Stacey R. Sueoka

Performance of polarization modulation and calibration optics for the Daniel K. Inouye Solar Telescope

The Daniel K. Inouye Solar Telescope (formerly Advanced Technology Solar Telescope) will be the worlds largest solar telescope and polarimeter when completed in 2019. Efficient use of the telescope to address key science priorities calls for polarization measurements simultaneously over broad wavelength ranges and calibration of the telescope and polarimeters to high accuracy. Broadband polarization modulation and calibration optics utilizing crystal optics have been designed for this application. The performance of polarization modulators and calibration retarders is presented along with a discussion of the unique challenges of this application. Polarimeters operate over the ranges of 0.38-1.1 microns, 0.5-2.5 microns, and 1.0-5.0 microns. Efficient polarization modulation over these broad ranges led to modulators utilizing multiple wave plates and that are elliptical, rather than linear, retarders. Calibration retarders are linear retarders and are constructed from the same sub-component wave plate pairs as the polarization modulators. Polarization optics must address efficiency over broad wavelength ranges while meeting beam deflection, transmitted wave front error, and thermal constraints and doing so with designs that, though large in diameter, can be affordably manufactured.

Construction status of the Daniel K. Inouye Solar Telescope

We provide an update on the construction status of the Daniel K. Inouye Solar Telescope. This 4-m diameter facility is designed to enable detection and spatial/temporal resolution of the predicted, fundamental astrophysical processes driving solar magnetism at their intrinsic scales throughout the solar atmosphere. These data will drive key research on solar magnetism and its influence on solar winds, flares, coronal mass ejections and solar irradiance variability. The facility is developed to support a broad wavelength range (0.35 to 28 microns) and will employ state-of-the-art adaptive optics systems to provide diffraction limited imaging, resolving features approximately 20 km on the Sun. At the start of operations, there will be five instruments initially deployed: Visible Broadband Imager (VBI; National Solar Observatory), Visible SpectroPolarimeter (ViSP; NCAR High Altitude Observatory), Visible Tunable Filter (VTF (a Fabry-Perot tunable spectropolarimeter); Kiepenheuer Institute for Solarphysics), Diffraction Limited NIR Spectropolarimeter (DL-NIRSP; University of Hawaii, Institute for Astronomy) and the Cryogenic NIR Spectropolarimeter (Cryo-NIRSP; University of Hawaii, Institute for Astronomy). As of mid-2016, the project construction is in its 4th year of site construction and 7th year overall. Major milestones in the off-site development include the conclusion of the polishing of the M1 mirror by University of Arizona, College of Optical Sciences, the delivery of the Top End Optical Assembly (L3), the acceptance of the Deformable Mirror System (Xinetics); all optical systems have been contracted and are either accepted or in fabrication. The Enclosure and Telescope Mount Assembly passed through their factory acceptance in 2014 and 2015, respectively. The enclosure site construction is currently concluding while the Telescope Mount Assembly site erection is underway. The facility buildings (Utility and Support and Operations) have been completed with ongoing work on the thermal systems to support the challenging imaging requirements needed for the solar research. Finally, we present the construction phase performance (schedule, budget) with projections for the start of early operations.

Characterization of DKIST retarder components with polarization ray tracing

Super achromatic retarders and polychromatic modulators are required to meet the polarimetry specifications of the Daniel K. Inouye Solar Telescope. These components have been analyzed and toleranced using a birefringent polarization ray trace over wavelength and field of view.

Polarization modeling and predictions for Daniel K. Inouye Solar Telescope part 1: telescope and example instrument configurations

Abstract. We outline polarization performance calculations and predictions for the Daniel K. Inouye Solar Telescope (DKIST) optics and show Mueller matrices for two of the first light instruments. Telescope polarization is due to polarization-dependent mirror reflectivity and rotations between groups of mirrors as the telescope moves in altitude and azimuth. The Zemax optical modeling software has polarization ray-trace capabilities and predicts system performance given a coating prescription. We develop a model coating formula that approximates measured witness sample polarization properties. Estimates show the DKIST telescope Mueller matrix as functions of wavelength, azimuth, elevation, and field angle for the cryogenic near infra-red spectro-polarimeter (CryoNIRSP) and visible spectro-polarimeter. Footprint variation is substantial and shows vignetted field points will have strong polarization effects. We estimate 2% variation of some Mueller matrix elements over the 5-arc min CryoNIRSP field. We validate the Zemax model by showing limiting cases for flat mirrors in collimated and powered designs that compare well with theoretical approximations and are testable with lab ellipsometers.

Polarization modeling and predictions for DKIST part 2: application of the Berreman calculus to spectral polarization fringes of beamsplitters and crystal retarders

Abstract. We outline polarization fringe predictions derived from an application of the Berreman calculus for the Daniel K. Inouye Solar Telescope (DKIST) retarder optics. The DKIST retarder baseline design used six crystals, single-layer antireflection coatings, thick cover windows, and oil between all optical interfaces. This tool estimates polarization fringes and optic Mueller matrices as functions of all optical design choices. The amplitude and period of polarized fringes under design changes, manufacturing errors, tolerances, and several physical factors can now be estimated. This tool compares well with observations of fringes for data collected with the spectropolarimeter for infrared and optical regions at the Dunn Solar Telescope using bicrystalline achromatic retarders as well as laboratory tests. With this tool, we show impacts of design decisions on polarization fringes as impacted by antireflection coatings, oil refractive indices, cover window presence, and part thicknesses. This tool helped DKIST decide to remove retarder cover windows and also recommends reconsideration of coating strategies for DKIST. We anticipate this tool to be essential in designing future retarders for mitigation of polarization and intensity fringe errors in other high spectral resolution astronomical systems.

