer $\mathbf{M}{\Delta}$, diattenuator $\mathbf{M{D}}$, and retarder $\mathbf{M_{R}}$, as shown in the following Equation,

$$\mathbf{M} = \mathbf{M}{\Delta}\mathbf{M{R}}\mathbf{M_{D}}. $$

This function is critical for separating depolarization from the Mueller matrix, which enables the integration of polarization aberration in the laboratory into simulated data (generated by a polarization ray tracer, e.g. Poke [@Ashcraft_2023], Pyastropol [@Pruthvi.2020]). Katsu also adopts the flexible interchangeable backend system of prysm [@Dube2019;@Dube:22] for hot-swappable numpy-like backends [@harris2020array] including cupy for GPU-accelerated computing [@cupy_learningsys2017] and jax for automatic differentiation [@jax2018github].

Statement of need

Polarimetric characterization in the laboratory is critical to the performance of astronomical instrumentation. The next generation of astronomical observatories has identified that wavefront errors induced by polarization, called polarization aberrations, could be a limiting factor in direct exoplanet imaging. Ample modeling has been done at the observatory level to understand the nominal polarization aberrations that arise in the telescope [@anche2023polarization;@gaudi2020habitable;@Will_polarization_luvoir] but less work has been done to characterize instrumentation in the laboratory.

Katsu has recently been used as the primary backend of the Gromit polarimeter at the UA Space Astrophysics Laboratory [@gromit], and used to measure the spatially-varying polarization aberrations of the Space Coronagraph Optical Bench (SCoOB) [@ashcraft2024]. The measurement and polarimetric data reduction capabilities available in Katsu enabled expeditious full Mueller polarimetry of an astronomical coronagraph testbed. In the future, we aim to add more polarimetric data reduction capabilities to Katsu, like a recently-published modification of DRRP data reduction to leverage insights from dual-channel polarimetry [@melby2024] for passive insensitivity to variations in total intensity.

Acknowledgements

This work was sponsored by a NASA Space Technology Graduate Research Opportunity. J.N.A. acknowledges support by NASA through the NASA Hubble Fellowship grant #HST-HF2-51547.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy.

References