From Lawrence Berkeley National Lab: “3 Sky Surveys Completed in Preparation for Dark Energy Spectroscopic Instrument”

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From Lawrence Berkeley National Lab

July 8, 2019

Glenn Roberts Jr.
geroberts@lbl.gov
(510) 486-5582

Researchers will pick 35 million galaxies and quasars to target during DESI’s 5-year mission

It took three sky surveys – conducted at telescopes in two continents, covering one-third of the visible sky, and requiring almost 1,000 observing nights – to prepare for a new project that will create the largest 3D map of the universe’s galaxies and glean new insights about the universe’s accelerating expansion.

This Dark Energy Spectroscopic Instrument (DESI) project will explore this expansion, driven by a mysterious property known as dark energy, in great detail. It could also make unexpected discoveries during its five-year mission.

LBNL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory starting in 2018

NOAO/Mayall 4 m telescope at Kitt Peak, Arizona, USA, Altitude 2,120 m (6,960 ft)

The surveys, which wrapped up in March, have amassed images of more than 1 billion galaxies and are essential in selecting celestial objects to target with DESI, now under construction in Arizona.

The latest batch of imaging data from these surveys, known as DR8, was publicly released July 8, and an online Sky Viewer tool provides a virtual tour of this data. A final data release from the DESI imaging surveys is planned later this year.

It took three sky surveys – conducted at telescopes in two continents, covering one-third of the visible sky, and requiring almost 1,000 observing nights – to prepare for a new project that will create the largest 3D map of the universe’s galaxies and glean new insights about the universe’s accelerating expansion.

This Dark Energy Spectroscopic Instrument (DESI) project will explore this expansion, driven by a mysterious property known as dark energy, in great detail. It could also make unexpected discoveries during its five-year mission.

The surveys, which wrapped up in March, have amassed images of more than 1 billion galaxies and are essential in selecting celestial objects to target with DESI, now under construction in Arizona.

The latest batch of imaging data from these surveys, known as DR8, was publicly released July 8, and an online Sky Viewer tool provides a virtual tour of this data. A final data release from the DESI imaging surveys is planned later this year.

Scientists will select about 33 million galaxies and 2.4 million quasars from the larger set of objects imaged in the three surveys. Quasars are the brightest objects in the universe and are believed to contain supermassive black holes. DESI will target these selected objects for several measurements after its start, which is expected in February 2020.

DESI will measure each target across a range of different wavelengths of light, known as spectrum, from the selected set of galaxies repeatedly over the course of its mission. These measurements will provide details about their distance and acceleration away from Earth.

A collection of 5,000 swiveling robots, each carrying a fiber-optic cable, will point at sets of pre-selected sky objects to gather their light (see a related video [below]) so it can be split into different colors and analyzed using a series of devices called spectrographs.

Three surveys, 980 nights

“Typically, when you apply for time on a telescope you get up to five nights,” said David Schlegel, a DESI project scientist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), which is the lead institution in the DESI collaboration. “These three imaging surveys totaled 980 nights, which is a pretty big number.”

The three imaging surveys for DESI include:

The Mayall z-band Legacy Survey (MzLS), carried out at the Mayall Telescope at the National Science Foundation’s Kitt Peak National Observatory near Tucson, Arizona, over 401 nights. DESI is now under installation at the Mayall Telescope.

The Dark Energy Camera Legacy Survey (DECaLS) at the Victor Blanco Telescope at NSF’s Cerro Tololo Inter-American Observatory in Chile, which lasted 204 nights.

Dark Energy Survey


Dark Energy Camera [DECam], built at FNAL


NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

Timeline of the Inflationary Universe WMAP

The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

The Beijing-Arizona Sky Survey (BASS), which used the Steward Observatory’s Bok telescope at Kitt Peak National Observatory and lasted 375 nights.

2.3-metre Bok Telescope at the Steward Observatory at Kitt Peak in Arizona, USA, altitude 2,096 m (6,877 ft)

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This map shows the sky areas covered (blue) by three surveys conducted in preparation for DESI. (Credit: University of Arizona)

On-site survey crews – typically two DESI project researchers per observing night for each of the surveys – served in a sort of “lifeguard” role, Schlegel said. “When something went wrong they were there to fix it – to keep eyes on the sky,” and researchers working remotely also aided in troubleshooting.

On the final night of the final survey …

In early March, Eva-Maria Mueller, a postdoctoral researcher at the U.K.’s University of Portsmouth, and Robert Blum, former deputy director at the National Optical Astronomy Observatory (NOAO) that manages the survey sites, were on duty with a small team in the control room of the NSF’s Victor Blanco Telescope on a mile-high Chilean mountain for the final night of DECaLS survey imaging.

Seated several stories beneath the telescope, Mueller and Blum viewed images in real time to verify the telescope’s position and focus. Mueller, who was participating in a five-night shift that was her first observing stint for the DESI surveys, said, “This was always kind of a childhood dream.”

