New Telescope Instrument Will Watch the Sky with 5,000 Eyes

Dark Energy is the mysterious force driving the expansion of the Universe. We don’t know what dark energy is, even though it makes up about 68% of the Universe. And the expansion is accelerating, which only adds to the mystery.

A new instrument called the Dark Energy Spectroscopic Instrument (DESI) will study dark energy. It’s doing so with 5,000 new robotic “eyes.”

Scientists and engineers gave the Mayall Telescope at the Kitt Peak National Observatory 5,000 new fiber-optic eyes. The 4 meter (13 ft.) telescope has been operating since 1973, but is being re-commissioned with one purpose: to explore dark energy as the Dark Energy Spectroscopic Instrument (DESI.) Its 5,000 new eyes will map the distance between Earth and millions of galaxies and quasars.

“The unprecedented maps from DESI will allow us to measure how the universe has expanded over time, to see how gravity and dark energy compete to pull and push material apart.”

Professor Daniel Eisenstein, Center for Astrophysics | Harvard & Smithsonian, DESI co-Spokesperson

DESI is a collaboration between almost 500 researchers at 75 different institutions in 13 countries. It’s taken a decade to get to this first image of light from distant galaxies. But this sample image is only a taste of what’s to come.

At the heart of DESI are the 5,000 fiber optic sensors, each one controlled robotically. The sensors sit behind the focal plane of the lenses that make up the rest of the scope.

DESI automatically points at pre-selected sets of galaxies, gathers their light, and then splits that light into narrow bands of color to precisely map their distance from Earth. It can measure how much the universe has expanded as this light traveled to Earth. In ideal conditions, DESI can cycle through a new set of 5,000 galaxies every 20 minutes.

DESI’s 5000 spectroscopic “eyes” can cover an area of sky about 38 times larger than that of the full moon, as seen in this overlay of DESI’s focal plane on the night sky (top). Dustin Lang, Aaron Meisner, DESI Collaboration/Imagine Sky Viewer; NASA/JPL-Caltech/UCLA; and Legacy Surveys project

“After a decade in planning and R&D, installation and assembly, we are delighted that DESI can soon begin its quest to unravel the mystery of dark energy,” said DESI Director Michael Levi of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), the lead institution for DESI’s construction and operations.

“Most of the universe’s matter and energy are dark and unknown, and next-generation experiments like DESI are our best bet for unraveling these mysteries,” Levi added. “I am thrilled to see this new experiment come to life.”

The interior of the 4 meter Mayall Telescope at Kitt Peak National Observatory in Arizona. Image Credit: Kitt Peak/DESI

DESI’s power comes from its ability to measure 5,000 galaxies at a time, one with each of its robotic fiber optic sensors. Each sensor is the width of a human hair, and in good conditions they can focus on a new target in about two seconds. That means that in only about one minute, DESI can switch from one set of 5,000 galaxies to another set of 5,000 galaxies.

From the sensors, the light from these distant galaxies and quasars travels along the 241 km (150 miles) network of fiber optic cables to spectrographs. The spectrographs split the light into thousands of wavelengths. The Universe is expanding, and distant galaxies are moving away from us faster than nearer galaxies. By splitting the light from these distant galaxies and measuring the stretched wavelengths, scientists can determine how quickly each galaxy is moving away from us.

One of DESI’s ten “petals” or “wedges,” with some of its sensors installed. Each wedge holds 500 fiber optic sensors, with each sensor controlled robotically to target a separate target. All together, DESI can measure 5,000 galaxies or quasars at a time. Image Credit: Berkeley Lab.

“Galaxies aren’t scattered randomly in space, but instead form a complex pattern from which we can learn about the composition and history of the universe.”


DESI uses the velocities and distances of the galaxies to create a 3-D map of the Universe. Because it’s so fast, and can target a set of 5,000 different galaxies and quasars so quickly, DESI will map 20 times more objects than any predecessor. All together, DESI will measure and map 35 million galaxies and 2.4 millions quasars.

Professor Daniel Eisenstein of the Center for Astrophysics | Harvard & Smithsonian serves as co-Spokesperson of the collaboration. In a press release he said, “Galaxies aren’t scattered randomly in space, but instead form a complex pattern from which we can learn about the composition and history of the universe. The unprecedented maps from DESI will allow us to measure how the universe has expanded over time, to see how gravity and dark energy compete to pull and push material apart.”

But DESI’s 3-D map of the Universe will do more than shed light on dark energy. It will produce an enormously valuable data-set of spectroscopic information that can be used to tackle other astrophysical research problems.

Dark energy is the mysterious force that drives the expansion of the Universe, and that expansion is gaining speed. Image Credit: NASA’s Goddard Space Flight Center

Professor Douglas Finkbeiner of the Center for Astrophysics | Harvard & Smithsonian explains, “A beautiful aspect of DESI is the ability to combine precision calibration and novel statistical tools with the raw size of the data set. I look forward to the DESI opportunity not just to study dark energy, but the Milky Way, galaxies, quasars, and everything in between.”

Designing, building, installing, and testing a device like DESI is an enormous undertaking. But from another perspective, the work only really starts once scientists get their hands on DESI’s data. And as we’ve seen with some undertakings, like the Cassini mission for example, the scientific effort to understand the data continues long after the mission is over. And that data can be full of surprises.

“I’m really excited to see what we find that we weren’t even looking for.”

Parker Fagrelius, Project Scientist, Berkeley National Laboratory

“These large projects take years to construct, years to operate, and years to understand,” Eisenstein says. “DESI wouldn’t be possible without the commitment of hundreds of people, coming together collectively to pursue these goals. In many ways, that’s the joy and legacy of these surveys.”

“The amount of data we’ll be able to collect I think is going to open up even more questions and answers in astrophysics and cosmology that we don’t yet know to ask,” said Parker Fagrelius, Project Scientist at the Berkeley National Laboratory. “I’m really excited to see what we find that we weren’t even looking for.”


Evan Gough

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