Largest Imaging Spectro-Polarimeter Achieves First Light at the NSF Daniel K. Inouye Solar Telescope

Undulating waves and cells of gold, orange and red fill this image. Dark black areas are surrounded by deep red and orange indicating sunspots. — A narrow-band image of the Sun at a wavelength of λ=588.9nm – 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 – and imply how thoroughly they can be examined by combining all data (image, spectroscopy, and polarimetry) available from the VTF. Each pixel in the original version of the image corresponds to 10 km (or 6.2 miles) on the Sun. Credit: VTF/KIS/NSF/NSO/AURA

Maui, HI – The U.S. National Science Foundation Daniel K. Inouye Solar Telescope, the world’s most powerful solar telescope, operated by the NSF National Solar Observatory (NSO) near the summit of Maui’s Haleakalā, reached a major milestone: achieving first light with its most advanced instrument, the new Visible Tunable Filter (VTF). The solar image it produced shows early promise to the instrument’s scientific capabilities. Designed and built by the Institut für Sonnenphysik (KIS) in Freiburg, Germany, the VTF is the world’s largest imaging spectro-polarimeter, emerging as a centerpiece to the Inouye’s instrument suite.

First Light Achieved

After arriving last year, the KIS team, in collaboration with NSF NSO scientists and engineers, rebuilt and integrated the VTF into the Inouye’s Coudé Lab, marking the completion of the telescope’s originally designed suite of five first-generation instruments. Following extensive optic calibration and alignment, the team successfully carried out the instrument’s first on-Sun observations.

The newly released image reveals a cluster of sunspots on the Sun’s surface with a spatial sampling of 10 km (or 6.2 miles) per pixel. Sunspots, areas of intense magnetic activity, often lead to solar flares and coronal mass ejections. This image, taken during technical testing as part of first light, shows early promise for the VTF’s full capabilities. While it is not yet fully operational, science verification and commissioning are expected to begin in 2026. The Inouye was built for instruments like the VTF – of such magnitude that it took over a decade to develop. These successful first light observations underscore the unique quality and functionality of the instrument, setting the stage for exciting findings in solar physics in the coming decades.

“After all these years of work, VTF is a great success for me,” said Dr. Thomas Kentischer, KIS Co-Principal Investigator and key architect behind the instrument’s optical design. “I hope this instrument will become a powerful tool for scientists to answer outstanding questions on solar physics.”

“The significance of the technological achievement is such that one could easily argue the VTF is the Inouye Solar Telescope’s heart, and it is finally beating at its forever place,” added Dr. Matthias Schubert, KIS VTF Project Scientist.

The Instrument

The VTF is an imaging spectro-polarimeter that captures two-dimensional snapshots of the Sun at specific wavelengths. Different wavelengths of light appear to our eyes as different colors – and light increases in wavelength as it moves from violet to red in the optical range of the electromagnetic spectrum. Unlike traditional spectrographs that spread light into a full spectrum like a rainbow, the VTF uses an etalon – a pair of precisely spaced glass plates separated by tens of microns – that allows it to tune through colors. By adjusting this spacing at the nanometer scale (i.e., as tiny as a billionth of a meter), the VTF sequentially scans different wavelengths, similar to taking a series of photographs using different color filters. It takes several hundred images in just a few seconds with three high-accuracy synchronized cameras, at different colors, and combines these images to build a three-dimensional view of solar structures and analyzes their plasma properties.

The VTF features the largest Fabry-Pérot etalons ever built for solar research, with a second etalon expected to arrive from KIS by year’s end.

“Seeing those first spectral scans was a surreal moment. This is something no other instrument in the telescope can achieve in the same way,” said Dr. Stacey Sueoka, Senior Optical Engineer at NSO. “It marked the culmination of months of optical alignment, testing, and cross-continental teamwork. Even with just one etalon in place, we’re already seeing the instrument’s potential. This is only the beginning, and I’m excited to see what’s possible as we complete the system, integrate the second etalon, and move toward science verification and commissioning.”

Why It Matters

“When powerful solar storms hit Earth, they impact critical infrastructure across the globe and in space. High-resolution observations of the sun are necessary to improve predictions of such damaging storms,” said Carrie Black, NSF program director for the NSF National Solar Observatory. “The NSF Inouye Solar Telescope puts the U.S. at the forefront of worldwide efforts to produce high-resolution solar observations and the Visible Tunable Filter will complete its initial arsenal of scientific instruments.”

The Sun is a plasma laboratory right on our doorstep. Everyone is familiar with aurorae, for instance, which show the influence of solar activity on Earth – a consequence of energy and small particles released by the Sun interacting with our planet’s magnetic field. Similar to weather forecasts on Earth, it should be possible to predict the geomagnetic disturbances caused by energy eruptions on the Sun responsible for these beautiful aurorae – which can also have other unwelcoming implications. Space weather refers to the changing conditions in space, driven by the Sun’s behavior, that affect Earth and space-based technologies. On our increasingly technological Earth, sudden solar storms can cause devastating damage to critical infrastructure, and disable large portions of the electrical power grid, communications networks, or space systems.

“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,” said Christoph Keller, NSO Director.