Blum, who had logged many evenings at the Blanco telescope for DECaLS, said, “It’s really exciting to think about finishing this phase.” He noted that this final night was focused on “cleaning up little holes” in the previous imaging. Blum is now serving in a new role as acting operations director for the Large Synoptic Survey Telescope under installation in Chile.

New software designed for the DESI surveys, and precise positioning equipment on the telescopes, has helped to automate the image-taking process, setting the exposure time and filters and compensating for atmospheric distortions and other factors that can affect the imaging quality, Blum noted. During a productive evening, it was common to produce about 150 to 200 images for the DECaLS survey.

Cool cosmic cartography experiment

The data from the surveys was routed to supercomputers at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), which will be the major storehouse for DESI data.

NERSC

NERSC Cray Cori II supercomputer at NERSC at LBNL, named after Gerty Cori, the first American woman to win a Nobel Prize in science

NERSC Hopper Cray XE6 supercomputer


LBL NERSC Cray XC30 Edison supercomputer


The Genepool system is a cluster dedicated to the DOE Joint Genome Institute’s computing needs. Denovo is a smaller test system for Genepool that is primarily used by NERSC staff to test new system configurations and software.

NERSC PDSF


PDSF is a networked distributed computing cluster designed primarily to meet the detector simulation and data analysis requirements of physics, astrophysics and nuclear science collaborations.

Future:

Cray Shasta Perlmutter SC18 AMD Epyc Nvidia pre-exascale supeercomputer

NERSC is a DOE Office of Science User Facility.

More than 100 researchers participated in night shifts to conduct the surveys, said Arjun Dey, the NOAO project scientist for DESI. Dey served as a lead scientist for the MzLS survey and a co-lead scientist on the DECaLS survey with Schlegel.

“We are building a detailed map of the universe and measuring its expansion history over the last 10 to 12 billion years,” Dey said. “The DESI experiment represents the most detailed – and definitely the coolest – cosmic cartography experiment undertaken to date. Although the imaging was carried out for the DESI project, the data are publicly available so everyone can enjoy the sky and explore the cosmos.”

BASS survey supported by global team

Xiaohui Fan, a University of Arizona astronomy professor who was a co-lead on the BASS survey conducted at Kitt Peak’s Bok Telescope, coordinated viewing time by an international group that included co-leads Professor Zhou Xu and Associate Professor Zou Hu, other scientists from the National Astronomical Observatories of China (NAOC), and researchers from the University of Arizona and from across the DESI collaboration.

4
The Bok (left) and Mayall telescopes at Kitt Peak National Observatory near Tucson, Arizona. DESI is currently under installation at the Mayall telescope. (Credit: Michael A. Stecker)

BASS produced about 100,000 images during its four-year run. It scanned a section of sky about 13 times larger than the Big Dipper, part of the Ursa Major constellation.

“This is a good example of how a collaboration is done,” Fan said. “Through this international partnership we were bringing in people from around the world. This is a nice preview of what observing with DESI will be like.”

Fan noted the DESI team’s swift response in updating the telescope’s hardware and software during the course of the survey.

“It improved a lot in terms of automated controls and focusing and data reduction,” he said. Most of the BASS survey imaging concluded in February, with some final images taken in March.

Next steps toward DESI’s completion

All of the images gathered will be processed by a mathematical code, called Tractor, that helps to identify all of the galaxies surveyed and measure their brightness.

With the initial testing of the massive corrector barrel, which houses DESI’s package of six large mirrors, in early April, the next major milestone for the project will be the delivery, installation, and testing of its focal plane, which caps the telescope and houses the robotic positioners.

Dey, who participated in formative discussions about the need for an experiment like DESI almost 20 years ago, said, “It’s pretty amazing that our small and dedicated team was able to pull off such a large survey in such a short time. We are excited to be turning to the next phase of this project!”

NERSC is a DOE Office of Science User Facility.

More:

Explore Galaxies Far, Far Away at Internet Speeds

Scientists have released an “expansion pack” for a virtual tour of the universe that you can enjoy from the comfort of your own computer. The latest version of the publicly accessible images of the sky roughly doubles the size of the searchable universe from the project’s original release in May.

News Center


In this video, Dark Energy Spectroscopic Instrument (DESI) project participants share their insight and excitement about the project and its potential for new and unexpected discoveries.

DESI is supported by the U.S. Department of Energy’s Office of Science; the U.S. National Science Foundation, Division of Astronomical Sciences under contract to the National Optical Astronomy Observatory; the Science and Technologies Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the National Council of Science and Technology of Mexico; the Ministry of Economy of Spain; the French Alternative Energies and Atomic Energy Commission (CEA); and DESI member institutions. The DESI scientists are honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation. View the full list of DESI collaborating institutions, and learn more about DESI here: desi.lbl.gov.

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