The „Visible Tunable Filter“(VTF) is a spectro-polarimeter that is presently being built at KIS, and will be installed at the US-American Daniel K. Inouye Solar Telescope as a focal plane instrument in about two years from now. The VTF will be used to measure magnetic fields with high spatial resolution on the Sun. To this end, the VTF measures precisely the state and the degree of polarization of the incoming solar light.

The boundary between the umbra and the penumbra of sunspots has untill recently been defined by an intensity value only. Using Hinode satellite data, Jurcak et. al 2018 found an empirical law of the magnetic nature of the umbra-penumbra boundary in stable sunspots: an invariant vertical component of the magnetic field.

In the dark umbra of a sunspot, the convective motion is largely suppressed by the strong magnetic field. In the past, numerous studies have reported on negligible convective flows (e.g., Beckers 1977). Based on this, the umbra typically served as the zero-velocity reference for the calculation of Doppler velocities of an active region.

The presence of magnetic fields in the Sun lead to a plethora of magneto-dynamic phenomena that have triggered (and yet does) the interest of many solar physicist. The study of these physical phenomena relies on the accurate inference of the physical properties from the recorded spectra.

The Sun has a variable magnetic field. The most peculiar manifestation of this variability is the 11-year sunspot cycle. Besides the number of sunspots, the frequencies of the seismic waves that propagate through the solar interior are changing.

The magnetic topology in umbral dots has been studied by means of the inversion

of spectropolarimetric data recorded with the Hinode satellite and the GREGOR

solar telescope. The SP instrument attached to the 0.5-meter SOT telescope

on-board the Hinode satellite records spectropolarimetric data of two Fe I

lines at 630 nm, which are formed about 60-80 km higher than the three Fe I

lines at 1565 nm observed with the GRIS instrument attached to the 1.5-meter

ground solar telescope GREGOR.

The thermal structure of the penumbra below its visible surface has important implications for our present understanding of sunspots and their penumbrae: which magneto-convective mode is transporting energy, and how magneto-acoustic wave modes are converted in sunspot seismology.

With the waning 24th sunspot cycle, only few active regions and sunspots appear on the solar surface. Magnetic fields which currently reach the solar surface only produce small pores, which prevail for a few days before they disappear. A region of emerging flux with a few pores was observed on May 24, 2017, with the broad band imager (BBI) at the 1.5m GREGOR solar telescope on Tenerife. The BBI observes the solar surface with two cameras simultaneously, with highest spatial and time resolution, in several regions of the solar spectrum.

Sunspots are the longest-known manifestation of solar activity, and their magnetic nature has been known for more than a century. Despite this, the boundary between umbrae and penumbrae, the two fundamental sunspot regions, has hitherto been solely defined by an intensity threshold.

In early November 2017, the main frame for the Visible Tunable Filter (VTF) was installed at the large laboratory in the Jacob-Burckhardt-Straße 1 (JB1). All optical components of the VTF will be mounted to this complex structure. The steel frame extends over two floors, has an overall height of 5 meters and weighs about 2 tons. After the completion of the VTF in Freiburg, the instrument will be dismounted, packed up, and shipped to the Daniel K. Inouye Solar Telescope (DKIST) on the Hawaiian island of Maui.