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Telescope and Adaptive Optics Induced Polarization

To investigate the ability of the telescope, the NFIRAOS adaptive optics system and a notional instrument to support future polarimetric observations, several studies have been carried out. The studies involved modeling of the induced polarization of the telescope optics using Zeemax and analytic models to check that different modeling methods yielded equivalent results. The results from Zeemax and analytic models were equivalent and this gave confidence that the Zeemax modeling of the NFIRAOS adaptive optics system was valid. All modeling was set up for the baseline TMT design, most importantly, the telescope optics coatings that follow the Gemini Protected Silver recipe.

Given the expected levels of induced polarization, the ability to calibrate the full system was considered. This involved observers that have extensive experience with a variety of different observatories with polarimetric instruments on their Nasmyth platforms, some fed by adaptive optics systems. The limits on the precision of polarimetric measurements were estimated as a function of wavelength and for different observing modes (imaging and spectropolarimetry), field position and science target position on the sky.

A large set of possible polarimetric observing programs (science cases) covering all science areas and possible observing modes was gathered. The required polarimetric precision for each observing program was compared against the estimated limits on the precision to assess whether a particular program is possible (assuming that a suitable instrument is developed in the future).

Technical requirements on telescope optical coatings are being explored based on the expected performance of the baseline coatings and the many science cases that are expected to be achievable.