Deriving Telescope Mueller Matrices Using Daytime Sky Polarization Observations

Telescopes often modify the input polarization of a source so that the measured circular or linear output state of the optical signal can be significantly different from the input. This mixing, or polarization cross talk, is defined by the optical system Mueller matrix. We describe an efficient method here for recovering the input polarization state of the light and the full 4 x 4 Mueller matrix of the telescope with an accuracy of a few percent without external masks or telescope hardware modification. Observations of the bright, highly polarized daytime sky using the Haleakala 3.7 m AEOS telescope and a coude spectropolarimeter demonstrate the technique.