Inferring Telescope Polarization Properties Through Spectral Lines Without Linear Polarization

Derks, Alysa

Undergraduate Honors Thesis

Inferring Telescope Polarization Properties Through Spectral Lines Without Linear Polarization Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/fb494872c

Abstract

Polarimetric observations taken with ground- or space-based telescopes usually need to be corrected for changes of the polarization state induced in the optical path. In this paper, I present a new technique to determine the polarization properties of a telescope through observations of spectral lines that have no or negligible intrinsic linear polarization signals. For such spectral lines, any observed linear polarization must be induced by the telescope optics. I apply the technique to observations taken with the Spectropolarimeter for Infrared and Optical Regions (SPINOR) at the Dunn Solar Telescope (DST) and demonstrate that I can retrieve the characteristic polarization properties of the DST at three selected wavelengths of 459, 526 and 615 nm. I determine the amount of cross-talk between the intensity, Stokes I, and the linear and circular polarization states, Stokes Q, U and V , and between Stokes V and Stokes Q and U in spectropolarimetric observations of active regions on the solar surface. I fit a set of parameters that describe the polarization properties of the DST to the observed cross-talk values. I compare my results to parameters that were derived using a conventional telescope calibration unit (TCU). The values for the ratio of reflectivities X = rs/rp and the retardance τ of the DST turret mirrors from the analysis of the cross-talk match those derived with the TCU within the error bars. I find a negligible contribution of retardance from the entrance and exit windows of the evacuated part of the DST. Residual cross-talk after applying a correction for the telescope polarization stays at a level of 3-10% regardless of which parameter set is used, but with an rms fluctuation in the input data of already a few percent. The accuracy in the determination of the telescope properties is thus more limited by the quality of the input data than the method itself. It is possible to derive the parameters that describe the polarization properties of a telescope from observations of spectral lines without intrinsic linear polarization signal. Such spectral lines have a dense coverage (about every 50 nm) in the visible part of the spectrum (400–615 nm), but none were found at longer wavelengths. Using spectral lines without intrinsic linear polarization is a promising tool for the polarimetric calibration of current or future solar telescopes such as the Daniel K. Inouye Solar Telescope (DKIST).

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