Optical design and polarimetric performance of a SmallSat UV polarimeter to study interstellar dust: PUFFINS

The Polarimetry in the Ultraviolet to Find Features in INterStellar dust (PUFFINS) is a SmallSat mission concept designed to obtain ultraviolet (UV) spectropolarimetric observations to probe the interstellar dust grain properties and to understand wavelength-dependent extinction and star formation. PUFFINS plans to observe 70 UV bright target stars at varying distances within a 1800-3200Å wavelength range with 0.02% polarimetric accuracy. PUFFINS uses a simple telescope design with all reflective optics coated with protected aluminum to enhance reflectivity in the UV. The telescope and the spectropolarimeter, which consists of a Wollaston prism and a half-wave retarder, have been carefully selected to be greater than Technology Readiness Level 6 (TRL6). The telescope is designed to exhibit negligible instrumental polarization and crosstalk, significantly reducing the time needed for polarimetric calibration in orbit. The optimum and careful selection of the target stars will enable PUFFINS to observe an expanded and well-defined sample to test the predictions by interstellar grain alignment theory in the observation phase of 9 months. This paper outlines the details of the optical and optomechanical design and evaluates the polarimetric performance of PUFFINS.