Abstract
Daniel K. Inouye Solar Telescope (DKIST) is designed to deliver accurate spectropolarimetric solar data across a wide wavelength range, covering a large field of view simultaneously using multiple facility instruments for solar disk, limb, and coronal observations. We show successful design and implementation of National Solar Observatory Coudé Laboratory Spectropolarimeter, a custom metrology tool for efficient continuous broadband polarization calibration of the telescope mirrors through a coudé laboratory focus. We compare multiple fitting techniques for the 10 to >140 variable DKIST system polarization models. We compare results with the first DKIST solar calibration observations and find small thermally forced retardance changes of ±0.2 deg and ±0.5 deg for two separate SiO2 retarders. Modulation matrices derived are stable to < ± 0.01 per element during the first on-Sun calibration tests. We achieve good fit agreement to our metrology-based model over a 390- to 1600-nm bandpass. The solutions are robust and efficient using only 10 input Stokes vectors from elliptical calibration retarders. We developed a custom polarizer assembly used with metrology tools to orient the DKIST polarization coordinates to better than 0.1-deg clocking angle.
Highlights
Daniel K. Inouye Solar Telescope (DKIST) and Polarization Models for CalibrationThe National Science Foundation’s Daniel K
We use Eq (4) to define the Mueller matrix for the Gregorian Optical System (GOS) as a combination of the polarizer rotated into local coordinates by the angle denoted as “pol” and an elliptical retarder rotated into local coordinates by the angle denoted as “ret.” We discuss the sequence of polarization states generated by the GOS during calibration in more detail later
We began with 17-variable model fits to show that the 24-variable modulation matrix matches metrology when fitting a grouping of mirrors and the rotating polycarbonate retarder installed in National Solar Observatory Coudé Laboratory Spectropolarimeter (NCSP)
Summary
We show here that the design and implementation of a custom dual-channel SP we call the National Solar Observatory Coudé Laboratory Spectropolarimeter (NCSP) We deployed this metrology tool on the telescope to calibrate the DKIST system optics polarization for wavelengths between 390 and 1600 nm with continuous coverage at subnanometer sampling. We recently have investigated spatial variation of retardance across multi-layer retarders made of polished crystals, stretched polycarbonate, and ferro-electric liquid crystals by Harrington and Sueoka 2018b (HS18b66) This variation was included in the DKIST optical model to show PolCal errors as functions of field angle and wavelength. We demonstrate efficient modulation with NCSP using a copy of the polycarbonate modulator deployed in our lab metrology tool as well as in DL-NIRSP This modulator combines with a complex optical feed using 10 mirrors, multiple lenses, and fiber fed spectrographs to provide a representative optical system for demonstrating our calibration process
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