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The Daniel K. Inouye Solar Telescope, the largest solar telescope globally, located in Hawaii, has reached a significant milestone in solar observation. After nearly 15 years in development, the Visible Tunable Filtergraph (VTF), an advanced instrument created by the Institute for Solar Physics (KIS) in Freiburg, Germany, has successfully captured its first images. This project also involves collaboration with the Max Planck Institute for Solar System Research (MPS) in Göttingen, Germany.
The recently published data were obtained during the instrument’s technical commissioning phase. The VTF provides an unprecedented analysis of sunlight, extracting critical information about solar plasma flow velocities and magnetic field strengths at the visible surface of the Sun and in adjacent layers of gas. Even during this initial testing phase, the VTF is capable of revealing incredibly small structures. Once fully operational and with further data processing, the resolution is expected to increase even more.
With a primary mirror measuring four meters in diameter, the Inouye Solar Telescope holds the title for the largest telescope of its kind. Its location on the Hawaiian volcano Haleakala offers optimal observational conditions, complemented by advanced methods for image stabilization and reconstruction. Since starting operations in 2022, the telescope has begun to deliver astonishingly detailed observations of the Sun, uncovering its smallest structures. In an effort to maximize the amount of information obtained from solar observations, the Inouye Solar Telescope is systematically incorporating additional scientific instruments. Four out of five planned instruments are operational, with the VTF being the most advanced. The recent images captured mark a significant milestone, referred to by researchers as a “technical first light.”
“The Inouye Solar Telescope is designed to delve into the fundamental physics of the Sun and its role in space weather. The VTF is an exceptional instrument ideal for this endeavor,” remarked Christoph Keller, Director of the National Solar Observatory, which oversees the telescope’s operations.
Exploring the Sun’s dynamic processes
The primary objective of the VTF team is to gain deeper insights into the dynamic processes occurring on the Sun. The Sun frequently experiences powerful eruptions that eject particles and radiation into space, leading to beautiful auroras on Earth while potentially disrupting technological infrastructure and satellites. The VTF aims to investigate the regions from which these eruptions originate: the visible surface of the Sun, known as the photosphere, and the adjacent chromosphere. Understanding the interactions of hot plasma flows and fluctuating magnetic fields in these areas is crucial for comprehending the mechanisms that trigger solar eruptions. Key properties such as plasma flow velocity, magnetic field strength, pressure, and temperature can be precisely measured using the VTF.
A monumental instrument for solar observation
The introduction of the VTF marks a significant technological leap for the Inouye Solar Telescope. “The commissioning of the VTF is like giving the solar telescope its heartbeat at last,” stated Matthias Schubert, VTF project scientist at KIS.
Weighing in at 5.6 tons and occupying a space comparable to a small garage, the VTF was conceived at KIS and its installation at the Inouye Solar Telescope began at the start of last year. The total development process spanned approximately 15 years, coinciding closely with the overall development time of the telescope itself.
The VTF’s function is to capture images of the Sun with the highest possible spatial, temporal, and spectral resolution. Two unique Fabry-Pérot interferometers enable the instrument to isolate narrow wavelength ranges from the incoming sunlight, allowing for spectrally scanning the sunlight with remarkable accuracy. Furthermore, the VTF can select specific polarization states of light, resulting in two-dimensional images for each wavelength and polarization. This data contributes to determining temperature, pressure, velocity, and magnetic field strength at various altitudes on the Sun, achieving a spatial resolution of around 10 kilometers per pixel and capturing hundreds of images per second.
“The VTF allows us to achieve images of unparalleled quality, marking a new era in ground-based solar observation,” emphasized Sami K. Solanki, director at the MPS.
A snapshot of solar activity
The initial published image recorded by the VTF utilizes sunlight at a wavelength of 588.9 nanometers, depicting a dark sunspot along with its intricately structured penumbra over a section of the solar surface measuring around 25,000 kilometers on each side. Sunspots appear on the Sun’s surface with varying frequency and are linked to particularly strong magnetic fields that inhibit the rise of hotter plasma. The image maintains a fine spatial resolution of 10 kilometers per pixel.
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