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The Institut für Sonnenphysik (KIS) located in Freiburg Germany has installed a high-precision measuring instrument for the world's largest solar telescope, the U.S. National Science Foundation (NSF) Daniel K. Inouye Solar Telescope on the Haleakalā volcano located on Maui/Hawaii. After thoroughly calibrating the optical elements, researchers were able to record first observation data of the sun in two typical wavelengths, thus demonstrating the functionality of the instrument. In addition, the measurements offer initial insights into the image quality and the scientific knowledge gain that can be expected for solar physics worldwide in the coming decades.
From Freiburg to Maui (Hawaii)
The German institute for solar physics develops, builds and operates unique high-tech research instruments for the detailed study of the sun. Thanks to these instruments, it has become possible to answer fundamental astrophysical questions and to study the influence of the sun on the earth's magnetic field. For this purpose, the light is broken down into its individual color components and examined with high precision.
With the recently completed Visible Tunable Filter (VTF), the dynamics of solar plasma can now be fundamentally analyzed in high resolution. Similar to weather forecasts on earth, this will make it possible in future to predict massive geomagnetic disturbances caused by energy eruptions on the sun. Life on earth has become increasingly technologized and sudden solar storms can cause devastating damage to satellite navigation or energy grids. Research for this type of predictions requires optical components whose surfaces are manufactured with atomic precision and control engineering tools that work with the same precision.
The instrument
The most recent milestone in the instrumental work at the institute is the development of VTF, an imaging spectrometer for the polarimetric investigation of the smallest magnetic structures on the sun. At the heart of the instrument are two high-precision, tunable Fabry-Pérot interferometers (FPI, figure 1) which spectrally scan the light with an accuracy of a few picometers, i.e., a trillionth of a meter.
The instrument provides two-dimensional intensity maps for each spectral scanning step (figure 2) which are additionally filtered with a polarizer. This procedure can be used, for example, to create and analyze precise velocity and magnetic field maps to study the dynamic effects of the sun in different wavelength ranges. During each observation, some 12 million spectra are recorded which are then used to determine the temperature, pressure, speed and magnetic field strength at different heights in the solar atmosphere. This is achieved by taking several hundred images within a few seconds. Thanks to these measurements, new insights can be gained into the temporal development of the solar atmosphere.
The Inouye solar telescope makes it possible to conduct observations with an unprecedented spatial resolution which opens up a new world of science. This is partly due to its location at an altitude of over 3000 meters at the Haleakalā summit (figure 3) and partly due to the size of the primary mirror with a diameter of 4 meters. The filter instrument (VTF) developed specially to meet these requirements can explore this new world thanks to the high surface accuracy and stability of its interferometers. The spectral transmission range is defined by its plate spacing, with the reflectivity of the plate coatings defining the filter width. Together with the achievable distance control accuracy, this means that for the instrument to have the necessary sensitivity, the variation of the plate spacing over the entire active image field must be small-scale, not exceeding a few nanometers.
If, by way of illustration, these orders of magnitude were transferred to a lake with a diameter of 30 km, the deviations on the surface would correspond to a maximum wave height of less than one millimeter. The development of the interferometers with the diameter required for a large telescope posed a particular challenge. Never before had it been possible to build a Fabry-Pérot interferometer with an active area measuring almost 0.3 meters and with the required optical quality.
The success was the result of a collaboration led by the German institute and involving various globally active high-tech companies from the fields of optics production, surface coating and control engineering. The design of the interferometer, the construction of the mechanics and the development of the complex control technology were entirely in the hands of the institute. The interlocking coordination of all the tasks with the partners was particularly challenging. The realization of this demanding project was also due to the successful collaboration with international research institutions, primarily the NSF National Solar Observatory (NSO, USA), which operates the Inouye Solar Telescope, the Max-Planck-Institut für Sonnensystemforschung (MPS, Germany) and the Istituto Ricerche Solari Aldo e Cele Daccò (IRSOL, Switzerland)
Dr. Matthias Schubert
matthias.schubert@leibniz-kis.de
https://www.leibniz-kis.de/en/research/scientific-instrumentation/vtf/
https://nso.edu/telescopes/inouye-solar-telescope/
https://nso.edu/blog/vtf-shipment-arrives-at-inouye-solar-telescope/
https://nso.edu/for-public/nso-in-hawaii/
Fig. 1: Measurement setup of first FPI showing the two glass plates with a diameter of 0.3 m and the ...
KIS
Fig. 2: Solar atmosphere in the filtergram channel at a wavelength of λ = 588.9 nm. In the darker ar ...
VTF/KIS/NSF/NSO/AURA
Fig. 3: Near the summit of Maui’s Haleakalā, the NSF Daniel K. Inouye Solar Telescope - and its set of cutting-edge solar instruments, such as the Visible Tunable Filter.
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