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 W beam and a 5 arc min 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 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-deg net circular retardance and spectral oscillations over ±2 deg for optics specified as strictly linear retarders. Spatial variations on scales >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 of the on axis beam is not impacted once circular retardance is included. The 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-deg retardance for field angles greater than roughly 1 arc min for both quartz and MgF2 retarders at visible wavelengths with significant variation between the three DKIST calibration retarders. We present polishing error maps to inform calibration techniques attempting to deliver absolute accuracy of system calibration below effective cross talk levels of 1 deg retardance.
Highlights
We showed in this paper how the field of view available for calibration at the high accuracy for Daniel K. Inouye Solar Telescope (DKIST) is impacted by of spatial nonuniformity for our six-crystal super achromatic calibration retarders
Wide wavelength requirements and high heat loads can drive designs to large apertures and manycrystal polychromatic solutions mounted near focal planes that create elliptical retardance and couples in spatial variation to calibration accuracy
Spectral metrology tools showed that net circular retardance over 5 deg and spectral oscillations of circular retardance of over Æ2 deg were detected in the DKIST calibration retarders
Summary
Later designs for superachromats used three compounds or bicrystalline achromats in place of A and B for six total crystals.[7] This increased the wavelength range when requiring achromatic linear retardance of various specifications to achieve high efficiency of modulation or calibration. The longer wavelength designs used MgF2 crystals around 2-mm physical thickness, giving 40-waves net retardance bias at 633 nm for this higher birefringence material Another first light instrument was designed to include infrared capabilities at wavelengths as long as 5000 nm. The optics we designate for the DL-NIRSP has a wavelength range for reasonably efficient modulation from 500 to 2500 nm with the calibration retarder covering from 900 to 2500 nm. We show retardance spatial nonuniformity of polycarbonate and ferroelectric liquidcrystal-type retarders
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