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Capturing the colors of the cosmos

The Dark Energy Spectroscopic Instrument is a new robotic array of 5,000 fiber-optic “eyes” designed to explore the mystery of dark energy, which makes up about 68% of the universe and is speeding up its expansion.

Adapted from a release by Lawrence Berkeley National Laboratory.

A new instrument mounted atop a telescope in Arizona aimed its robotic array of 5,000 fiber-optic “eyes” at the night sky to capture the first images showing its unique view of galaxy light.

It was the first test of the Dark Energy Spectroscopic Instrument, known as DESI, with its nearly complete complement of components. The long-awaited instrument is designed to explore the mystery of dark energy, which makes up about 68% of the universe and is speeding up its expansion.

DESI’s components are designed to automatically point at preselected sets of galaxies and gather their light. In a process known as “spectroscopy,” the light is then split into narrow bands of color to precisely map their distance from Earth and gauge how much the universe expanded as this light traveled to Earth.

In ideal conditions, DESI can cycle through a new set of 5,000 galaxies every 20 minutes. Sarah Eftekharzadeh, postdoctoral researcher, and Kyle Dawson, professor, both in the Department of Physics & Astronomy at the University of Utah, were key contributors to the software that commands DESI to capture the light from each of these 5,000 galaxies. Over the course of months, this amounts to tens of millions of carefully crafted code to satisfy DESI’s program.

This time-lapse footage shows the Mayall Telescope at Kitt Peak National Observatory moving within its dome. DESI is installed on the Mayall Telescope.

“We will be looking at different objects in all parts of the sky to sample as much of the universe as possible,” said Eftekharzadeh. “Over the next few months, the DESI collaboration will be testing the whole system to prove that we can complete this ambitious goal.”

Like a powerful time machine, DESI will peer deeply into the universe’s infancy and early development—up to about 11 billion years ago—to create the most detailed 3-D map of the universe.

Installation of DESI began in February 2018 at the Nicholas U. Mayall Telescope at Kitt Peak National Observatory near Tucson, Arizona. Over the past 18 months, institutions from around the globe shipped a bevy of DESI components to the site and for installation on the telescope.

DESI’s focal plane, which carries 5,000 robotic positioners that swivel in a choreographed “dance” to individually focus on galaxies, is at the top of the telescope. These little robots—which each hold a light-gathering fiber-optic cable that is about the average width of a human hair—serve as DESI’s eyes. It takes about two seconds for the positioners to swivel to a new sequence of targeted galaxies. With its unprecedented surveying speed, DESI will map over 20 times more objects than any predecessor experiment.

The focal plane, which is composed of a half-million individual parts, is arranged in a series of 10 wedge-shaped petals that each contain 500 positioners and a little camera to help the telescope point and focus.

A simulation of simultaneously reconfiguring 37 robotic positioners on the Dark Energy Spectroscopic Instrument.

Eftekharzadeh was key to the development and testing of the software that tells DESI’s “eyes” where to look. She and Dawson designed 15 different observing plans to commission the hardware and software components of  DESI. In all, Eftekharzadeh compiled the list of final destinations for the robots’ movements to target more than 30 million objects in the sky.

DESI follows eBOSS, the recently completed cosmological survey within the Sloan Digital Sky Survey (SDSS). Dawson is the principal investigator of eBOSS, which is currently the largest spectroscopic survey of the universe.

“Looking deep into the universe is like looking into a time machine,” said Dawson. “Light takes longer to travel from the most distant objects, so we are seeing those objects as they were many billions of years ago. By looking at millions of different galaxies over a wide range of distance, we can explore the evolution of the cosmos layer by layer as if peeling an onion.”

By repeatedly mapping the distance to 35 million galaxies and 2.4 million quasars across one-third of the area of the sky over its five-year run, DESI will teach us more about dark energy. Quasars, among the brightest objects in the universe, allow DESI to look deeply into the universe’s past.

DESI will provide very precise measurements of the universe’s expansion rate. Gravity had slowed this rate of expansion in the early universe, though dark energy has since been responsible for speeding up its expansion.

DESI’s 5,000 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). Each one of these robotically controlled eyes can fix a fiber-optic cable on a single object to gather its light. The gathered light collected from a small region in the Triangulum galaxy (bottom) by a single fiber-optic cable (red dot) is split into a spectrum (bottom) that reveals the fingerprints of the elements present in the galaxy and aid in gauging the distance to the galaxy. The test spectrum shown here was collected by DESI on Oct. 22, 2019.

“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.

The DESI collaboration has participation from nearly 500 researchers at 75 institutions in 13 countries.

The focal plane, corrector barrel, and other components add about 10 tons of weight to the Mayall Telescope’s movable arm, which itself weighs about 250 tons and rises 90 feet above the floor in the Mayall’s 14-story dome.

Among the more recent arrivals at Kitt Peak is the collection of spectrographs that are designed to split up the gathered light into three separate color bands to allow precise distance measurements of the observed galaxies across a broad range of colors.

These spectrographs, which allow DESI’s robotic eyes to “see” even faint, distant galaxies, are designed to measure redshift, which is a shift in the color of objects to longer, redder wavelengths due to the objects’ movement away from us. Redshift is analogous to how the sound of a fire engine’s siren shifts to lower tones as it moves away from us.

There are now eight spectrographs installed, with the final two arriving before year-end. To connect the focal plane with the spectrographs, which are located beneath the telescope, DESI is equipped with about 150 miles of fiber-optic cabling.

In the representation of a DESI petal at left that shows its coverage of the night sky, each blue circle represents a single robotic “eye” in the form of an automatically positioned, light-gathering fiber-optic cable. In a test collection of light from the galaxy M33, the positioners in this DESI petal detect the signature of hydrogen emission in the galaxy, as shown in the color map at right. Brighter colors indicate brighter emission.

“This is a very exciting moment,” said Nathalie Palanque-Delabrouille, a DESI spokesperson and an astrophysics researcher at France’s Atomic Energy Commission (CEA) who has participated in the selection process to determine which galaxies and other objects DESI will observe.

“The instrument is all there. It has been very exciting to be a part of this from the start,” she said. “This is a very significant advance compared to previous experiments. By looking at objects very far away from us, we can actually map the history of the universe and see what the universe is composed of by looking at very different objects from different eras.”

Palanque-Delabrouille’s institution, CEA, contributed a specialized cooling system to optimize the performance of the light sensors that enable DESI’s broad color-sampling range.

Gregory Tarlé, a physics professor at the University of Michigan who led the student teams that assembled the robotic positioners for DESI and related components, said it’s gratifying to reach a stage in the project where all of DESI’s complex components are functioning together.

The positioners were installed in the focal plane petals at Berkeley Lab, and after assembly and testing, the completed petals were shipped to Kitt Peak and installed one at a time on the Mayall Telescope.

Eftekharzadeh is excited to see her contributions to the first phase of DESI.

“This is indeed a big honor; when I first started with DESI, I could not guess what my contributions would even be. It is a pleasure to be involved in such a massive human endeavor. I am keen to see how DESI pushes our knowledge further in understanding the interplay between matter and  dark energy in the universe.”