Galileo Galilei used glass lenses to build the first telescope capable of studying the night sky in 1609, improving on a Dutch invention. However, mirrors took over soon after Isaac Newton built the first reflecting telescope later that century: today, astronomy is dominated by telescopes with giant mirrors up to 10 meters wide.
With Dragonfly, a telescope in New Mexico made up of two fly-eye arrays of 24 Canon telephoto lenses, Galileo’s approach has made a comeback. Last month, the Dragonfly team announced a $3.65 million upgrade that will add 120 more lenses, making it the world’s largest lens-based telescope in terms of light-gathering power. Dragonfly promises to capture the faint glow of vast, tenuous gas clouds that hold clues to the universe’s unseen dark matter, with a field of view far greater than that of an equivalent reflecting telescope. “Dragonfly is going to provide a completely new view of the universe,” says team member Deborah Lokhorst of the University of Toronto (UT).
Large telescope mirrors excel at gathering photons from the distant universe and zooming in on particular objects with sharp resolution. But they tend to have small fields of view, and scattered starlight from internal reflections can swamp the faint signals from extended structures like diffuse nebulae.
A decade ago, UT astronomer Roberto Abraham and Yale University astronomer Pieter van Dokkum discovered that an array of dozens of commercial telephoto lenses could gather as much light as a 1-meter telescope while maintaining the lenses’ wide fields of view. Canon also had just produced a 14-centimeter-wide lens with a special coating that reduced scattered light. Abraham, van Dokkum, and their colleagues were able to find dim objects that were hiding in plain sight by pointing all 48 lenses at the same part of the sky and stacking their exposures. “Dragonfly is a very interesting project,” says Joop Schaye, a Leiden Observatory astronomer. “The low–surface-brightness universe has long been neglected.”
So far, Dragonfly’s main claim to fame has been the discovery of numerous ultra-diffuse galaxies (UDGs), some of them as large as the Milky Way, but with hardly any stars. In 2016, the team discovered one that is spinning far faster than it should, given how few stars there are to hold the galaxy together by gravity. That suggested that fully 98% of its mass must be in the form of dark matter, the mysterious stuff that, on average, constitutes some 85% of all gravitating mass in the universe.
In 2018, the Dragonfly team discovered the fast-spinning galaxy’s opposite: a galaxy rotating so slowly that it must contain hardly any dark matter at all. “UDGs are very interesting galaxies,” says Reynier Peletier, an astronomer at the University of Groningen who is looking for similar objects with the much larger European VLT Survey Telescope in Chile. “One way or the other, they may hold the key to the mystery of dark matter.” Indeed, van Dokkum says, the contrasting galaxies discovered by Dragonfly are already posing problems for theories that try to explain away dark matter by modifying the laws of gravity.
The new lenses that will be installed over the next 18 months will allow the Dragonfly team to pursue a new target: faint gas clouds surrounding galaxies. These clouds are the dense ends of gaseous filaments that connect far-flung galaxies in a cosmic web which is thought to have coalesced around regions rich in dark matter. To make these wisps of hot, tenuous gas stand out more clearly, the new lenses’ filters will transmit only the feeble red light of glowing hydrogen or the green glow of ionized oxygen.
The observations will help the team’s understanding of how gas flows into galaxies to form new stars, as well as how stellar explosions blow gas back into space. They will also help guide efforts to model the properties of the dark matter–laden cosmic web and how galaxies took shape within it.
Abraham hopes to expand Dragonfly to a 2000-lens array, giving it the same light-gathering power as a 6-meter telescope, in order to map the much fainter filaments of the cosmic web that lie farther away from galaxies. “It’s not crazy,” he says. “Dragonfly could well be the first example of a completely new class of optical telescope.”
READ MORE: The Beginner’s Guide to Choosing a Telescope