Polarimetry and PSFs: Behind the Scenes with PUNCH Data Analysis
The Polarimeter to UNify the Corona and Heliosphere is scheduled to launch in Spring 2025. PUNCH will image the outer corona and inner heliosphere to view them as elements of a single unified system, complementing the work of Parker Solar Probe. PUNCH science makes use of the physics of Thomson scattering to extract 3D information about the corona. This requires sensitive photometry and polarimetry, and merging of data from four separate cameras into a single “virtual focal plane”, which in turn drive rigorous treatment of the images from each camera, including corrections for distortion and PSF effects; I’ll give an overview of unusual aspects of the image calibration pipeline to address these needs. Further, PUNCH will report both “B” and “pB” (“polarized brightness”) — but that second term is somewhat ill-defined in the literature. I’ll describe the two major forms of pB currently in use, why PUNCH chose the more Stokes-like version, how PUNCH handles pB calculation and background subtraction, and an unexpected reason why three-polarizer coronagraphs give robust results. I will also highlight a surprising (to me) and helpful analogy between representations of linear polarization and of color, to further build intuition about polarimetry and its measurement.
Craig DeForest got his start in studying plasma physics in the mid 1980s, working at the D3D tokamak facility in San Diego. Rumours that he was bit by a radioactive lizard there and now fights crime by night are greatly exaggerated. He holds a 1989 Bachelor’s Degree from Reed College and a 1995 doctorate from Stanford University, and over the past 25 years has made a career of developing novel instrumentation and analysis techniques. He chairs the AAS/SPD and leads the PUNCH mission. He’s here today to talk about some of the techniques used by PUNCH to merge data from four separate cameras into polarimetric, background-subtracted data products.