The sun hangs low over the expansive plains of Castile and León, nestled in the north-central region
of Spain. But from a rugged slope of the Pico Trigaza mountain, it appears that a few stubborn, puffy white clouds have begun to drift across the solar disk. For the small team gathered here, it is a vexing development.
A group of researchers, some wearing cardboard eclipse glasses, clusters nervously around sets of portable telescopes and cameras connected to glowing laptops. All of this precision gear is scattered across makeshift tabletops perched precariously on the mountainside. On the computer screens, the sun’s disk resembles a deep orange crescent that is slowly waning. That is because the moon began to crawl across it roughly half an hour ago. In just 25 minutes, it will obscure the sun entirely, producing an eerie, wondrous phenomenon: a total solar eclipse.
In the oppressive late-summer, late-afternoon heat, the researchers watch anxiously as those fluffy clouds linger stubbornly around the sun. From an abandoned stone hut on the slope a few meters above, where the research team has set up base camp, one of the graduate students emerges and nods toward the cloud formation.
“Do you think it will dissolve?” he blurts out, his voice tinged with stress.
“It will,” Miloslav Druckmüller replies, glancing calmly at his doctoral candidates as they perform final equipment checks in preparation for totality.
Here in Spain, on this 12th day of August, Druckmüller is getting ready for his 19th solar eclipse expedition. He knows from long, hard experience that weather conditions can turn on a dime when the moon’s sudden shadow abruptly drops local temperatures. Furthermore, there is little sense in riling up the graduate students any more than they already are.
During totality, the moon blocks the blinding, overwhelming glare of the solar disk, revealing delicate, ghostly structures in the sun’s outer atmosphere, known as the corona. Using custom-built telescope systems and advanced image-processing techniques that Druckmüller has spent decades developing, the researchers aim to gain critical insights into the physical processes governing this enigmatic region, which is otherwise notoriously difficult to study. In addition to imaging the visible coronal light, the team will attempt to track the dispersal of superhot iron ions into interplanetary space, a feat that could yield rare clues regarding the underlying patterns of heat emanating from the corona.
They will have exactly 107 seconds to accomplish all of that.
A Mathematician’s Path to Solar Eclipse Photography
Miloslav Druckmüller is a professor of mathematics and image processing at the Brno University of Technology in the Czech Republic, and he is a leading figure in the Solar Wind Sherpas, an elite international team dedicated to studying the sun’s corona during total solar eclipses. Within the specialized global community of eclipse chasers, he is nothing short of a legend, albeit a deeply reluctant one.
He photographed his first solar eclipse in 1999, embarking on what he initially believed would be a one-off scientific lark. During that event, the moon’s deep shadow narrowly missed Druckmüller’s native Czechia by just a few tens of kilometers, prompting him to chase the phenomenon to Hungary armed with a 30-year-old Soviet MTO 1,000-millimeter catadioptric lens. All he truly remembers from that adrenaline-fueled day is the frantic struggle to locate the sun in the sky using the lens’s minuscule field of view, and desperately trying to keep it in focus during the fleeting two minutes of totality.
He readily insists that he has never genuinely enjoyed any of the eclipses he has photographed.
“In fact, I have never properly seen any of those eclipses,” Druckmüller reflects. “You are just watching the screens, making sure that it’s all doing what it’s supposed to do. There are so many things that can go wrong and you can’t redo it.”
Despite Druckmüller’s outward tranquility on this Spanish hillside, the stakes are undeniably high. Just 45 minutes prior, his group received troubling news from their colleagues stationed in Iceland, where viewing conditions had initially promised to be far superior to those in Spain. Instead, a vast low-pressure system had completely blanketed the Icelandic sky with thick clouds, leaving all three research teams there entirely without a view.
What Solar Eclipses Reveal About the Corona
The solar corona has remained a profound scientific mystery for centuries. Although it is vastly dimmer—by a factor of a million—than the sun’s visible surface, the corona is paradoxically scorching hot, reaching temperatures of about one million degrees Celsius compared to a mere 5,500 degrees on the solar surface itself. Because the sun’s nuclear fusion engine churns intensely at the star’s core, basic intuition might suggest that temperatures should naturally decrease the farther one travels from that central heat source.
