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

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 AE0.2 deg and AE0.5 deg for two separate SiO 2 retarders. Modulation matrices derived are stable to < AE 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. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Polarization modeling and predictions for DKIST, part 8: calibration polarizer spatial variation impacts

“…69 For now, the mirrors are only positioned to a single static position with beam pointing variations as the system changes azimuth, elevation, and coudé table angle. As we noted in H21, 68 the DKIST coudé lab beam was only stabilized to roughly AE9.7 arc sec field angle during some of our early polarization calibrations. Recent alignment work and on-going optical installations make this number dependent on construction project activities, changing frequently, with a range over AE15 arc sec anticipated.…”

“…Misalignment produces spectral oscillations in retardance, which introduce a coupling between temperature changes and the spectral drift of these elliptical retardance oscillations. We showed in H21 68 detection of the clocking oscillations outlined in H20 86 with calibrations both on-sun and with the DKIST calibration lamp. We also showed in H21 68 successful on-sun use of the optically contacted calibration retarder (OCcal) and system calibration with time-efficient calibration sequences.…”

“…We showed in H21 68 detection of the clocking oscillations outlined in H20 86 with calibrations both on-sun and with the DKIST calibration lamp. We also showed in H21 68 successful on-sun use of the optically contacted calibration retarder (OCcal) and system calibration with time-efficient calibration sequences. We compile here additional information about the interference fringe properties anticipated for our polarizers.…”

“…We show additional mirror measurements and new ellipsometric metrology from an outside vendor (J. A. Woollam) in H21 68 at a range of incidence angles matching the DKIST mirrors to improve the system polarization model.…”

“…We have a dedicated metrology tool called the National Solar Observatory Coudé lab Spectro-Polarimeter (NCSP) described in H21. 68 This system calibrates the full telescope using either our Gregorian focus calibration lamp or the solar beam itself. The calibration optic misalignments present limitations in calibration accuracy even for the on bore sight beam.…”