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

Abstract

Modern observatories and instruments require optics fabricated at larger sizes with more stringent performance requirements. The Daniel K. Inouye Solar Telescope (DKIST) will be the world's largest solar telescope at 4.0 m aperture delivering a 300 Watt beam and a 5 arc-minute field. Spatial variation of retardance is a limitation to calibration of the full field. Three polarimeters operate seven cameras simultaneously in narrow bandpasses from 380 nm to 1800 nm. The DKIST polarization calibration optics must be 120 mm in diameter at Gregorian focus to pass the beam and operate under high heat load, UV flux and environmental variability. Similar constraints apply to the three retarders for modulation within the instrument suite with large beams near focal planes at F/ 18 to F/ 62. We assess how design factors can produce more spatial and spectral errors simulating elliptical retardance caused by polishing errors. We measure over 5 • net circular retardance and spectral oscillations over ±2 • for optics specified as strictly linear retarders. Spatial variations on scales larger than 10 mm contain 90% of the variation. Different designs can be a factor of 2 more sensitive to polishing errors with dissimilar spatial distributions even when using identical retardance bias values and materials. The calibration for the on axis beam is not impacted once circular retardance is included. Calibration of the full field is limited by spatial retardance variation unless techniques account for this variation. We show calibration retarder variation at amplitudes of 1 • retardance for field angles greater than roughly one arc minute for both quartz and MgF 2 retarders at visible wavelengths with significant variation between the three DKIST calibration retarders. We present polishing error maps to inform new calibration techniques attempting to deliver absolute accuracy of system calibration below effective cross-talk levels of 1 • retardance.

Polarization modeling and predictions for Daniel K. Inouye Solar Telescope, part 7: preliminary NCSP system calibration and model fitting

“…A series of fold mirrors pick off the beam converging at F∕18 within the CN instrument mirror path. We drop the beam down below the DKIST coudé beam height and package this CN F∕18 focus on a small optical bench containing a system quite similar to our NLSP described in HS18b, 66 H19, 67 and H20. 80 Figure 1 shows a CAD layout of the beam starting in the coudé laboratory with DKIST M7, collimated by M8 and folded to a pupil image inside the CN optics.…”

The Daniel K. Inouye Solar Telescope – Observatory Overview

“…The magnitude of the Stokes vector can be impacted by the depolarization row and the polarizance column. The demodulation matrix can influence all of these parameters (see, e.g., Section 7, Harrington and Sueoka, 2018a). Several of the science use-cases call out a continuum polarization stability (accuracy) specification of 0.05% for the polarizance column.…”

Polarization modeling and predictions for Daniel K. Inouye Solar Telescope part 5: impacts of enhanced mirror and dichroic coatings on system polarization calibration

“…16 We recently have investigated spatial variation of retardance across multi-layer retarders made of polished crystals, stretched polycarbonate and ferro-electric liquid crystals in HS18b. 17 This variation was then included in the DKIST optical model to show polarization calibration errors as functions of field angle and wavelength. We used a definition of calibration efficiency to show how we can use a single calibration retarder to simultaneously and efficiently calibrate all DKIST instruments from 380 nm to 1650 nm, representing the entire first-light AO corrected suite.…”

“…The Cryo-NIRSP feed optics and modulator properties are described in our prior references. 16,17,57,65 For the rest of the polarimetric instruments, the beam must propagate through the AO system as shown in Equation 4. The DL-NIRSP, ViSP and VTF all see M7, M8 and M9 as well as the tenth mirror as the adaptive optics system deformable mirror (DM = M10).…”

“…Knowing the range of expected polarization properties also helps validate any DKIST-derived calibrations when using our custom retarders we have mapped spatially and assessed for thermal instabilities. 13,16,17 In September of 2018, we had acquired witness samples for every telescope mirror in the DKIST path that has been coated to date. This includes the optics M2 through M10.…”