Fiber-Optic Cable Image
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. (DESI Collaboration)

A new instrument mounted on a telescope in Arizona aimed its robotic array of 5,000 fiber-optic “eyes” at the night sky on Oct. 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 68 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 5,000 galaxies every 20 minutes.

The DESI collaboration has participation from nearly 500 researchers at 75 institutions in 13 countries, including scientists from KIPAC, a joint institute of DOE’s SLAC National Accelerator Laboratory and Stanford University. The latest milestone marks the beginning of DESI’s final testing before the formal start of observations in early 2020.

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’ll also shed more light on how the expansion affects the way galaxies and other astrophysical objects form and grow over time.

“DESI will let us measure certain properties of the universe like never before,” said Chia-Hsun Chuang, a KIPAC research scientist working with Risa Wechsler, former spokesperson of the DESI collaboration and director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). He chairs a group of DESI scientists who are currently running simulations of how dark energy has impacted baryon acoustic oscillation – fluctuations in the density of matter caused by sound waves in the early 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, 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. 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 10 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 previous experiment.

The focal plane, which contains half a 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 guide, focus and align (GFA system). KIPAC researchers Kevin Reil and Aaron Roodman were involved in various aspects of the development and commissioning of the GFA camera and its software.

Among the more recent arrivals at Kitt Peak is the collection of spectrographs that are designed to split the gathered light into thousands of wavelengths 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 that are moving away from us to longer, redder wavelengths. 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 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.

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