Calibration Approach for the Polstar Spectropolarimeter

The Polstar Mission uses time domain FUV and NUV spectropolarimetric observations to provide insight into how our galaxy became able to support a solar system like ours. Polstar will study the evolution of massive stars, including the inter-stellar dust and gas that they interact with and the dynamics of protoplanetary disks and lower resolution near UV (180nm – 320 nm) to study interstellar reddening mechanism. Polstar will simultaneously measure at high spectral resolving power all four Stokes parameters (I, Q, U, V)^T to high accuracy (0.001) with a precision better than 0.0001 using an instrument comprising a modulator with rotating MgF2 retarders and a fixed MgF2 Wollaston prism analyzer that concurrently measures the two orthogonal polarizations of the retarder modulated signal at six optimized rotational angles on a common detector. The output of the Wollaston prism either directly enters an echelle spectrograph, which provides spectral resolution of ~30K with a wavelength range from 122 nm to 200 nm, or is reflected to a prism spectrograph, which provides spectral resolution ranging from 140–4000 with a wavelength range from 180 nm to 320 nm. In this paper we outline the instrument calibration approach to obtain the Mueller matrices over the full wavelength range that permit us to determine the Stokes parameters, and to spectrally calibrate the instrument. We also describe the ground support equipment for calibration.