To observe the faint corona in visible light, researchers must find a way to block the sun’s ferocious, blinding glare. Specialized telescopes known as coronagraphs accomplish this by employing an opaque disk called an occulter. However, because light inevitably bends around the edges of the occulter through an optical phenomenon called diffraction, the instrument must block an area significantly larger than the solar disk itself to prevent stray light from spilling over and ruining the observations. Unfortunately, doing so means obstructing the innermost part of the corona, which happens to harbor the most crucial physical processes underpinning the puzzling behavior of the sun’s upper atmosphere.
This fundamental limitation is why total eclipses matter so profoundly to solar physicists. By a remarkable stroke of cosmic coincidence, the moon is precisely the right size and situated at just the right distance from Earth to completely cover the solar disk during a total eclipse. Because the moon sits far out in space, the view from the ground is entirely unmarred by instrumental diffraction, allowing the delicate corona to reveal itself nearly all the way down to the sun’s actual surface.
While space probes can image the solar surface in other wavelengths, those spaceborne views also come with distinct limitations. Spacecraft such as NASA’s Solar Dynamics Observatory are capable of viewing the innermost parts of the corona in extreme ultraviolet light, explains Shadia Habbal, a solar physicist at the University of Hawaii and leader of the Solar Wind Sherpas project.
“But it cuts off very quickly, so you don’t see its extension into space as well as we can see it in the visible emission lines” during an eclipse, Habbal notes. “That’s the biggest advantage of solar eclipse photography.”
While the European Space Agency’s Proba-3 mission, launched in 2024, simulates total solar eclipses by flying two spacecraft in tight formation 150 meters apart—with one acting as an artificial occulter and the other serving as the telescope—the instruments aboard that satellite still possess operational limitations compared to the specialized, ground-based equipment deployed by the Solar Wind Sherpas.
How Eclipse Images Sparked a Scientific Partnership
When Druckmüller finally developed the film from his 1999 eclipse expedition, the resulting photographs fell frustratingly short of capturing the event’s eerie, sublime beauty. Refusing to accept the limitation, he spent countless evenings applying his advanced expertise in numerical image processing, attempting to authentically re-create in his photographs the breathtaking sights he had witnessed from that Hungarian cornfield.
It took two arduous years, but he ultimately succeeded. Where previous images showed only dull, lifeless views of a blackened lunar disk surrounded by blunt, featureless wisps of coronal plasma, Druckmüller’s processed photographs revealed the subtlest, most intricate details of magnetic field lines stretching from the sun far out into interplanetary space. As these revolutionary photographs began to circulate among the international community of solar physicists, invitations to join eclipse expeditions started rolling in.
Habbal first learned about the stunning images through a Ph.D. student who had encountered them at a conference presentation that unfortunately failed to credit Druckmüller. When Habbal’s research team subsequently published the images in a scientific paper, they inadvertently misattributed them to another researcher. Druckmüller found out about the error through an astrophysicist friend.
“I wrote her a rather disappointed email and she responded within 10 minutes,” Druckmüller recalls. “She really didn’t have a clue.”
The oversight was promptly corrected, and within weeks, Habbal and Druckmüller embarked on a professional collaboration that soon blossomed into a lasting friendship.
“Milos’s processing brings out details that nobody has seen before, and they tell you a lot about very tiny features in the corona that you don’t see otherwise,” Habbal says.
How Image Processing Reveals the Solar Corona
Capturing the corona’s delicate, multifaceted structures requires finding an innovative way to display both its blindingly bright features and its faintest outer extensions within the exact same image.
“During the solar eclipse, the contrast between the darkest and brightest parts of the corona is enormous, possibly one to a million,” Druckmüller explains. “But a computer monitor only allows you to display a contrast ratio of one to about 800.” While some modern monitors, particularly OLED displays, can deliver higher contrast ratios, the hardware gap remains immense.
The human biological eye naturally adjusts to this massive range of brightness, allowing a terrestrial observer to simultaneously perceive the glowing corona and the faint stars and planets that briefly emerge in the darkened twilight sky. To successfully combine these extremes within a single photograph, Druckmüller employs custom-designed image-processing algorithms to merge dozens of individual exposures that capture varying segments of that dynamic range.
