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DESI (Dark Energy Spectroscopic Instrument)

Science overview: Since the beginning of the 20th century, our understanding of the universe and our place in it has changed dramatically. From understanding that our solar system is in a galaxy amongst many and that our universe is expanding, we have developed a model that can explain to great accuracy our observations of the universe, how it evolved from an earlier, hotter, and denser period to form the web of galaxies we observe today. 1)

However successful it has been, it leaves some fundamental questions unanswered. Two of these concern the universe's energy budget: only about 5 percent of its total mass and energy can be accounted for by ordinary matter; 27 percent is dark matter that can only be inferred by its gravitational effects; and 68 percent is dark energy, responsible for the accelerating expansion of the universe.

By accurately measuring the expansion history over the past 11 billion years, DESI's scientific goal is to constrain possible models of dark energy. In order to accomplish this goal, DESI will measure the position and receding velocity of about 40 million galaxies. These galaxies will have been pre-selected by studying deep images of the DESI survey area, or footprint, taken during observing campaigns before DESI starts up.

The DESI survey: DESI seeks to map the large-scale structure of the universe over an enormous volume and a wide range of look-back times (based on "redshift," or the shift in the light of distant objects toward redder wavelengths of light). Targeting about 30 million pre-selected galaxies across one-third of the night sky, scientists will use DESI's redshifts data to construct 3D maps of the universe. 2)

The DESI survey is based on four primary classes of galaxies, here described in order of distance from us:

1) DESI will provide a detailed map using bright galaxies, stretching out to redshift 0.4 and including galaxies to 20th magnitude. These objects are bright enough to observe when the moon is near full. While these are the easiest targets, they are still of high value because they probe the universe most recently, when the accelerating expansion is strongest.

2) Next, DESI will use luminous red galaxies (LRGs). These are the most massive galaxies, composed largely of old stars. They can be seen at substantial distances and their red color makes them easy to select in imaging. With them, the survey will reach redshift 1.

3) The largest sample in DESI relies on emission line galaxies (ELGs). These are fainter and more distant, but their vigorous star formation and hot young stars create strong emission in distinct wavelengths that DESI can detect out to redshift 1.6.

4) To go yet further, DESI will survey quasars. These are galaxies where the central supermassive black hole is accreting large amounts of gas that is glowing as it reaches relativistic speeds. Quasars outshine the stars of the galaxy and achieve luminosities that allow DESI to detect them out to redshift 3.5 and beyond.

An important extra application of the quasars is that their spectra is altered by the light traveling through the intergalactic gas between the quasar and us. The neutral hydrogen in this gas produces absorption at a key wavelength in the ultraviolet (121.6 nanometers), and that absorption pattern gets shifted into visible wavelengths due to the very high redshift of the gas. This produces a so-called Lyman-alpha forest and means that each quasar spectrum not only reveals its location in the map, but also a map of the intergalactic hydrogen along the line of sight.

Finally, DESI will observe stars in our own galaxy, adding to our understanding of the chemical abundances and gravitational dynamics of the Milky Way, particularly to help our study of the role of dark matter.

 

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. 3)

The DESI (Dark Energy Spectroscopic Instrument) 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 2019, 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) 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.

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Figure 1: This map shows the sky areas covered (blue) by three surveys conducted in preparation for DESI (image 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 UK.'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.

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Figure 2: An aerial image of the Cerro Tololo Interamerican Observatory in Chile, with the silvery dome of the 4-meter Blanco telescope pictured at lower right (image credit: NOAO/AURA/NSF)

Cool cosmic cartography experiment

The data from the surveys was routed to supercomputers at Berkeley Lab's NERSC (National Energy Research Scientific Computing Center), 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 NAOC (National Astronomical Observatories of China), 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 2019.

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Figure 3: The Bok (left) and Mayall telescopes at Kitt Peak National Observatory near Tucson, Arizona. DESI is currently under installation at the Mayall telescope (elevation of 2096 m), image credit: Michael A. Stecker

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.

