Daytime sky polarization calibration limitations

Abstract

The daytime sky has been recently demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K Inouye Solar Telescope (DKIST) and other large telescopes under construction can benefit from precise polarimetric calibration of large mirrors. Several atmospheric phenomena and instrumental errors potentially limit the techniques accuracy. At the 3.67m AEOS telescope on Haleakala, we have performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derived Mueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to multiple-scattering in the sky provided calibration observations are done in regions of high polarization degree. The technique is also insensitive to assumptions about telescope induced polarization provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between the functional dependence of polarization predictions and the corresponding on-sky calibrations.

Polarization modeling and predictions for Daniel K. Inouye Solar Telescope part 4: calibration accuracy over field of view, retardance spatial uniformity, and achromat design sensitivity

“…As an example, we have used five layer polycarbonate retarders at spectral resolving powers up to 50,000 in the High Resolution Visible and Infrared Spectrograph (HiVIS) at the 3.6m Advanced Electro-optical System (AEOS) on Haleakala, Maui. [66][67][68][69][103][104][105] No fringes have ever been seen in the HiVIS data. Both VTF and HiVIS mount these optics in an F/ 40 beam near a focal plane.…”

“…In earlier works, we have used an Euler formalism to describe rotation matrices. 43,[64][65][66][67][68][69][70] In the Euler angle rotation formalism, there are three successive rotations about specific coordinate axes. We denote the three Euler angles as (α, β, γ) and use a short-hand notation where cos(γ) is shortened to c γ .…”

Polarization modeling and predictions for DKIST part 3: focal ratio and thermal dependencies of spectral polarization fringes and optic retardance

“…A collaboration has been using this instrument for high precision spectropolarimetry where fringes must be very well separated from stellar signals. 76,77 As part of using this instrument in 2014, we performed a range of additional calibrations to characterize the internal cross-talk using the daytime sky [78][79][80][81] as well as many internal calibrations to establish orientations of the retarders. We found a spectral resolving power of R∼ 2500 at 800nm wavelength rising to R∼ 3500 at 1000 nm wavelength.…”

Polarization modeling and predictions for DKIST part 1: telescope and example instrument configurations

“…We have recently applied this analysis to the 4m AEOS telescope and compared the predictions to polarization calibrations derived from the daytime sky. 52 Depending on the sensitivity and computational speed required, the pupil sampling, field sampling, wavelength coverage, and telescope pointing step size can be increased to sample the beam to a desired accuracy. Typically sampling the footprint in a 20x20 grid of rays achieves <0.0001 level numerical precision, consistent with our simulation needs and is a good compromise between computation speed and numerical accuracy.…”