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Polstar combines, for the first time, the complementary benefits of spectroscopy and polarimetry to probe the complex interface between massive stars and the interstellar medium. Furthermore, it leverages an innovative combination of effective area and time coverage, to reach the diversity of targets necessary to transform our understanding of the ecology of star and planet creation. Detailed knowledge of these bright, yet distant objects, is crucial for understanding the transformation of our galaxy, from the barren landscape of the early Big Bang, into the chemically enriched environment that produced the solar system we call home. Polstar will map stellar wind and magnetospheric structures by uniting time domain, polarimetry and spectroscopy capability in the near- and far-UV (NUV and FUV), which are densely populated with high-opacity resonance lines encoding a rich array of diagnostic information. UV spectropolarimetry is equally important for probing interstellar dust and protoplanetary disks. The instrument combines advances in high reflectivity UV coatings and delta-doped CCDs with high quantum efficiencies to provide dedicated FUV spectropolarimetry for the first time in 25 years. The FUV channel (Ch1), covers 122-200nm at resolution R>30k, while the NUV channel (Ch2) covers 122-320nm at R~140-4,000. The instrumental polarization stability is built to provide signal-to-noise ratios (SNR) for UV polarimetry precision of 1x10-3 per exposure per resolution element (resel). Precision can be further improved with spectral binning and/or stacking multiple exposures. Polstar spectral resolution in Ch1 is >30x better than WUPPE, with 10x better effective area, while reaching shorter wavelength than WUPPE to access strong lines of species like NIV and SiIV. The 3-year mission of Polstar is 100x longer than WUPPE with orders of magnitude gains in stellar and interstellar observations.
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