 


 

The DESI instrument

DESI is a Stage IV ground-based dark energy experiment that will study BAOs (Baryon Acoustic Oscillations) and the growth of structure through redshift-space distortions (RSD) with a wide-area galaxy and quasar redshift survey. DESI is the successor to the successful Stage-III BOSS (Baryon Oscillation Spectroscopic Survey) redshift survey and complements imaging surveys such as the Stage-III Dark Energy Survey (DES, operating 2013{2018) and the Stage-IV Large Synoptic Survey Telescope (LSST, planned start early in the next decade). DESI is an important component of the DOE Cosmic Frontier program, meeting the need for a wide-field spectroscopic survey identified in the 2011 "Rocky-III" dark energy community planning report. In addition to providing Stage IV constraints on dark energy, DESI will provide new measurements that can constrain theories of modified gravity and inflation, and that will measure the sum of neutrino masses. 4)

DESI, the first Stage IV dark energy experiment, will use the redshifts of 25 million galaxies to probe the nature of dark energy and test General Relativity. DESI builds on the successful Stage-III BOSS redshift survey, which has established BAO as a precision technique for dark energy exploration. DESI will make an order-of-magnitude advance over BOSS in volume observed and galaxy redshifts measured by using 5,000 robotically controlled fiber-positioners to feed a collection of spectrographs covering 360 nm to 980 nm. Newly designed optics for the National Optical Astronomy Observatory's 4-m Mayall telescope at Kitt Peak, Arizona, will provide a FOV (Field of View) of 8º x 8º. The telescope dome and telescope with the existing MOSAIC corrector are shown in Figure 4. 5)

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Figure 4: On the left is the Mayall 4m telescope dome structure. On the right is the telescope. The black cylinder is the MOSAIC corrector that will be replaced as part of the DESI project to instrument a 3 degree diameter FOV (image credit: DESI Collaboration)

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Figure 5: DESI block diagram (image credit: DESI Collaboration)

 

Over its 5-year observing lifetime, DESI will measure the spectra of more than 30 million galaxies and quasars covering 14,000 square degrees. The DESI instrument will provide unprecedented multi-object spectroscopy incorporating a novel design. 6)

DESI's new corrector optics provides a 3º diameter field of view that feeds a focal plate containing of 5,000 robotic positioners. The positioners can be reconfigured within 3 minutes to measure the spectra of a new set of galaxies. Optical fibers mounted to the positioners extend 50 meters down the telescope to feed 10 broadband spectrographs, each containing three detectors. The spectrographs cover a spectral range of 360 - 980 nm with a resolution of 2,000 to 5,000, enabling DESI to probe redshifts up to 1.7 for emission line galaxies and 3.5 for the lyman α spectra from quasars.

All commands and data transfers are run by the Instrument Control System, while the data acquisition system ensures accurate targeting and fast analysis of large amounts of data.

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Figure 6: Illustration of DESI in the Mayall Telescope (image credit: LBL)

The instrument has been mounted on the 4-meter Mayall Telescope in 2018. The construction and integration of the instrument is the result of collaborations between several institutions.

The DESI project is sponsored by the US DOE (Department of Energy), the UK STFC (Science and Technology Facilities Council), the Heising-Simons Foundation, the Gordon and Betty Moore Foundation, AURA (Association of Universities for Research in Astronomy), the NSF (National Science Foundation) and NOAO (National Optical Astronomy Observatory).

DESI is the product of an international collaboration that brings together more than 450 researchers from more than 70 institutions including Australia, Canada, China, Colombia, France, Mexico, Spain, Switzerland, the U.K., and the U.S. The collaboration is led by Lawrence Berkeley National Laboratory, which is managed by the University of California for the U.S. Department of Energy's Office of Science. DESI is located at Kitt Peak National Observatory near Tucson, Arizona. Kitt Peak is part of the NSF's National Optical-Infrared Astronomy Research Laboratory (NSF's OIR Lab).

 

DESI development

• October 28, 2019: A new instrument mounted atop a telescope in Arizona has 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. 7)

- 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 percent of the universe and is speeding up its expansion.

Figure 7: This video highlights the components and statistics that make DESI, the Dark Energy Spectroscopic Instrument, unique. Installed on the Mayall Telescope at Kitt Peak National Observatory near Tucson, Arizona, DESI brings high-speed automation to its galaxy-mapping mission. In five years DESI will capture the light from 35 million galaxies and 2.4 million quasars to produce the largest 3D map of the universe (video credit: Marilyn Chung/Berkeley Lab)

- 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 Earth. In ideal conditions DESI can cycle through a new set of 5,000 galaxies every 20 minutes.

- The latest milestone, achieved Oct. 22, marks the opening of DESI's final testing toward the formal start of observations in early 2020.

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Figure 8: DESI's 5000 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 (image credit: DESI Collaboration; Legacy Surveys; NASA/JPL-Caltech/UCLA)

- 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 3D map of the universe.

- 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.

- "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.

- "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," Levi added. "I am thrilled to see this new experiment come to life."

- 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 Science Foundation's National Optical-Infrared Astronomy Research Laboratory.

- Over the past 18 months, a bevy of DESI components were shipped to the site from institutions around the globe and installed on the telescope.

