Reviewed by Lexie CornerApr 25 2025
The US National Science Foundation (NSF) Daniel K. Inouye Solar Telescope in Hawaii, the largest solar telescope globally, has reached a key milestone. The Visible Tunable Filtergraph (VTF), a German instrument designed for the telescope, has taken its first images after nearly 15 years of preparation.
A narrow-band image of the Sun at a wavelength of λ=588.9 nm, that of a well-known solar sodium line also known as the “NaD line.” The image was acquired during recent first light efforts with the VTF at the Inouye and shows how precisely the structures within a sunspot are resolved. Each pixel in the original version of the image corresponds to 10 km (or 6.2 miles) on the Sun. Image Credit: VTF/KIS/NSF/NSO/AURA
The imaging spectro-polarimeter was developed and constructed at the Institute for Solar Physics (KIS) in Freiburg, Germany, with the Max Planck Institute for Solar System Research (MPS) in Göttingen, Germany, as a project partner. The current data was obtained during the instrument’s technical commissioning.
VTF, more than ever before, is analyzing sunlight captured by the Inouye Solar Telescope. It collects data on the magnetic field strength and the solar plasma flow velocity at the Sun’s visible surface and the surrounding gas layers. During the current technical test phase, VTF is identifying even the smallest structures. The resolution will further improve in subsequent scientific operations after extensive post-processing of the data.
The Inouye Solar Telescope, the largest in the world with a primary mirror diameter of four meters, has been providing highly detailed images of the Sun since 2022. Thanks to the ideal observational conditions on Haleakalā volcano in Hawaii and the application of advanced image stabilization and reconstruction techniques, the telescope reveals even the smallest structures.
Additional scientific instruments are being progressively added to the Inouye Solar Telescope to maximize the amount of data collected from sunlight. These instruments process incoming light by analyzing various wavelength ranges and light polarization states independently. Of the five instruments planned, four are currently operational.
The newest and most powerful addition is the Visible Tunable Filtergraph (VTF), the largest spectro-polarimeter of its kind in the world. VTF has now captured the first images of the Sun as part of its technical commissioning. This milestone is considered a technical "first light" by researchers.
The Inouye Solar Telescope was designed to study the underlying physics of the Sun as the driver of space weather. In pursuing this goal, the Inouye is an ideal platform for an unprecedented and pioneering instrument like the VTF.
Christoph Keller, Director, National Solar Observatory, Max Planck Institute for Solar System
A Look at the Dynamic Nature of the Sun
The VTF team wants to learn more about the star's dynamic nature. The Sun frequently exhibits powerful eruptions that release radiation and particles into space. While these solar events can produce auroras on Earth, they can also disrupt satellites and other technological systems.
Using the Inouye Solar Telescope, VTF will focus on the regions of the Sun where eruptions originate: the photosphere, the visible surface of the Sun, and the chromosphere, the adjacent layer of the solar atmosphere.
A better understanding of the mechanisms behind these eruptions depends on analyzing the complex interactions between hot plasma flows and shifting magnetic fields in these regions. VTF can measure key parameters such as temperature, pressure, magnetic field strength, and plasma flow velocity to provide critical insights.
A Colossus Peering at the Sun
The commissioning of VTF represents a significant technological advance for the Inouye Solar Telescope. The instrument is, so to speak, the heart of the solar telescope, which is now finally beating at its final destination.
Matthias Schubert, Project Scientist, Institute for Solar Physics
VTF is a large and complex instrument. It consists of two levels, weighs 5.6 tons, and occupies a footprint roughly the size of a small garage. Developed over several years at the Institute for Solar Physics in Freiburg, Germany, its installation at the Inouye Solar Telescope began last year. The project took over 15 years to complete, nearly as long as the solar telescope itself.
VTF's primary goal is to capture high-resolution images of the Sun, with the highest possible spectral, temporal, and spatial resolution. The instrument employs two Fabry-Pérot interferometers, which are unique in terms of size and precision, to filter narrow wavelength bands from the incoming visible sunlight.
This allows the sunlight to be spectrally scanned with picometer-level accuracy. Additionally, VTF selects specific polarization states, determining the direction of the light's oscillation.
By capturing two-dimensional images of the Sun for each wavelength and polarization state, VTF can measure temperature, pressure, speed, and magnetic field strength at various altitudes of the Sun. The observational data provides a temporal resolution of hundreds of images per second and a spatial resolution of about 10 kilometers per pixel.
VTF enables images of unprecedented quality and thus heralds a new era in ground-based solar observation.
Sami K. Solanki, Director, Max Planck Institute for Solar System
A First Glimpse
The recently released image uses sunlight with a wavelength of 588.9 nm. It shows a dark sunspot with a detailed penumbra in a region of the solar surface approximately 25,000 km by 25,000 km in size. Sunspots appear on the Sun's surface at varying intervals and are associated with strong magnetic fields that prevent heated plasma from rising from the Sun’s interior. The image has a spatial resolution of 10 km per pixel.
About the Telescope and Instrument
The Daniel K. Inouye Solar Telescope is operated by the National Solar Observatory (NSO) and is supported by the US National Science Foundation (NSF). The Visible Tunable Filtergraph (VTF) spectro-polarimeter was designed and constructed by the Institute for Solar Physics in Freiburg, Germany. Project collaborators include the Istituto ricerche solari Aldo e Cele Daccò (IRSOL) in Switzerland and the Max Planck Institute for Solar System Research (MPS) in Göttingen, Germany.