German instrument at the largest solar telescope put into operation

The spectro-polarimeter Visible Tunable Filtergraph (VTF) of the Daniel K. Inouye Solar Telescope on the Hawaiian island of Maui has recorded images of the sun for the first time. The instrument was developed in Germany.

The Visible Tunable Filtergraph instrument was developed and built by the Leibniz Institute for Solar Physics (KIS) in Freiburg. It is designed to image the sun "with the highest possible spatial, temporal and spectral resolution", according to the Max Planck Institute for Solar System Research (MPS). The Göttingen-based institute is a partner in the project.

The VTF is currently the largest spectro-polarimeter. It weighs 5.6 tons and has the footprint of a small garage. It took around 15 years to develop and its installation in the Daniel K. Inouye Solar Telescope (DKIST) began last year.

Accuracy in the picometer range

In order to filter out individual, very narrow wavelength ranges from the visible sunlight, the VTF has two Fabry-Pérot interferometers, which are unique worldwide in terms of their size and precision. This allows the sunlight to be scanned spectrally with an accuracy of a few picometers.

Two-dimensional images of the sun are created for each wavelength and polarization state, from which the temperature, pressure, flow velocity of the solar plasma and magnetic field strength on the surface of the sun and in the gas layers above it can be determined. The spatial resolution is around 10 kilometers per pixel and the temporal resolution is several hundred images per second.

With the VTF, the solar telescope can observe the region of the sun where the eruptions occur more precisely than ever before: the visible surface of the sun, called the photosphere, and the chromosphere, which is the layer of the solar atmosphere above it. By observing the interplay of hot plasma currents and the changing magnetic fields, scientists hope to gain a better understanding of the processes that trigger solar flares.

New phase of Earth-based solar observation

"VTF enables images of unprecedented quality and thus heralds a new phase in Earth-based solar observation", said Sami Solanki, Director of the Sun and Heliosphere Department at the MPS.

The DKIST, named after a former senator for the state of Hawaii, is currently the largest solar telescope with a four-meter primary mirror. Test operation began at the end of 2019 and it has been in regular operation since 2022. In order to be able to observe the sun even better, it will be successively equipped with additional instruments that process the incoming light, for example by examining individual wavelength ranges or oscillation directions of the light separately. The VTF is the fourth and penultimate of these instruments.

The VTF is still in the test phase, but the images already make very small structures visible. When the instrument is in regular scientific operation, the data will be post-processed so that the resolution will be even higher.

(wpl)