5,000 Fiber-Optic “Eyes” Pointed at the Sky

A view of DESI’s fully installed focal plane, which features 5,000 automated robotic positioners, each carrying a fiber-optic cable to gather galaxies’ light. (Source: DESI Coll.)

A new instrument mounted on a telescope in Arizona aimed its robotic array of 5,000 fiber-optic “eyes” at the night sky on October 22 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, which is now almost completed. The long-awaited instrument is designed to explore the mystery of dark energy, which makes up about seventy percent of the universe and is speeding up its expansion.

DESI’s components are designed to automatically point at preselected sets of galaxies, gather their light, and then split that light into narrow bands of color to precisely map their distance from Earth and gauge how much the universe expanded as this light traveled to us. In ideal conditions DESI can cycle through a new set of five thousand galaxies every twenty minutes.

Top: DESI can cover an area of sky about 38 times larger than that of the full moon, as seen in this overlay of a part of the focal plane on the night sky. Each one of these robotically controlled eyes can fix a fiber-optic cable on a single object to gather its light (the red circle marks the location of a single positioner). Bottom left: the petal that includes the galaxy M33. Blue circles demarcate the region of sky that can be studied by each of the 500 fibers in the petal; each fiber can point to a star or galaxy within its circle. Right: The hydrogen line emission detected by DESI from M33 as a color map. (Source: DESI Coll.)

“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, 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.”

Risa Wechsler, former spokesperson of the DESI collaboration and director of the Kavli Institute for Particle Astrophysics and Cosmology, said, “It’s extremely exciting to see the instrument become a reality and the collaboration get ready for its data. DESI will be truly transformational. It’ll allow us to go eleven billion years back in time and measure ancient light with unprecedented precision. The results will be a big leap in our understanding of dark energy.”

The DESI collaboration has participation from nearly 500 researchers at 75 institutions in 13 countries. The latest milestone marks the beginning of DESI’s final testing before the formal start of observations in early 2020.

A time machine to study dark energy

Like a powerful time machine, DESI will peer deeply into the universe’s infancy and early development to create the most detailed 3D map of the universe yet.

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. In particular, quasars – active galaxies that are among the brightest objects in the universe – will allow DESI to look deeply into the universe’s past.

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

But the DESI data will go beyond determining the rate of cosmic expansion. It shall also shed more light on how the expansion affects the way galaxies and other astrophysical objects form and grow over time.

Pointing 5,000 “eyes” at the sky

Installation of DESI began in February 2018 at the Nicholas U. Mayall Telescope at Kitt Peak National Observatory near Tucson, Arizona. “With DESI we are combining a modern instrument with a venerable old telescope to make a state-of-the-art survey machine,” said Lori Allen, director of Kitt Peak National Observatory at the National Optical-Infrared Astronomy Research Laboratory.

Over the past 18 months, a bevy of DESI components was shipped to the site from institutions around the globe and installed on the telescope. One particular technological masterpiece is DESI’s focal plane, which carries 5,000 robotic positioners that swivel in a choreographed “dance” to focus on individual galaxies.

DESI’s roundish focal plane is made up of ten wedge-shaped petals, each holding 500 robotic positioners that individually target galaxies to collect their light via thin fiber-optic cables. (Source: C. Soto, OIR Lab)

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 ten seconds for the positioners to swivel to a new sequence of targeted galaxies. With its unprecedented surveying speed, DESI will map over twenty times more objects than any previous experiment.

The focal plane, which contains half a million individual parts, is arranged in a series of ten wedge-shaped petals that each contain five hundred positioners and a little camera to help the telescope guide, focus and align (GFA system).

Analyzing ancient light

Among the more recent arrivals at Kitt Peak is the collection of spectrographs that are designed to allow precise redshift distance measurements of the observed galaxies across a broad range of colors.

There are now eight DESI 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.

“This is a very exciting moment,” said Nathalie Palanque-Delabrouille, a DESI spokesperson and an astrophysics researcher at the French Alternative Energies and 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.”

Gregory Tarlé, a physics professor at the University of Michigan who led the student teams that assembled the robotic positioners and related components, said it’s gratifying to reach a stage in the project where all of DESI’s complex components are functioning together. UM delivered a total of 7,300 robotic positioners, including spares. During the production peak, the teams were churning out about fifty positioners a day.

Now that the hard work of building DESI is largely done, Tarlé said he looks forward to its discoveries. “I want to find out what the nature of dark energy is,” he said. “We finally have a shot at really trying to understand the nature of this stuff that dominates the universe.” (Sources: SLAC / AURA)

Link: The Dark Energy Spectroscopic Instrument (DESI), Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA, USA


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