A two-dimensional spectropolarimeter as a first-light instrument for the Daniel K. Inouye Solar Telescope

J. Astron. Telesc. Instrum. Syst.

2018

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.

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Section: Introductionmentioning

confidence: 99%

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

J. Astron. Telesc. Instrum. Syst.

2018

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“…A train of dichroic beam splitters allows for rapid changing of instrument configurations and simultaneous operation of three polarimetric instruments covering 380 to 1800 nm. [7][8][9][10] Complex modulation and calibration strategies are required for such a mulitinstrument system. 7,8,[11][12][13][14] The planned 4-m European Solar Telescope (EST), though on-axis, will also require similar calibration considerations.…”

Section: Predicting Dkist Polarizationmentioning

confidence: 99%

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

J. Astron. Telesc. Instrum. Syst

2017

“…the ferro electric liquid crystal modulator in VTF is capable of kHz rates versus discrete modulation on timescales <0.01 Hz for coronal observations with DL-NIRSP or Cryo-NIRSP). 9,54 Expected solar magnetic field strengths can range from a fraction of a Gauss spatially unresolved below the DKIST diffraction limit to several kiloGauss covering the entire instantaneous field of view of a DKIST instrument. Translation of an error in magnetic field to an error in measured Stokes vectors implies some numerical techniques to relate changes in modeled field properties to errors in a measurement through some kind of atmospheric model and inversion process.…”

Section: Dkist Optics and Polarization Models For Calibrationmentioning

confidence: 99%

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

J. Astron. Telesc. Instrum. Syst.

White

2019