Universe Today
Telescopes can have more than one sensor. Those sensors can utilize some of the same infrastructure, like lenses and mirrors, and specialize in collecting different data. A good example of this is the Inouye Solar Telescope (IST). It is the largest solar telescope in the world, with a primary mirror diameter of 4 meters. It also has five separate instruments, four of which are currently in operation. The latest of these to come online is the Visible Tunable Filtergraph (VTF), which just collected its first light according to a press release by the Max Planck Institute for Solar Research, one of the project partners.
The five instruments in the IST all have separate roles to play in the telescope's overarching mission - to image the Sun. Most specialize in a given wavelength - for example, the Visible Broadband Imager (VBI) captures light in the visible ranges to keep track of the Sun's surface and lower atmosphere. In contrast, the Cryogenic Near-InfraRed Spectro-Polarimeter (Cryo-NIRSP) focuses on the near infrared, allowing it to track the Sun's dynamic magnetic fields.
But the VTF is the largest of all the instruments—as the press release puts it, it is the "heart" of the IST. The instrument weighs 5.6 tons and takes up the equivalent space of a two-story garage. It was developed over 15 years by the Institute for Solar Physics in Freiburg. Finally, it captured its "first light," which was translated into a picture capturing a 25,000 km x 25,000km area of the Sun's surface.
Fraser tries to answer the question that IST is collecting data on - how hot is the Sun?
VTF's primary goal is to track the Sun's dynamics to understand space weather better. To do this, it is designed to capture as high-resolution an image as possible as quickly as possible, and it does so more effectively than any other solar telescope. Images from the VTF can cover only 10km in a single pixel, about seven times better than the estimated resolution of the Swedish Solar Telescope, the next closest.
To do so, the VTF relies on two versions of a specialized piece of equipment, a Fabry-Perot Interferometer (FPI). FPIs bounce light between two reflective surfaces, and, by carefully controlling both the distance between the surfaces and the angle at which the light hits the first surface, they separate the light into multiple parallel beams that either interfere with or enhance each other. They have very high spatial resolution and can isolate specific wavelengths down to a picometer, allowing them to capture light information about phenomena in those particular wavelengths.
They are also very fast—FPIs allow the VTF to capture images hundreds of times a second, which is critical when tracking fast-moving transients on the Sun's surface. Ultimately, the VTF uses them to create two-dimensional pictures of each wavelength of interest, allowing researchers to build a "spectral map" of a given area of the Sun and watch the features there progress on time scales of either a few seconds or a few weeks.
We are soon about to get an even higher resolution image of the Sun - Fraser discusses what the last one, from five years ago, meant scientifically.
Ultimately, the real test of the VTF, and the IST more generally, will come when the instrument completes its commissioning process. The recent first light milestone and the image it created are only a part of a larger commissioning project. It is due to wrap up sometime in 2026. At that point, when the VTF fully comes online and starts pumping out data, scientists will be able to tell us further whether the 15-year development cycle to track the Sun's solar dynamics was worth the wait.
Learn More:
Max Planck Institute for Solar System Research - The heart of world’s largest solar telescope begins to beat
UT - Groundbreaking New Maps of the Sun's Coronal Magnetic Fields
UT - New Detailed Images of the Sun from the World’s Most Powerful Ground-Based Solar Telescope
UT - High-Resolution Images of the Sun Show How Flares Impact the Solar Atmosphere