The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) is the largest solar telescope in the world, utilizing a 4m off-axis primary mirror sending light to a ∼16m wide rotating multi-instrument coudé laboratory supported by a highly complex active and adaptive optics system, delivering a diffraction limited beam. The resulting mount size, long optical pathways, various moving components, and complex thermal design leaves DKIST with a very tight optical error budget that is susceptible to vibration-related degradation. Prior to and throughout the early stages of DKIST operations, there has been an ongoing survey to identify and address vibration sources affecting the optical path of the telescope. Using data from our High Order Adaptive Optics (HOAO) and Power Spectral Density (PSD) data taken from accelerometers placed throughout the site, we have been able to record and track noteworthy frequencies as they appear throughout various phases of operations. Efforts within the last year have allowed for improvements in this vibration survey with increased monitoring via expansion in both the frequency and scale of data collection. This has enabled us to distinguish and categorize several vibration sources that encompass both high impact individual frequencies and overall noise, in order to prioritize solutions for those with the highest impact on image motion. Components of the DKIST facility thermal system and end consumer internal thermal processes, requisitely located throughout the telescope mount and coud´e in order to remove waste heat from temperature sensitive areas, often prove to be the sources of such vibration. Presented herein are recent examples of sources with significant impact, including the details on how we tracked and identified them, and the solutions that were implemented in order to reduce jitter. As DKIST continues operations, future vibration mitigation efforts will be supported by additional data from other instruments in order to identify opportunities for optimization and further isolate localized vibration within our optics systems.
The US National Science Foundation 4m Daniel K. Inouye Solar Telescope (DKIST) on Haleakala, Maui is the largest solar telescope in the world. DKIST’s superb resolution and polarimetric sensitivity will enable astronomers to explore the origins of solar magnetism, the mechanisms of coronal heating and drivers of flares and coronal mass ejections. DKIST operates as a coronagraph at infrared wavelengths, providing crucial measurements of the magnetic field in the corona. During its Operations Commissioning Phase, DKIST has already conducted a significant number of shared-risk observations for community researchers. The complex raw data are calibrated by the DKIST Data Center located in Boulder and distributed to the science community. We’ll present examples of science results and discuss lessons learned. Ongoing instrument development efforts include, an upgrade of the single-conjugate adaptive optics system to a multi-conjugate AO, the implementation of image slicers for the DL-NIRSP instrument and development of infrared detectors the DL- and CRYO-NIRSP instruments.
The NSF’s Daniel K Inouye Solar Telescope (DKIST) is the world’s largest solar telescope at the summit of Haleakalā. All large observatories are subject to the negative impacts of vibrations, therefore, one of the goals during the operations and commissioning phase is to collect data to identify and mitigate image jitter. DKIST has five high spatial resolution facility instruments spread across a 16-meter rotating platform. Vibration sources such as moving instrument components, environmental control systems, and active optics can induce image jitter differently across large distances, causing non-common path errors uncorrectable by AO systems. We built a new tool called the Vibrometer, a high speed image tracker designed to measure image motion in order to assess the system optical vibrations at 2kHz rates. We will present how the Vibrometer played a vital role in eliminating the image jitter observed in the Visible Spectro-Polarimeter (ViSP) instrument's slit scanning images. The image jitter was caused by mechanical motion of the Visible Broadband Imager's (VBI) large two-axes camera stage while performing image mosaic scans during simultaneous measurements.
The Daniel K. Inouye Solar Telescope, with its 4m aperture, is the largest telescope for observations of the Sun, and is currently in its Operations Commissioning Phase. During this phase of the project, the five DKIST first light instruments, the Visible Broadband Imager (VBI), the Visible Spectro-Polarimeter (ViSP), the Diffraction-Limited Near-Infrared Spectro-Polarimeter (DL-NIRSP), the Cryogenic Near-Infrared Spectro-Polarimeter (Cryo-NIRSP) and the Visible Tunable Filter (VTF) are used in selected modes to acquire scientific data. We provide an overview of the DKIST instrumentation system and its inherent flexibility. We further report on lessons learned during commissioning, and present sample data products.
