Design drivers for the Polstar spectropolarimeter: an FUV/NUV design achieving high spectral resolving power with precise 4-Stokes measurements

Abstract

The Polstar NASA medium explorer (MIDEX) design configuration and implementation is strongly driven by the requirement to measure the state of polarization of stellar objects using a space-based sensor. Constraints include, but are not limited to, symmetry of geometry and coatings of the collecting aperture, angle of incidence at optical surfaces, coating uniformity, line of sight jitter and drift, orbit properties, thermal stability, and ground calibration. The Polstar MIDEX will observe scientifically interesting stars. Polstar will simultaneously measure all four Stokes parameters (I, Q, U, V) to high accuracy and precision (~0.001%) of the Stokes vector at high spectral resolving power. The 600-mm diameter aperture telescope images a selected star at the entrance slit of a spectrometer. Polstar offers two spectral channels within one spectrometer: a Far UV 122 nm to 200 nm Channel 1 with R~30K spectral resolving power and a low spectral resolution in Channel 2 channel covering 180 nm to 320 nm with R ~ 120 to 4K and spectroscopy over 115 nm to ~1,000nm. Channel 1 uses a cross-dispersed echelle spectrometer design. Channel 2 achieves its spectral dispersion with a MgF2 prism disperser. The two channels share a common array detector. The spectrometer includes rotating MgF2 retarders and a fixed MgF2 Wollaston prism analyzer to implement a dual beam polarization sensing function. Two orthogonal polarization states are imaged onto the array detector as interleaved echellograms (Channel 1) and as parallel spectra (Channel 2). This paper presents the design resulting from these design constraints and describes the approaches to calibrate the design pre-flight and during flight.