Inside DESI, an Ambitious Project to Map the Universe in 3D

DESI, the Dark Energy Spectroscopic Instrument, aims to map the universe in three dimensions and shine a light on the mysterious force of nature we call dark energy. Its five-year sky survey will begin in 2020, but the project achieved an important milestone this fall when collaborators started assembling key pieces of equipment at Kitt Peak National Observatory in Arizona. A picture may be worth a thousand words, but exploring DESI by pictures and numbers offers an in-depth glimpse into this unique, ambitious instrument—and a peek at the excitement to come.

An illustration of what DESI will look like once fully installed on the 4-meter Mayall telescope at Kitt Peak National Observatory. The corrector is shown in gray, pointing out of the dome opening, and the spectrographs are shown in yellow.
Image Credit: R. Lafever, J. Moustakas/DESI Collaboration.

45 years ago, a world-class 4-meter telescope named for astronomer Nicholas U. Mayall turned toward the sky for the first time. Today, the Mayall telescope is in the midst of a complete overhaul. Despite its age and the rapid pace at which astronomy has progressed, the telescope is headed for world-class status once again: the instrument is on the edge of creating the most detailed map of the sky in history. Scientists hope that this map will provide important insight into dark energy, the mysterious force causing the expansion of the universe to accelerate.

Left: Astronomer Nicholas U. Mayall looks through the telescope that would soon bear his name, spring 1973. Right: The Mayall telescope.
Image Credit: (R) Tom Eglin/NOAO/AURA/NSF, (L) Mark Hanna/NOAO/AURA/NSF.

35 million galaxies and quasars will be mapped by the telescope once it’s fully outfitted with DESI. The goal of DESI, as its name implies, it to learn more about dark energy through spectroscopy—the measurement of the light emitted by an object (aka its spectrum). From this information, astronomers can determine the position of a galaxy in three dimensions, and how quickly it is moving away from us, a value described by its redshift. Therefore, by measuring the spectra of millions of objects, astronomers can map how matter is distributed in the universe and find the relationship between distance and redshift in the early universe. Since dark energy influences the expansion of the universe, these results will help astronomers refine models of dark energy.

The Hubble Ultra Deep Field Image contains nearly 10,000 galaxies.
Image Credit: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team.

2.4 metric tons—that’s how much “the corrector” weighs. The corrector is a piece of equipment that has replaced the original top end of the Mayall telescope (the original end was lifted off of the telescope by crane and taken out through the dome opening). The corrector has six large lenses and significantly increases the telescope’s field of view, enabling DESI to image a large area of sky. The corrector traveled from its construction site in London to Arizona over the summer, was installed this fall, and will be tested this winter. Light from the sky will enter DESI through the corrector and be focused onto a focal plane.

A team at a vendor in Santa Rosa, California poses behind a lens that now sits in the corrector.
Image Credit: VIAVI Solutions © 2010 The Regents of the University of California, through the Lawrence Berkeley National Laboratory.

10 petals fit together like pie-shaped wedges to form the focal plane. Each petal is packed with robotic positioners that will eventually collect light from distant galaxies and send it by a fiber optic cable to one of DESI’s ten spectrographs, where the actual measurements are made and the data is processed. One of the spectrographs has already been shipped to the observatory, the rest will be soon on their way from their building site in France.

One of ten petals machined for DESI. These were taken before the robotic positioners were added, except for the one shown on the bottom right.
Image Credit: Joe Silber/Berkeley Lab.

5,000 robotic positioners—500 per petal—are the key to mapping so many objects in just five years. Each positioner can be independently controlled and collects data from a preprogramed set of objects. Because they are packed so closely together, the motion of the positioners must be choreographed in a precise dance. To-date, DESI scientists have finished installing positioners on seven of the ten petals.

A petal fully stocked with 500 slender robotic positioners.
Image Credit: R. Lafever, J. Moustakas/DESI Collaboration. © 2010 The Regents of the University of California, through the Lawrence Berkeley National Laboratory.

600 researchers from more than 70 institutions and 10 countries are collaborating on this intricate project, with Lawrence Berkeley National Laboratory taking the lead. To learn more, check out Berkeley Lab’s new video, 5,000 Robots Merge to Map the Universe in 3D.

Kitt Peak. The Mayall telescope is in the largest dome, 18 stories tall.
Image Credit: Jeremy Keith (CC BY 2.0)

Kendra Redmond

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