Modern astronomical polarimeters often require simultaneous operation of multiple instruments over broad wavelength ranges. The 4 m DKIST solar telescope will soon cover 0.38 to 4.6 μm with at least 12 independent narrow band polarimeters, all in quasi-simultaneous operation. Calibration can be efficiently performed over this entire bandpass using our elliptical retarder design, achieved with just two optically contacted MgF2 crystal retarder pairs. Calibration requires very well-characterized, uniform, defect-free retarders and polarizers. I report here on the successful development of four extremely large aperture (d = 120 mm) optically contacted MgF2 retarder pairs used to make a DKIST calibrator and a modulator for the Cryo-NIRSP instrument. All four crystal pairs have clear apertures free of defects. New procedures deliver fast axis alignment in the range of 0.1 deg to 0.2 deg post contact bonding. For the calibrator crystals, a new process was developed using deterministic fluid jet polishing driven by retardance mapping to achieve stringent retardance spatial uniformity. I show that transmitted wavefront error is not a sufficient proxy for retardance polishing. Polishing softer MgF2 retarder crystals required substantial development to simultaneously achieve flatness, roughness, and retardance uniformity. The optical contact bond ensures there are no bonding agents (oils, epoxies) with spectral absorption bands in the entire 0.3 to 6 μm bandpass without any possibility for leaks or degradation. These four crystals will be used in DKIST and Cryo-NIRSP in a 300 W solar beam and are anticipated to mitigate heating, stability, and UV irradiation issues. I use the Berreman calculus to compute retarder depolarization, with >10 % magnitudes found at the shortest wavelengths after including typical crystal optic axis cutting errors and incidence angle variation in converging beams.
Astronomical mirror coatings are often metals protected by multiple layers of dielectrics. Varying the thickness and layering of dielectrics causes a significant dependence on the polarization properties (retardance, diattenuation, and depolarization) of reflected light across all wavelengths. Polarization further varies with angle of incidence and mirror shape. In models predicting polarization performance, assumptions on the properties and uniformity of coated optical surfaces are usually made. Here, we present how a non-uniformly applied coating affects polarization performance and causes depolarization across an aperture. We then assess the differences from assuming a uniform surface. Using the NSF’s Daniel K. Inouye Solar Telescope as an example of a complex, many-optic, articulated system, we also compare depolarization effects of mirror coating non-uniformity to other known sources of systematic polarization error on DKIST.
The National Science Foundation’s 4m Daniel K. Inouye Solar Telescope (DKIST) on Haleakala, Maui is now the largest solar telescope in the world. DKIST’s superb resolution and polarimetric sensitivity will enable astronomers to unravel many of the mysteries the Sun presents, including the origin of solar magnetism, the mechanisms of coronal heating and drivers of flares and coronal mass ejections. Five instruments, four of which provide highly sensitive measurements of solar magnetic fields, including the illusive magnetic field of the faint solar corona. DKIST operates as a coronagraph at infrared wavelengths where the sky background is low and bright coronal emission lines are available. The high-order, single-conjugate adaptive optics system (AO) provides diffraction limited imaging and the ability to resolve features approximately 20 km on the Sun. A multi-conjugate AO upgrade is in progress. With these unique capabilities DKIST will address basic research aspects of Space Weather and help improve predictive capabilities. DKIST has completed construction and is now in the early phases of operations. Community proposal-based shared-risk observations are conducted by the DKIST operations team.
Astronomical instruments greatly improve wavelength multiplexing capabilities by using beam splitters. In the case of the 4-m National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) solar telescope, over 70 W of optical power is distributed simultaneously to four instruments, each with multiple cameras. Many DKIST observing cases require simultaneous observations of many narrow bandpasses combined with an adaptive optics system. The facility uses five dichroic optical stations to allow at least 11 cameras and two wavefront sensors to simultaneously observe ultraviolet to infrared wavelengths with flexible reconfiguration. The DKIST dichroics required substantial development to achieve very tight specifications over very large apertures of 290 mm diameter. Coating spectral variation occurs over <1 nm wavelength, comparable with instrument bandpasses. We measure retardance spectral variation of up to a full wave and diattenuation varying over ±10 % per nm. Spatial variation of Mueller matrix elements for coatings in both transmission and reflection requires careful metrology. We demonstrate coatings from multiple vendors exhibit this behavior. We show achievement of 5-nm root mean square (RMS) reflected wavefront and 24-nm RMS power with coatings over 8 μm thick. We show mild impacts of depolarization and spectral variation of polarization on modulation efficiency caused by the dichroic coatings. We show an end-to-end system polarization model for the visible spectropolarimeter instrument, including the dichroics, grating, analyzer, and all coated optics. We show detailed performance for all DKIST dichroics for community use in planning future observations.
