Visible Tunable Filtergraph (VTF) at the Daniel K. Inouye Solar Telescope
The Visible Tunable Filtergraph (VTF) is one of five scientific instruments on the world's largest solar telescope, the U.S. National Science Foundation Daniel K. Inyoue Solar Telescope (DKIST) in Hawaii, built and operated by the National Solar Observatory in Boulder, Colorado, USA. VTF is an imaging spectro-polarimeter that was developed and built at the Institute for Solar Physics (KIS) in Freiburg (Germany). The MPS is a partner in the project.
With a primary mirror diameter of four meters, DKIST is the largest solar telescope in the world. Thanks to the optimal observation conditions on the Hawaiian volcano Haleakala and state-of-the-art image stabilization and reconstruction methods, it allows for detailed images of the Sun. DKIST is also equipped with five scientific instruments. They process the incident light further by, for example, examining individual wavelength ranges or polarizations of the light separately.
VTF is the most powerful of the five instruments and thus the heart of the telescope. VFT's task is to image the Sun with the highest possible spatial, temporal, and spectral resolution. In order to filter out individual, very narrow wavelength ranges from the incident visible sunlight, the instrument uses a Fabry-Pérot interferometer that is unique in the world in terms of its size and precision. In this way, it is possible to scan the sunlight spectrally with an accuracy of a few picometers. In addition, VTF detects the complete polarization state of the light. For each wavelength and polarization state, two-dimensional images of the Sun are then created, from which the temperature, pressure, gas and plasma velocities, and magnetic field strength at different altitudes of the Sun can be determined. The observational data achieves a spatial resolution of less than 30 kilometers and a temporal sequence of hundreds of images per second.
VFT was developed and built at KIS. The planning and construction phase took about 15 years. Installation of the instrument at the Daniel K. Inyoue Solar Telescope began in early 2024, and technical first light was achieved in early 2025.