| Abstract : | Polar-Areas Stellar-Imaging in Polarization High-Accuracy Experiment (PASIPHAE)
is an optopolarimetric survey aiming to measure the polarization of millions of stars, hence creating a three-dimensional tomographic map of the magnetic field within the Milky Way. Wide Areas Linear Optical Polarimeter (WALOP) will be used as the polarimeter in this experiment. It has a large field of view (30′ × 30′), and it can measure the Stokes parameters simultaneously. WALOP targets to achieve 0.15% polarimetric accuracy in the SDSS-r filter band. To achieve this much accuracy in polarization measurement, we need to adopt an accurate photometry method. The most conventional and easiest method for photometry is aperture photometry, which performs well for bright stars (magnitude 12), but for faint stars (magnitude 16), we need to follow the better alternative way of photometry, the Point Spread Function (PSF) photometry. PSF photometry is a two-dimensional fitting of a star image using a PSF. If the PSF is known, we can measure the photon count very accurately, minimizing the function, defined as (image − flux × PSF)2. The main challenging thing to do is to construct PSF. There are many methods in literature like 2D Gaussian function, Moffat function, Zernike polynomials, principle component analysis (PCA) to construct the PSF from a star image. Here, we present a robust method to model the PSF from multiple images of bright stars and use it to do photometry for faint stars to achieve the desired accuracy goal of the survey. |
