Abstract
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.
Highlights
This simple r−2 envelope will be used in later sections to estimate fringe amplitude reduction for many-crystal retarders in converging beams such as Daniel K. Inouye Solar Telescope (DKIST)
Crystal retarders show the expected interference of fringes between ordinary and extraordinary beams, but otherwise the behavior of fringe amplitude reduction with F/ number matches the r−2 envelope. We will take these r−2 envelopes and assess the six-crystal DKIST retarder designs over a range of wavelengths and for all fringe periods predicted in the Berreman calculus
We showed how the short-wavelength DKIST use cases at 396 nm can expect more than one magnitude amplitude reduction for the longest period fringes while expecting up to two orders of magnitude fringe suppression for the shortest period fringes during calibration
Summary
Spectral fringes in intensity and polarization are the dominant source of error. Fringes must be estimated in converging or diverging beams along with dependence on optical design properties such as cover windows, oil layers, and antireflection coatings This must be coupled to thermal behavior as environmental and. We recently adopted the Berreman calculus to model many-crystal retarders along with antireflection coatings, oils, and bonding materials, and we refer to this work as H17 here.[4] We use the Berreman calculus along with interferrometric calculations and thermal modeling to create fringe amplitude and Mueller matrix predictions for the DKIST instruments. We adapted the Berreman formalism to the six-crystal achromatic retarders used in DKIST along with many-layer antireflection coatings, oil layers, and cover windows.[4] In this paper, we use the Berreman calculus and add interference effects from converging and diverging beam variation across the aperture. Subsequent figures will display a matrix that is not formally a Mueller matrix but is convenient for displaying the separate effects of transmission, retardance, and diattenuation in simple forms
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