World’s largest imaging spectro-polarimeter achieves first light

30 Apr 2025

New Visible Tunable Filter is fitted on the NSF Daniel K. Inouye Solar Telescope, in Hawaii.

The U.S. National Science Foundation Daniel K. Inouye Solar Telescope – the world’s most powerful solar telescope – operated by the U.S. NSF National Solar Observatory (NSO), which is located near the summit of Maui’s Haleakalā, Hawaii, has achieved “first light” with its most advanced instrument – the new Visible Tunable Filter (VTF).

The solar image it has produced shows early promise to the instrument’s scientific capabilities. Designed and built by the Institut für Sonnenphysik (Solar Physics; “KIS”), in Freiburg, Germany, the VTF is the world’s largest imaging spectro-polarimeter.

After arriving in 2024, the KIS team, in collaboration with the NSF NSO scientists, rebuilt and integrated the VTF into the Inouye’s Coudé Lab, marking the completion of the telescope’s originally designed suite of five 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 (6.2 miles) per pixel. Sunspots, areas of intense magnetic activity, often lead to solar flares and coronal mass ejections. The image (below), taken during technical testing as part of first light, “shows early promise for the VTF’s full capabilities,” said the NSF statement.

VTF is Inouye Solar Telescope’s heart

“After all these years of work, VTF is a great success,” 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.” Dr. Matthias Schubert, KIS VTF Project Scientist, added, “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.”

The VTF is an imaging spectro-polarimeter that captures two-dimensional snapshots of the Sun at specific wavelengths. 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 wavelengths.

By adjusting this spacing at the nanometer scale, the VTF sequentially scans different wavelengths, similar to taking a series of photographs with different color filters. It takes several hundred images in just a few seconds with three high-accuracy synchronized cameras, then 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 the end of 2025.

Dr. Stacey Sueoka, Senior Optical Engineer at NSO, said, “This achievement marks 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. I’m excited to see what’s possible as we complete the system, integrate the second etalon, and move toward science verification and commissioning.”

Polarimetry explained

Additionally, light moves in waves that can oscillate in different directions. Polarimetry is the technique of measuring the direction in which these lightwaves oscillate. When you combine spectroscopy and polarimetry, you are not just looking at the colors of the light – you are also figuring out how lightwaves’ oscillations are oriented at each color.

If the light is polarized in a particular way, it can reveal hidden details about the solar magnetic field, which is crucial for understanding solar flares, and space weather. The VTF, with its unparalleled combination of imaging, spectral, and polarimetric capabilities, allows observers to obtain an unprecedented full picture from the light received from the Sun.

The central mission of the VTF is to spectroscopically isolate narrow-band images of the Sun at the highest possible spectral, spatial and temporal resolution provided by the Inouye – i.e., a spectral resolution able to resolve a range of wavelengths as small as 1/100,000th of the center wavelength.

VTF’s polarimetric capabilities enable users to measure the polarization of the light coming from the imaged areas, and thus, infer its magnetic properties. By correlating all this information – spatial, temporal, spectral, and magnetic – the team will gain an unprecedented understanding of the nature of the Sun, and the mechanisms driving solar phenomena.