World’s largest solar telescope takes a look at the Sun

Designed and built by the Institute for Solar Physics (KIS) in Freiburg, Germany, with support from the Max Planck Institute for Solar System Research (MPS) in Göttingen, the VTF will allow the Inouye Solar Telescope to examine the Sun’s surface and surrounding gas layers with unprecedented precision. The latest data were obtained during the technical commissioning of the instrument.

VTF analyses sunlight in greater detail than ever before, extracting key information such as plasma flow velocities and magnetic field strengths at and above the Sun’s visible surface. Even during the current test phase, the instrument has already been able to reveal extremely small structures, with resolution expected to improve further once scientific operations and post-processing of data are fully underway.

With its four-metre primary mirror, the Inouye Solar Telescope is the largest of its kind globally. Positioned on the Hawaiian volcano Haleakalā, it benefits from optimal observational conditions and advanced image stabilisation and reconstruction techniques. Since 2022, it has offered detailed observations of the Sun, revealing intricate structures invisible to previous solar observatories.

To expand its capabilities, the telescope is being equipped with a series of scientific instruments designed to analyse incoming light by separating its different wavelength ranges and polarisation states. Of the five instruments planned, four are already operational. The VTF is the most recent and the most powerful addition, representing a new benchmark in solar spectro-polarimetry.

During its technical commissioning phase, researchers recorded the instrument's first images of the Sun, referring to this achievement as a ‘technical first light’.

Christoph Keller, Director of the National Solar Observatory, which operates the Inouye Solar Telescope, commented: “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.”

Examining the dynamic Sun

The VTF project team aims to deepen understanding of the Sun’s dynamic nature. Solar eruptions, which frequently eject particles and radiation into space, can create striking auroras on Earth but also pose risks to satellites and critical infrastructure.

The VTF enables the Inouye Solar Telescope to closely investigate the regions of the Sun where such eruptions originate: the photosphere and the chromosphere. By examining the complex interactions between plasma flows and magnetic fields, researchers hope to gain greater insight into the mechanisms that trigger these events. VTF can precisely measure parameters such as plasma flow velocity, magnetic field strength, pressure, and temperature.

A colossal achievement

Matthias Schubert, VTF Project Scientist at KIS, stated: “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.”

Weighing 5.6 tonnes and spanning an area similar to that of a small garage, the VTF occupies two floors of the observatory. Development work at the Institute for Solar Physics began nearly 15 years ago, paralleling the construction timeline of the Inouye Solar Telescope itself. Installation of the VTF at the telescope site commenced early last year.

The VTF’s mission is to capture the highest possible spatial, temporal, and spectral resolution images of the Sun. It uses two Fabry-Pérot interferometers – unique in their size and precision – to filter extremely narrow wavelength bands from visible sunlight, allowing spectral scanning with picometre-level accuracy. In addition, the instrument isolates different polarisation states of light, creating two-dimensional solar images for each measured wavelength and polarisation. These images reveal temperature, pressure, speed, and magnetic field variations across different solar altitudes.

Sami K. Solanki, Director at the MPS, added: “VTF enables images of unprecedented quality and thus heralds a new era in ground-based solar observation.”

First images released

The first image published used sunlight at a wavelength of 588.9 nanometres and depicted a dark sunspot with its intricate penumbra over a solar surface area of approximately 25,000 kilometres by 25,000 kilometres. Sunspots, which appear with varying frequency, are regions associated with intense magnetic fields that inhibit the rise of hot plasma from within the Sun. The image reached a spatial resolution of 10 kilometres per pixel.

About the telescope and the instrument

The Daniel K. Inouye Solar Telescope is funded by the US National Science Foundation (NSF) and operated by the National Solar Observatory (NSO). The Visible Tunable Filtergraph (VTF) was developed and built by the Institute for Solar Physics in Freiburg, Germany, with the Max Planck Institute for Solar System Research (MPS) in Göttingen, Germany, and the Istituto ricerche solari Aldo e Cele Daccò (IRSOL) in Switzerland contributing as project partners.