To construct a single finished composite image, Druckmüller’s teams must capture roughly 200 individual frames during the fleeting moments of totality, utilizing exposure times ranging from a fraction of a millisecond to two seconds. The raw data must then be rigorously calibrated against a vast matrix of reference images to systematically filter out electronic noise and optical imperfections.
Flat-field frames, which depict a uniform light background, are utilized to correct optical defects caused by telescope optics, dust particles, and localized variations in pixel sensitivity. Dark frames, captured with light completely blocked from the sensor, help eliminate thermal artifacts generated by the camera’s image sensor itself.
“It takes the computer more than two days to process one image,” Druckmüller notes. When factoring in all necessary calibration data, a single finalized image requires processing approximately 3,000 individual input frames.
Choosing a Location for Solar Eclipse Photography
Since 2008, research teams directed by Habbal and Druckmüller have traveled to virtually every total solar eclipse across the globe, deliberately dispersing across multiple geographic locations to hedge their bets against unfavorable weather conditions. Statistically, about half of the deployed teams return with fully usable scientific data; for the rest, the expedition amounts to a short, highly stressful voyage with empty hands. For this particular 12 August eclipse, three groups were deployed to Iceland while two headed to Spain.
The teams consistently seek observation sites situated as close as possible to the center line of the band of totality, where the moon completely blocks the sun for the longest possible duration. In Spain, the band of totality cut cleanly across the northern portion of the country, and the central axis of the lunar shadow passed within a mere 11 kilometers of Pico Trigaza, promising Druckmüller and his colleagues a precious 107 seconds of totality.
However, duration is only one variable in the complex planning equation. Totality on Pico Trigaza would commence just 51 minutes before sunset, with the sun hanging a mere 8 degrees above the horizon. At such a low altitude, solar rays must travel through a significantly thicker layer of the Earth’s atmosphere than when the sun is positioned high overhead, intensifying the atmospheric light distortion that complicates precision photography. Operating their camp at an altitude of 1,600 meters above sea level on Pico Trigaza would help mitigate the air density, but challenges remained.
“Iceland would have been a much better place for us than Spain because the sun was higher up, some 25 degrees above the horizon,” Druckmüller says wistfully. Nevertheless, Iceland’s notoriously volatile and unpredictable weather made Spain an essential strategic backup.
On the mountain, local authorities and residents had spruced up a rustic, long-abandoned stone hut for the visiting researchers, running heavy-duty power cables from a nearby telecommunication outpost and hauling in a dedicated water tank. Five Ph.D. students drove more than 2,000 kilometers from Brno, spending nearly a week meticulously setting up and testing the delicate equipment. Druckmüller and his wife joined them two days before the celestial event, cooking meals on a small propane grill and sleeping in sleeping bags laid over yoga mats on the stone floor.
“We have plenty of space here to set up our telescopes, beautiful views, and a roof over our heads,” Druckmüller says with a wide grin. “It’s perfect.”
Yet, even the finest equipment and a meticulously vetted site mean virtually nothing if unexpected cloud cover rolls in at the critical moment.
“So much work goes into the preparations and then, if you end up with nothing, you just feel terrible sadness,” says Jana Hoderová, an assistant professor of mathematics at Brno University of Technology and a key collaborator in Druckmüller’s circle.
As the moon continues its relentless crawl in front of the solar disk, the pesky cumulus cloud that kept the entire team on edge finally begins to dissolve into the afternoon air. With only 10 minutes remaining until totality, the collective sense of relief across the camp is palpable.
Mapping the Solar Corona’s Temperature With Iron
The automated countdown sequences programmed into the computers controlling the camera arrays have officially begun. The monitor screens now display only a tiny, glowing orange sliver, and the mountain environment is becoming perceptibly dimmer and cooler.
More than 20 astrophotography cameras—most drawing stable power from a car battery that also buffers the sensitive electronics against any electrical grid instabilities—are locked onto the sun. Alongside the primary equipment capturing the corona’s visible white light, the team has assembled a unique optical instrument consisting of eight paired cameras designed to detect specific emission light spectra generated by various ionized species of iron present within the corona.
