Three sky surveys completed in preparation for Dark Energy Spectroscopic Instrument

Below is a press release issued by Lawrence Berkeley National Laboratory announcing the completion of three sky surveys that will prepare the way for the Dark Energy Spectroscopic Instrument. DESI seeks to further our cosmic understanding by creating the largest 3-D map of galaxies to date. The U.S. Department of Energy’s Fermi National Accelerator Laboratory is a key player in the construction of this instrument, drawing on more than 25 years of experience with the Sloan Digital Sky Survey and the Dark Energy Survey.

Researchers will pick 35 million galaxies and quasars to target during DESI’s five-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 3-D 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 preselected sky objects to gather their light (see a related video) 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.
• The Beijing-Arizona Sky Survey (BASS), which used the Steward Observatory’s Bok telescope at Kitt Peak National Observatory and lasted 375 nights.

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), which 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.

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.

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.

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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.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.

The National Optical Astronomy Observatory (NOAO) is the national center for ground-based nighttime astronomy in the United States and is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation Division of Astronomical Sciences.

The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 to promote the progress of science. NSF supports basic research and people to create knowledge that transforms the future.

The Heising-Simons Foundation is a family foundation based in Los Altos, California. The Foundation works with its many partners to advance sustainable solutions in climate and clean energy, enable groundbreaking research in science, enhance the education of our youngest learners, and support human rights for all people.

The Gordon and Betty Moore Foundation fosters path-breaking scientific discovery, environmental conservation, patient care improvements and preservation of the special character of the Bay Area. Visit moore.org and follow @MooreFound.

The Science and Technology Facilities Council (STFC) of the United Kingdom coordinates research on some of the most significant challenges facing society, such as future energy needs, monitoring and understanding climate change, and global security. It offers grants and support in particle physics, astronomy and nuclear physics; visit https://stfc.ukri.org/.

Established in 1958 and aiming at the forefront of astronomical science, the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) conducts cutting-edge astronomical studies, operates major national facilities and develops state-of the-art technological innovations.

In 2015, 700 scientists from around the world came together and established a collaboration on neutrino research that has grown to include more than 1,000 scientists from over 30 countries. And Spain was involved from the beginning.

“It was an exciting time,” said Inés Gil-Botella, member of Fermilab’s Physics Advisory Committee and senior scientist at Spain’s Center for Energy, Environment and Technology Research (CIEMAT). “It was a huge step toward a neutrino-based partnership between Fermilab and Spain, and the rest of the world.”

The early collaboration meetings set the stage for what would quickly become the Deep Underground Neutrino Experiment, a name Gil-Botella remembers the early collaboration voting on. Already involved in the development of similar technology at the European laboratory CERN, Spanish institutions brought technical expertise of photon detectors and liquid-argon cryostats to the collaboration.

“Spanish scientists and institutions have a long history of working with Fermilab and have made countless important contributions over the years,” said Fermilab Director Nigel Lockyer. “Projects such as the development of the photon detection system are critical to the success of DUNE, and we benefit from the expertise of our colleagues from Spain.”

Currently, Spain is developing a photon detection system for DUNE’s giant particle detector. This is key to identifying and recreating a particle interaction. The system will allow scientists to understand when an interaction took place inside the detector and to determine the energy of that interaction. Knowing this information helps scientists narrow down when and where the neutrinos came from — a supernova for instance. The system is currently being tested in one of the aptly named ProtoDUNE detectors at CERN. Scientists from Spain are also working on controls and instrumentation for the DUNE detector that let researchers adjust voltages and monitor temperatures, for example.

Well before DUNE was even an idea, Spain was participating in research at Fermilab — most notably with the CDF collaboration, which, along with the DZero collaboration, discovered the top quark in 1995 using the Tevatron particle collider. During this era, Spanish institutions contributed the expertise they gained at CERN in collider physics to Fermilab experiments. One link between the Tevatron era and the current DUNE era was scientist Mario Martinez of the Spanish Institute for High Energy Physics in Barcelona. A leader in the CDF experiment, he also previously served as a representative for Spain in the early days of LBNF/DUNE.

Today, more than 60 scientists at 15 Spanish institutions contribute their expertise to more than a dozen experiments at Fermilab. In addition to their research at the Large Hadron Collider, Spanish scientists bring knowledge about neutrino physics, liquid-argon cryostats, computing, cosmology and more to the Fermilab research program. CIEMAT and the University of Granada, for example, are helping build the Short-Baseline Near Detector, which is part of Fermilab’s Short-Baseline Neutrino Program and relies on the liquid-argon detector technology that is also used by DUNE.

Neutrinos have three “flavors,” or types: muon, electron and tau. The SBN program will measure how the neutrinos’ flavors change as they go through the three detectors, which allows scientists to look for the existence of a fourth type of neutrino. The Short-Baseline Near Detector, the SBN detector closest to the neutrino production point, will record over a million neutrino interactions per year, providing scientists with an enormous treasure trove of data for analysis.

Spain is contributing to the simulation side of the photon detection system of SBND. The two institutions hope to expand their involvement when the SBN program begins taking data.

“We are open to help with what they need,” said Gil-Botella. “Physics is the goal, and I think the world has a lot to gain.”

Spain also collaborates with Fermilab on theoretical physics. Scientists are searching for ways experiments like DUNE can go beyond the Standard Model of physics, the framework that describes nature’s fundamental forces and particles at the subatomic scale. Scientists from the Institute of Corpuscular Physics (IFIC) in Valencia, the Autonomous University of Madrid (UAM) and the Institute for High Energy Physics in Barcelona are frequent collaborators with Fermilab’s theoretical physics group. Fermilab has also established formal staff and student exchange programs with both IFIC and UAM and plans to establish a similar program with the University of Barcelona. Every year the Fermilab Theoretical Physics Department receives many students, postdocs and scientists from Spanish institutions who come to Fermilab for several weeks to perform their theoretical work and interact with Fermilab theorists and experimentalists.

“If you are a neutrino physicist, Fermilab is the laboratory of reference,” said Michel Sorel, a scientist at IFIC who has been involved in Fermilab neutrino experiments for the past 20 years. “It is the neutrino capital of the world.”

Groups at the Institute of Space Sciences, the Institute for High Energy Physics and CIEMAT also participate in the Fermilab-hosted Dark Energy Survey, which completed its sixth and final year of data-taking earlier this year. This survey is an international collaboration that mapped a 5,000-square-degree area of the sky, recording information from 300 million galaxies.

The contributions go both ways. For example, Fermilab scientists are working on NEXT, the Neutrino Experiment with a Xenon TPC. Located at the Canfranc Underground Laboratory in Spain, NEXT seeks to determine whether or not neutrinos are their own antiparticles.

“We are collaborating on a day-to-day basis,” Sorel said. “It is a very close relationship both for the institutions and the individual scientists.”

For more information on Spain’s contributions at Fermilab, look at some of the previously published articles.