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

We outline polarization performance calculations and predictions for the Daniel K. Inouye Solar Telescope (DKIST) optics and show Mueller matrices for two of the first light instruments. Telescope polarization is due to polarization dependent mirror reflectivity and rotations between groups of mirrors as the telescope moves in altitude and azimuth. The Zemax optical modeling software has polarization ray-trace capabilities and predicts system performance given a coating prescription. We develop a model coating formula that approximates measured witness sample polarization properties. Estimates show the DKIST telescope Mueller matrix as functions of wavelength, azimuth, elevation, and field angle for the Cryogenic Near Infra-Red Spectro-Polarimeter and for the Visible SpectroPolarimeter (ViSP). Footprint variation is substantial and shows vignetted field points will have strong polarization effects. We estimate 2% variation of some Mueller matrix elements over the 5 arc minute CryoNIRSP field. We validate the Zemax model by show limiting cases for flat mirrors in collimated and powered designs that compare well with theoretical approximations and are testable with lab ellipsometers.

Optical properties of solid-core photonic crystal fibers filled with nonlinear absorbers

A theoretical and experimental investigation of the transmission of solid-core photonic crystal fibers (PCFs) filled with nonlinear absorbers shows a sharp change in the threshold for optical limiting and in leakage loss as the refractive index of the material in the holes approaches that of the glass matrix. Theoretical calculations of the mode profiles and leakage loss of the PCF are in agreement with experimental results and indicate that the change in limiting response is due to the interaction of the evanescent field of the guided mode with the nonlinear absorbers in the holes.

Construction update of the Daniel K. Inouye Solar Telescope project

Construction of the Daniel K. Inouye Solar Telescope (DKIST) is well underway on the Haleakalā summit on the Hawaiian island of Maui. Featuring a 4-m aperture and an off-axis Gregorian configuration, the DKIST will be the world’s largest solar telescope. It is designed to make high-precision measurements of fundamental astrophysical processes and produce large amounts of spectropolarimetric and imaging data. These data will support research on solar magnetism and its influence on solar wind, flares, coronal mass ejections, and solar irradiance variability. Because of its large aperture, the DKIST will be able to sense the corona’s magnetic field—a goal that has previously eluded scientists—enabling observations that will provide answers about the heating of stellar coronae and the origins of space weather and exo-weather. The telescope will cover a broad wavelength range (0.35 to 28 microns) and operate as a coronagraph at infrared (IR) wavelengths. Achieving the diffraction limit of the 4-m aperture, even at visible wavelengths, is paramount to these science goals. The DKIST’s state-of-the-art adaptive optics systems will provide diffraction-limited imaging, resolving features that are approximately 20 km in size on the Sun. At the start of operations, five instruments will be deployed: a visible broadband imager (VTF), a visible spectropolarimeter (ViSP), a visible tunable filter (VTF), a diffraction-limited near-IR spectropolarimeter (DLNIRSP), and a cryogenic near-IR spectropolarimeter (cryo-NIRSP). At the end of 2017, the project finished its fifth year of construction and eighth year overall. Major milestones included delivery of the commissioning blank, the completed primary mirror (M1), and its cell. Commissioning and testing of the coudé rotator is complete and the installation of the coudé cleanroom is underway; likewise, commissioning of the telescope mount assembly (TMA) has also begun. Various other systems and equipment are also being installed and tested. Finally, the observatory integration, testing, and commissioning (IT&C) activities have begun, including the first coating of the M1 commissioning blank and its integration within its cell assembly. Science mirror coating and initial on-sky activities are both anticipated in 2018.

Visible, near infrared spectropolarimeter for characterization of the DKIST optical system and polarization properties (Conference Presentation)

We have developed a laboratory spectropolarimeter built to characterize the transmissive and reflective polarization properties of the Daniel K. Inouye Solar Telescope (DKIST) optical components. This includes the full Mueller matrix of retarders, polarizers, mirrors, dichroic coatings, and other optical elements that introduce polarization effects. Characterization is performed at various angles of incidence from 400nm to 1650nm with ~9nm spectral resolution and statistical noise limits >5000 using many automated stages. With this data set, we present tolerance analysis of typical as-built DKIST optics.

Progress in modeling polarization optical components for the Daniel K. Inouye Solar Telescope

The DKIST will have a suite of first-light polarimetric instrumentation requiring precise calibration of a complex articulated optical path. The optics are subject to large thermal loads caused by the ~300Watts of collected solar irradiance across the 5 arc minute field of view. The calibration process requires stable optics to generate known polarization states. We present modeling of several optical, thermal and mechanical effects of the calibration optics, the first transmissive optical elements in the light path, because they absorb substantial heat. Previous studies showed significant angle of incidence effects from the f/13 converging beam and the 5 arc minute field of view, but were only modeled at a single nominal temperature. New thermal and polarization modeling of these calibration retarders shows heating causes significant stability limitations both in time and with field caused by the bulk temperature rise along with depth and radial thermal gradients. Modeling efforts include varying coating and material absorption, Mueller matrix stability estimates and mitigation efforts.