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.
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.
[...] 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.
[...] 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; a spatial resolution that requires 10 km sampling to image the finest details on the sun accessible to the Inouye/VTF; and a temporal resolution of a few seconds within which the instrument acquires hundreds of images.
This means that it can take consecutive images of areas of the Sun by recording just a distinct small range of wavelengths tied to specific properties of solar phenomena. During one single observation, around 12 million spectra are recorded, which can then be used to determine the temperature, pressure, velocity, and magnetic field strength at different altitudes in the solar atmosphere. From this, high-precision velocity and magnetic field maps can be derived to track evolutionary changes of solar phenomena on spatial scales between 20-40,000 km (i.e., 12-25,000 miles).
Finally, it is VTF's polarimetric capabilities that allow us to measure the polarization of the light coming from the imaged areas, and from it, infer its magnetic properties. By correlating all this information – i.e., spatial, temporal, spectral, and magnetic – we get an unprecedented understanding of the nature of our home star, and the mechanisms driving solar phenomena.
"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."
More information can be found online at www.nso.edu.
An example of the sunspot image is provided on the press release.
