Realm of the mountain king
From afar, the dome of the MMT Observatory atop Mount Hopkins strikes a regal profile. It lies 90 miles (145 km) southwest of Mount Graham and 45 miles (75 km) southeast of Kitt Peak, sitting alone on the peak like a mountain king on a craggy throne.
The exposed position has a benefit, explains Grant Williams, the director of the MMT, which is part of the Fred L. Whipple Observatory on Mount Hopkins. “When there are prevailing winds, it’s a laminar [smooth] flow over the telescope rather than being turbulent,” he says — which means better seeing and image clarity.
When the MMT saw first light in 1979, it was the third-largest optical telescope in the world. Then, the telescope comprised six separate mirrors left over from a canceled military project; its acronym stood for Multiple Mirror Telescope. However, in 2000, the MMT was converted to a single mirror. And in 2002, it was enhanced with the world’s first adaptive optics system to use a deformable secondary mirror for correcting atmospheric turbulence. Previous systems made corrections further down the line in the optical train, but deforming the secondary mirror itself sends sharp images directly to science instruments.
On the night I visited, prevailing southwesterlies topped 35 mph (56 km/h). These gusts weren’t a problem, but computer gremlins were a showstopper. An instrument switchover earlier in the day from an infrared imager to a spectroscope was creating software issues that ultimately scrubbed operations for the night.
On a ridge 1,000 feet (300 m) below, though, three smaller domes housing 1.2-, 1.3-, and 1.5-meter telescopes were in business, searching for exoplanets. Several smaller scopes operating from a low hut and a nearby array of individually housed commercial scopes also took part in the hunt.
“The Ridge is almost all exoplanets,” says Pascal Fortin, director of Whipple Observatory. The 1.2-meter telescope performed the initial sky survey for Kepler, NASA’s wildly successful planet-hunting mission. “We found the best places for Kepler to look, and it discovered over 3,000 in that small field,” he says.
Breaking light’s speed limit
The Ridge was also the site for Whipple’s very first astronomical instrument, a 10-meter gamma-ray detector installed in 1968. In 2007, its successor was put in. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is located at the base of Mount Hopkins and consists of four detectors. Each features a segmented 12-meter mirror with a longer focal length and less optical aberration.
Gamma rays are the most energetic form of radiation on the electromagnetic spectrum. VERITAS focuses on the most extreme gamma rays of all, so powerful they can’t be reproduced in a lab. When these rays hit our atmosphere, they create thousands of electrons and positrons that exceed the speed of light in air. (The speed of light when unimpeded in a vacuum is still the ultimate speed limit of the universe.)
Similar to a sonic boom, breaking the local light-speed limit generates a brief burst of blue light called Cherenkov radiation. A cascading chain of electron-positron pairs collides with more air molecules, producing a glowing shower the size of two football fields, some 6 miles (10 km) above the ground. It lasts only nanoseconds, but VERITAS reacts just as quickly, capturing data about the extreme environments where such gamma rays are born: supernovae, neutron stars, and black holes.
By placing the four VERITAS telescopes some 300 feet (90 m) apart, scientists can pinpoint where the gamma rays originated as well as minimize superfluous noise from other particles. Each VERITAS telescope uses 350 mirrors to focus light on a single camera containing a dazzling array of 499 photomultiplier tubes, one per pixel.
VERITAS is at the base of the mountain because “there is a sweet spot of how far away you want to be from those showers,” Qi Feng, an astrophysicist and postdoc working at Whipple, explains. “If we are at 4 kilometers [2.5 miles] on the mountain, the showers will be right above us. That’s going to be huge, and it’s going to be very hard to design the optics to catch those. So, we can’t be too high.”