Astronomical spectropolarimeters require high accuracy polarizers with large aperture and stringent uniformity requirements. In solar applications, wire grid polarizers are often used as performance is maintained under high heat loads and temperatures over 200°C. DKIST is the NSF’s new 4-m aperture solar telescope designed to deliver accurate spectropolarimetric solar data across a wide wavelength range, covering a large field of view simultaneously using multiple facility instruments. Polarizers at 120 mm diameter are used to calibrate DKIST instruments but vary spatially in transmission, extinction ratio, and orientation of maximum extinction. We combine new spatial and spectral metrology for polarizers and retarders to simulate the accuracy losses with field angle and wavelength caused simultaneously by spatial variation of several optical parameters including beam decenter from misalignments. We also present testing of a new crystal sapphire substrate polarizer designed and fabricated to improve DKIST long wavelength calibrations. We assess spatial thickness variation of sapphire and fused silica wafer substrates using spectral interference fringes.
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.
The Daniel K. Inouye Solar Telescope (DKIST) is a 4-meter solar observatory under construction at Haleakala, Hawaii. The Gregorian Optical System (GOS) is located at the secondary focus of the telescope and actuates different apertures and optics into the beam in order to facilitate configuration of the telescope optical beam for science and calibration activities. Due its location near Gregorian focus, the GOS design addresses several thermal challenges in order to maintain safe operating temperatures and prevent local seeing effects. In this paper we describe these thermal challenges and explain how we used modeling and simulation analyses to guide design choices. We will review results and limitations from the GOS lab acceptance testing process, look at lessons learned from integration at the summit, and share initial results from on sun testing. We conclude by comparing on sun test results with predictions from our design phase analyses.
Interference fringes are a major source of systematic error in astronomical spectropolarimeters. We apply the Berreman formalism with recent spatial fringe aperture averaging estimates to design and fabricate new fringe-suppressed polarization optics for several Daniel K. Inouye Solar Telescope (DKIST) use cases. We successfully performed an optical contact bond on a 120-mm-diameter compound crystal retarder for calibration with wavelength-dependent fringe suppression factors of one to three orders of magnitude. Special rotational alignment procedures were developed to minimize spectral oscillations, which we show here to represent our calibration stability limit under retarder thermal perturbation. We developed a fabrication technique to deliver low beam deflection for our large aperture polycarbonate (PC) retarders. Modulators are upgraded in two DKIST instruments with minimal beam deflection and bandpass-optimized antireflection coatings for fringe suppression factors of hundreds. We confirm that PC retarders do fringe as expected when low deflection is achieved. We show that increased retardance spatial variation from PC does not degrade modulation efficiency.
The Daniel K. Inouye Solar Telescope (DKIST) is designed to deliver accurate spectropolarimetric calibrations across a wide wavelength range and large field of view for solar disk, limb, and coronal observations. DKIST instruments deliver spectral resolving powers of up to 300,000 in multiple cameras of multiple instruments sampling nanometer scale bandpasses. We require detailed knowledge of optical coatings on all optics to ensure that we can predict and calibrate the polarization behavior of the system. Optical coatings can be metals protected by many dielectric layers or several-micron-thick dichroics. Strong spectral gradients up to 60 deg retardance per nanometer wavelength and several percent diattenuation per nanometer wavelength are observed in such coatings. Often, optical coatings are not specified with spectral gradient targets for polarimetry in combination with both average- and spectral threshold-type specifications. DKIST has a suite of interchangeable dichroic beam splitters using up to 96 layers. We apply the Berreman formalism in open-source Python scripts to derive coating polarization behavior. We present high spectral resolution examples on dichroics where transmission can drop 10% with associated polarization changes over a 1-nm spectral bandpass in both mirrors and dichroics. We worked with a vendor to design dichroic coatings with relatively benign polarization properties that pass spectral gradient requirements and polarization requirements in addition to reflectivity. We now have the ability to fit multilayer coating designs which allow us to predict system-level polarization properties of mirrors, antireflection coatings, and dichroics at arbitrary incidence angles, high spectral resolving power, and on curved surfaces through optical modeling software packages. Performance predictions for polarization at large astronomical telescopes require significant metrology efforts on individual optical components combined with system-level modeling efforts. We show our custom-built laboratory spectropolarimeter and metrology efforts on protected metal mirrors, antireflection coatings, and dichroic mirror samples.
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.
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.
