Large solar telescopes can observe the smallest details on the surface of the Sun, but only in a small field of view. As a result, they miss the large-scale development of active regions. Smaller telescopes in space or in earth-spanning networks observe the entire solar disk around the clock, but cannot zoom into the complex and rapidly changing structures that are shaped by the magnetic field. This is where the Vacuum Tower Telescope (VTT) on Tenerife, which has been in operation since 1988, comes into play. It is characterized by a large field of view and good spatial resolution, thus closing the gap between these two types of telescopes.

With the help of the new, modern camera system of the Leibniz Institute for Astrophysics Potsdam (AIP), the entire field of view of the VTT has now been restored for the first time. For a restored image, 100 short time-exposure images with 8000 × 6000 pixels are required, which are recorded at 25 frames per second. This means that the camera system delivers reconstructed images with an 8K image resolution for the first time. The fast image sequence makes it possible to eliminate the disturbing influences of the Earth’s turbulent atmosphere from the solar images. As a result, the telescope’s theoretical spatial resolution of down to 100 km on the surface of the Sun can be achieved. Time-lapse recordings of the restored images also enable the investigation of dynamic processes on time scales of 20 seconds. The new camera system complements the HELioseismic Large Region Interferometric Device (HELLRIDE), Laser Absolute Reference Spectrograph (LARS), and Fast Multi-line Universal Spectrograph (FaMuLUS) instruments at the VTT, which are operated by the Thuringian State Observatory Tautenburg (TLS), the Institute for Solar Physics (KIS) in Freiburg, and the AIP, respectively. “In order to better understand solar activity, it is crucial not only to analyze the fundamental processes of the fine structure and the long-term development of global activity with various instruments,” says Rolf Schlichenmaier, scientist at the KIS, “but also to investigate the temporal evolution of the magnetic field in active regions.”

The new images show areas that correspond to around 1/7 of the Sun’s diameter, i.e., around 200,000 km. This makes it possible to observe large-scale structures of the active Sun such as plasma motions and sunspot groups. In comparison, large telescopes typically only provide image fields of around 75,000 km in diameter. “Our expectations of the camera system were more than fulfilled right from the start,” says Robert Kamlah, who carried out the project as part of his doctoral thesis at the AIP and the University of Potsdam. The G-band observations demonstrated how the sunspots are embedded in the supergranulation, i.e., a large-scale convective pattern. The non-radial orientation and twist of the penumbral filaments revealed the complex magnetic field structure, which was responsible for three major and many minor flares in the active region.
By using special filters, the smallest magnetic field signatures become visible as bright structures in the solar images. Time series in the light of the singly ionized calcium line at 393.3 nm and in the Fraunhofer G-band at 430.7 nm enabled the identification of areas with increased activity and the tracking of plasma motions in the active regions in two layers of the solar atmosphere (photosphere and transition to the chromosphere). In addition, the researchers investigated methods for measuring image and observation quality. “The results obtained show how, together with our partners, we are teaching an old telescope new tricks,” says Carsten Denker, Head of the Solar Physics Section at the AIP. Telescopes like the VTT can make important contributions to the study of solar activity, especially when information of a large active region and its surroundings needs to be captured, such as during solar flares and other eruptive events as part of space weather forecasting. In the future, low-cost CMOS camera systems with 8K image resolution will also play an important role for the next generation of instruments on 4-meter solar telescopes, as they will triple the field of view of current 4K camera systems.

The 0.7-meter VTT solar telescope is operated by a German consortium led by the Institute for Solar Physics (KIS) in Freiburg, in collaboration with the Leibniz Institute for Astrophysics Potsdam (AIP) and the Max Planck Institute for Solar System Research in Göttingen.