Visible tunable Filter (VTF) for the Daniel K. Inouye Solar Telescope

Fig. 1 Vertical cut through DKIST (Image: DKIST)
Fig. 2 DKIST dome in the morning sun. In the background visible are Mauna Kea and Mauna Loa on Big Island (Image: KIS)
Fig. 3 Areal view of the Haleakala Observatory with DKIST under construction in March 2016 (Image: W. Schmidt)
Fig. 4 Backside of the actively controlled DKIST main mirror cell. (Image: W. Schmidt)

General Project Information

The Daniel K. Inouye Solar Telescope (DKIST, formerly called ATST) is being built on the Hawaiian island of Maui under the leadership of the National Solar Observatory (NSO) with financial support from the National Science Foundation. The DKIST represents a consortium of 22 U.S. institutions, a large portion of solar physicists in the country. The commissioning of the telescope started in 2020.

This solar telescope has an aperture of 4 m to allow and diffraction-limited observations in the visible and infrared spectral range. The wavelength range includes the visible and infrared light up to a wavelength of 5 microns. In addition, observations of the corona in the infrared should be possible. In visible light, the DKIST will be able, with the help of a powerful adaptive optics to resolve structures on the Sun with a size of 0.03 arcsec or 20 km on the Sun.

The DKIST represents several technological challenges:  on the one hand, the 4m wide primary mirror must not heat up despite its orientation to the Sun above the ambient air temperature, otherwise local air turbulence (mirror seeing) may build up near the mirror, and secondly the enormous energy of the focused Sun light in the primary focus of approximately 30 kW has to be dissipated without creating air turbulence.

The  location for the DKIST is the volcano Haleakala on the Hawaiian island of Maui, USA with an elevation of 3000 m above sea level. A detailed examination of various location around the world had shown that on Haleakala and on La Palma, Canary Islands, excellent seeing conditions prevail. The coronal viewing conditions (low atmospheric stray light) on Haleakala are better than at La Palma.

The realization of the DKIST

In a report to the National Research Council of the USA, ATST was recommended as the most important project out of 6 telescope projects of medium size. the DKIST is also part of the 10-year research plan in the solar and space physics of the NCR. End of 2009, the U.S. National Science Foundation decided to build the DKIST, and provided the necessary funding. After clarification of administrative issues, construction began in 2012, with First Light in late 2019. For more information about this project can be found on the DKIST project website:


Participation of the KIS in the DKIST

The Leibniz-Institut for Solar Physics considers the participation in the DKIST as an important prerequisite to continue its prominent role in solar research.

KIS staff were actively involved in the past to test the possible locations for the DKIST, helped formulate the scientific objectives of the DKIST, and contributed to the considerations for a Fabry-Perot spectrometer.

The main contribution of KIS to DKIST is a tunable Fabry-Perot spectro-polarimeter (Visible Tunable Filter - VTF), an instrument for high spatial resolution 2D imaging spectropolarimetry and spectroscopy. This instrument will be one of the first-light instruments of the DKIST. It is under development at KIS, and on-site installation and commissioning is planned to start early in 2022. The development of the VTF is mainly financed by the funding agencies of KIS: the state of Baden-Wuerttemberg and the federal government. The VTF project is also supported from the Max-Planck Institute for Solar System Research (MPS) and the Swiss IRSOL observatory. After commissioning, the VTF will become a facility instrument of the DKIST.