Data products from high spectral resolution astronomical polarimeters are often limited by fringes. Fringes can skew derived magnetic field properties from spectropolarimetric data. Fringe removal algorithms can also corrupt the data if the fringes and object signals are too similar. For some narrow-band imaging polarimeters, fringes change the calibration retarder properties and dominate the calibration errors. Systems-level engineering tools for polarimetric instrumentation require accurate predictions of fringe amplitudes, periods for transmission, diattenuation, and retardance. The relevant instabilities caused by environmental, thermal, and optical properties can be modeled and mitigation tools developed. We create spectral polarization fringe amplitude and temporal instability predictions by applying the Berreman calculus and simple interferometric calculations to optics in beams of varying F/ number. We then apply the formalism to superachromatic six-crystal retarders in converging beams under beam thermal loading in outdoor environmental conditions for two of the world’s largest observatories: the 10-m Keck telescope and the Daniel K. Inouye Solar Telescope (DKIST). DKIST will produce a 300-W optical beam, which has imposed stringent requirements on the large diameter six-crystal retarders, dichroic beamsplitters, and internal optics. DKIST retarders are used in a converging beam with F/ ratios between 8 and 62. The fringe spectral periods, amplitudes, and thermal models of retarder behavior assisted DKIST optical designs and calibration plans with future application to many astronomical spectropolarimeters. The Low Resolution Imaging Spectrograph with polarimetry instrument at Keck also uses six-crystal retarders in a converging F / 13 beam in a Cassegrain focus exposed to summit environmental conditions providing observational verification of our predictions.
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.
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.
We outline polarization fringe predictions derived from a new application of the Berreman calculus for the Daniel K. Inouye Solar Telescope (DKIST) retarder optics. The DKIST retarder baseline design used 6 crystals, singlelayer anti-reflection coatings, thick cover windows and oil between all optical interfaces. This new 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 SPINOR spectropolarimeter at the Dunn Solar Telescope using bi-crystalline achromatic retarders as well as laboratory tests. With this new tool, we show impacts of design decisions on polarization fringes as impacted by anti-reflection 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.
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.
The daytime sky has recently been demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K. Inouye Solar Telescope and other large telescopes under construction can benefit from precise polarimetric calibration of large mirrors. Several atmospheric phenomena and instrumental errors potentially limit the technique’s accuracy. At the 3.67-m AEOS telescope on Haleakala, we performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derived Mueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to multiple-scattering in the sky, provided calibration observations are done in regions of high polarization degree. The technique is also insensitive to assumptions about telescope-induced polarization, provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between the functional dependence of polarization predictions and the corresponding on-sky calibrations.
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.
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.
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.
The daytime sky has been recently demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K Inouye Solar Telescope (DKIST) and other large telescopes under construction can benefit from precise polarimetric calibration of large off-axis mirrors. Several atmospheric phenomena and instrumental errors potentially limit the techniques accuracy. At the 3.67m AEOS telescope on Haleakala, we have performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derivedMueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to assumptions about telescope induced polarization provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between predictions and on-sky calibrations.
The Hokupa’a-85 curvature adaptive optics system components have been adapted to create a new AO-corrected
coud´e instrument at the 3.67m Advanced Electro-Optical System (AEOS) telescope. This new AO-corrected
optical path is designed to deliver an f/40 diffraction-limited focus at wavelengths longer than 800nm. A new
EMCCD-based dual-beam imaging polarimeter called InnoPOL has been designed and is presently being installed
behind this corrected f/40 beam. The InnoPOL system is a flexible platform for optimizing polarimetric
performance using commercial solutions and for testing modulation strategies. The system is designed as a
technology test and demonstration platform as the coud´e path is built using off-the-shelf components wherever
possible. Models of the polarimetric performance after AO correction show that polarization modulation at rates
as slow as 200Hz can cause speckle correlations in brightness and focal plane location sufficient enough to change
the speckle suppression behavior of the modulators. These models are also verified by initial EMCCD scoring
camera data at AEOS. Substantial instrument trades and development efforts are explored between instrument
performance parameters and various polarimetric noise sources.
We report the main conclusions from an interactive, multidisciplinary workshop on “Polarimetric Techniques and Technology”, held on March 24-28 2014 at the Lorentz Center in Leiden, the Netherlands. The work- shop brought together polarimetrists from different research fields. Participants had backgrounds ranging from academia to industrial RD. Here we provide an overview of polarimetric instrumentation in the optical regime geared towards a wide range of applications: atmospheric remote sensing, target detection, astronomy, biomedical applications, etc. We identify common approaches and challenges. We list novel polarimetric techniques and polarization technologies that enable promising new solutions. We conclude with recommendations to the polarimetric community at large on joint efforts for exchanging expertise.
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