Design drivers for the PolStar telescope and UV spectropolarimeter instrument

The Polstar Mission seeks to study the evolution of massive stars including their effect on the interstellar medium and their behavior in binary systems using a 60 cm telescope with a UV Spectropolarimeter within MIDEX mission constraints on cost cap, throughput, coating requirements, and system-level dimensional stability. The mission is in a high-earth orbit and must ensure precise and repeatable polarimetric observations. Design-to-cost paradigms are exercised throughout all design phases and heritage approaches to structure and mirrors are evoked. In terms of classical error budgets, designing for diffraction-limited performance at 1.2 μm is sufficient, however, there are special design concerns at these wavelengths which require maximizing throughput of photons. Special coatings and minimum reflections are mandatory with meticulous attention to cleanliness throughout the entire mission life cycle. Decontamination heaters must be employed shortly after launch, prior to opening the door, and periodically throughout the mission lifetime. Additionally, spectropolarimetry requirements impose constraints on symmetry and control of phase and amplitude. The secondary mirror must have adjustment capability in three degrees of freedom (tip, tilt, and focus) to address drifts from thermal perturbations, aging, and possibly even spacecraft jitter. We present in-process design approach and analyses to meet the challenges of ultraviolet wavelengths and polarization stability..