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.”
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. Certain features, like solar magnetic fields, are not obvious just by looking at the light’s colors; but if the light is polarized in a particular way, and we are able to measure it, that 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 us to get an unprecedented full picture from the light we receive 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; 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.
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.
In order to access the necessary measurements to make crucial predictions a reality, we need cutting-edge instruments manufactured with atomic precision. The pioneering image spectro-polarimeter VTF is an example of the necessary technological leaps needed to increase our ability to produce reliable space weather predictions.
More information can be found online at www.nso.edu.
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About the U.S. NSF Daniel K. Inouye Solar Telescope
The U.S. National Science Foundation (NSF) Daniel K. Inouye Solar Telescope is operated by the NSF National Solar Observatory (NSO), a federally funded research and development center focused on solar research, under management by the Association of Universities for Research in Astronomy (AURA). The Inouye and NSO are funded by NSF through a cooperative agreement with AURA. The Inouye Solar Telescope is located on land of spiritual and cultural significance to Native Hawaiian people. The use of this important site to further scientific knowledge is done so with appreciation and respect. For more information, visit www.nso.edu.
The Inouye is the largest solar telescope in the world. With a focus on understanding the Sun’s explosive behavior, observations of magnetic fields are at the forefront of this innovative telescope. A combination of an off-axis design, to reduce scattered light, and cutting-edge polarimetery produces the first ongoing measurements of the magnetic fields in the Sun’s corona. The Inouye’s 4-meter mirror provides views of the solar atmosphere like we have never seen before. Focusing on small observing changes, the cutting-edge instrument suite gathers unprecedented images from the Sun’s surface to the lower solar atmosphere. The Inouye Solar Telescope reveals features three times smaller than anything we can see on the Sun today, and does so multiple times a second. Not only do the world-class instruments and optical assembly allow spectacular imagery, but also have incredible spectroscopic capabilities. Observing the specific fingerprints of hundreds of atoms and ions throughout the solar surface and atmosphere will help us explain the dynamic nature of the Sun’s behavior.
About the Institut für Sonnenphysik (Institute for Solar Physics)
The Institut für Sonnenphysik (KIS) is a state and federally funded research facility located in the city of Freiburg in the south of Germany. Its scientific focus is astronomy and astrophysics with a particular interest in solar physics. The main pillars are fundamental research, operations of the German solar telescope infrastructure on Tenerife (Spain), scientific instrument development, and science data infrastructure. Furthermore, the institute is strongly engaged in the education of students of all levels at Freiburg University.
The mission of the institute is to advance scientific knowledge of the Sun, stars, and their space environments by developing state-of-the-art theories and instrumentation for the largest ground-based solar telescopes at the frontier of what is scientifically and technically feasible, and by providing the global solar physics community with access to these developments. The latest achievement was the successful development of the VTF for the Inouye Solar Telescope.
To advance the understanding of the sun, a wealth of science-ready data will be generated and interpreted through observations performed on ground-based solar telescopes, their instruments and sophisticated data analysis methods and pipelines developed at KIS. The aim is to convey this field of research, which is important for the understanding of the effect of solar activity on space weather and thus on Earth, also to young researchers and to the general public. Along those lines, the institute also develops and provides access points to calibrated observational data for scientific research for free.
About the U.S. NSF National Solar Observatory
The mission of the U.S. National Science Foundation (NSF) National Solar Observatory (NSO) is to advance knowledge of the Sun, both as an astronomical object and as the dominant external influence on Earth, by providing forefront observational opportunities to the research community. The mission includes the operation of cutting edge facilities, the continued development of advanced instrumentation both in-house and through partnerships, conducting solar research, and educational and public outreach. NSO is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF.
Contact
For media inquiries, please contact:
Evan Pascual
Communications Specialist
U.S. NSF National Solar Observatory
media@nso.edu
For technical inquiries, please contact:
Dr. Friedrich Woeger
Instrument Systems Scientist
U.S. NSF National Solar Observatory
fwoeger@nso.edu
Dr. Stacey Sueoka
Senior Optical Engineer
U.S. NSF National Solar Observatory
ssueoka@nso.edu
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