These specialized iron-sensing instruments were constructed with remarkable resourcefulness and thrift. A colleague of Druckmüller’s laser-cut the custom aluminum plates required to hold the carefully aligned optical lenses in exchange for nothing more than “two bottles of whisky.” The various mounting brackets and structural attachments were precision 3D-printed right at the Brno University of Technology. Meanwhile, the optical lenses themselves, originally manufactured for the defunct camera brand Contax, were acquired at steep, secondhand discounts on eBay. By far the most expensive components of the entire assembly are the specialized optical interference filters required to isolate the razor-thin wavelengths emitted by the targeted iron ions.
“That’s something nobody else in the world can do,” Druckmüller says with visible satisfaction.
This unique technological setup, codeveloped alongside Habbal’s Hawaii research team, has already empowered scientists to map the precise distribution of temperatures across the entire corona and to discover previously unseen structures—dubbed “smoke rings”—within the coronal plasma. Furthermore, these high-resolution photographs have successfully revealed the critical boundary region where the outward-flowing solar plasma becomes so extraordinarily tenuous that its constituent ions finally cease colliding with one another.
“Iron is the most abundant of the heavy elements that we see in the solar corona,” Habbal explains. “And because the corona is so hot, the iron atoms lose electrons and get ionized. The larger the loss, the higher the temperature.”
For instance, achieving a temperature of approximately 1.6 million degrees Celsius is necessary to produce an iron ion that has been stripped of 12 electrons, whereas a temperature of 1.4 million degrees is sufficient to remove 11 electrons from an iron atom. Once an ion enters a specific electrical state, it retains that exact ionization level even as it streams outward into space embedded within the solar wind. Each of these distinct ionic states emits a sharply defined wavelength of light, allowing the researchers to map not only internal thermal variations within the corona, but also the precise pathways by which these iron ions propagate into interplanetary space and distribute thermal energy. Habbal and her colleagues had spent years attempting to image these elusive iron ions during eclipses, but it wasn’t until Druckmüller joined forces with them that they finally developed the necessary instrumentation and mathematical techniques to achieve success.
Three, Two, One—Totality
With a single minute remaining until totality, the researchers move swiftly to remove the protective solar filters from their camera lenses. It is now 50 minutes before sunset, and the waning sun has begun painting the distant horizon clouds with vibrant shades of orange. Then, with startling suddenness, the evening begins a rapid, premature descent into deep twilight.
Along the razor-thin edge of the fading solar crescent, brilliant points of light known as Baily’s Beads flash brilliantly and then vanish—the final fragments of direct photospheric light peering through deep valleys in the lunar terrain. As totality officially commences, the observers pull off their solar viewing glasses. The warm air rapidly cools. The graduate students remain quietly and intensely focused on their glowing displays.
Druckmüller looks up briefly from his camera rig to survey the darkened landscape. Surrounding the blackened disk of the moon rises the magnificent, sprawling golden halo of the solar corona. In the profound hush that falls across the mountainside, it feels as though even the local wildlife has grown still. The horizon is ablaze with fiery, saturated colors where the deep shadow of the moon terminates, while Venus burns brilliantly in the southwestern sky as a sharp, piercing white pinpoint against the darkened heavens.
The sharp, synchronized sound of dozens of camera shutters instantly fills the crisp mountain air. Then, just as quickly as it descended, the manufactured night begins to recede, bidding farewell with another brilliant cascade of golden Baily’s Beads.
Instantly, the gathered crowd erupts into cheers. The Ph.D. students throw their arms around one another in ecstatic celebration.
“The students did most of the work this time, so I actually allowed myself to really look at it,” Druckmüller reflects warmly. “It was beautiful. Pico Trigaza may not have been the best location for pure science photography, but from an aesthetic point of view, it was extraordinary.”
Yet, for Druckmüller, the culmination of the event marks only the beginning of the real work. While competing eclipse chasers race to rapidly release their imagery, he intends to take a deliberate, meticulous approach. He expects to publish his first processed visible-light images of the corona in approximately two weeks. Processing the complex iron-ion data arrays, however, and unlocking whatever profound astrophysical insights they may ultimately contain, will require months—perhaps even a full year.
“Probably until the next eclipse in 2027,” Druckmüller notes, looking ahead to the next expedition in North Africa.
And who knows? Perhaps by then, he might even manage to truly enjoy the experience.
