Inferring Telescope Polarization Properties Through Spectral Lines Without Linear Polarization

We present a 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. We apply the technique to observations taken with SPINOR at the Dunn Solar Telescope (DST) and demonstrate that we can retrieve the characteristic polarization properties of the DST at three wavelengths of 459, 526, and 615 nm. The values for the ratio of reflectivities X = rs/rp and the retardance τ of the DST turret mirrors from the analysis of the crosstalk match those derived with the TCU within the error bars.

We conclude that 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 50 nm separation) 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).