- Among the early arrivals was an assembly of lenses packaged in a large steel barrel, together weighing in at three tons. This corrector barrel sits over the 4-meter primary mirror of the Mayall Telescope and provides an expansive field of view. The lenses, each measuring about a meter across, were successfully tested in April.

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Figure 9: A machine positions a focal plane "petal" in preparation for its installation. Ten wedge-shaped petals make up DESI's focal plane (image credit: DESI Collaboration)

- 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.

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Figure 10: 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 (image credit: DESI Collaboration)

- 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 predecessor experiment.

- The focal plane, which is comprised 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.

- The focal plane, corrector barrel, and other DESI components weigh 11 tons, and the Mayall telescope's movable arm that DESI is installed on, has a mass of 250 tons and rises ~27 m above the floor in the Mayall's 14-story dome (Figure 6).

- 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.

- 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.

- "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 (known as CCDs or charge-coupled devices) that enable DESI's broad color-sampling range.

- Gregory Tarlé, a physics professor at the University of Michigan (UM) 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.

- UM delivered a total of 7,300 robotic positioners, including spares. During the production peak, the teams were churning out about 50 positioners a day.

- "It was quite a process," Tarlé said. "We were at the limits of precision for these production parts."

- 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.

- Now that the hard work of building DESI is largely done, Tarlé said he looks forward to DESI 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."

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Figure 11: Workers install DESI's spectrographs, which are used to split the light collected from DESI's focal plane into separate color bands (image credit: DESI Collaboration)

• April 12, 2019: DESI, the Dark Energy Spectroscopic Instrument, will mobilize 5,000 swiveling robots – each one pointing a thin strand of fiber-optic cable – to gather the light from about 35 million galaxies. 8)

- The little robots are designed to fix on a series of preselected sky objects that are as distant as 12 billion light-years away. By studying how these galaxies are drifting away from us, DESI will provide precise measurements of the accelerating rate at which the universe is expanding.

- This expansion rate is caused by an invisible force known as dark energy, which is one of the biggest mysteries in astrophysics and accounts for an estimated 68 percent of all mass and energy in the universe.

- In this video, DESI project participants share their insight and excitement about the project and its potential for new and unexpected discoveries.

Figure 12: In this video, DESI project participants share their insight and excitement about the project and its potential for new and unexpected discoveries (video credit: Marilyn Chung/Berkeley Lab, DESI Collaboration)

 


1) "Dark Energy Spectroscopic Instrument," LBL (Lawrence Berkeley Lab), URL: https://www.desi.lbl.gov/science/

2) "Dark Energy Spectroscopic Instrument, the DESI survey," URL: https://www.desi.lbl.gov/the-desi-survey/

3) Glenn Roberts Jr. "3 Sky Surveys Completed in Preparation for Dark Energy Spectroscopic Instrument, Researchers will pick 35 million galaxies and quasars to target during DESI's 5-year mission," LBL (Lawrence Berkeley Lab), 8 July 2019, URL: https://newscenter.lbl.gov/2019/07/08/3-sky-surveys-completed-
in-preparation-for-dark-energy-spectroscopic-instrument/

4) DESI Collaboration: Amir Aghamousa, Jessica Aguilar, Steve Ahlen, Shadab Alam,Lori E. Allen, Carlos Allende Prieto, James Annis, Stephen Bailey, Christophe Balland,Otger Ballester, Charles Baltay, Lucas Beaufore, Chris Bebek, Timothy C. Beers, EricF. Bell, Jos Luis Berna, Robert Besuner, Florian Beutler, Chris Blake, HannesBleuler, Michael Blomqvist, Robert Blum, Adam S. Bolton, Cesar Briceno, DavidBrooks, Joel R. Brownstein, Elizabeth Buckley-Geer, Angela Burden, Etienne Burtin,Nicolas G. Busca, Robert N. Cahn, Yan-Chuan Cai, Laia Cardiel-Sas, Raymond G.Carlberg, Pierre-Henri Carton, Ricard Casas, Francisco J. Castander, Jorge L.Cervantes-Cota, Todd M. Claybaugh, Madeline Close, Carl T. Coker, Shaun Cole, JohanComparat, Andrew P. Cooper, M.-C. Cousinou, Martin Crocce, Jean-Gabriel Cuby,Daniel P. Cunningham, Tamara M. Davis, Kyle S. Dawson, Axel de la Macorra, Juan DeVicente, Timothée Delubac, Mark Derwent, Arjun Dey, Govinda Dhungana, ZhejieDing, Peter Doel, Yutong T. Duan, Anne Ealet, Jerry Edelstein, Sarah Eftekharzadeh, et al., "The DESI Experiment Part I: Science,Targeting, and Survey Design," Instrumentation and Methods for Astrophysics, October 2016, URL: https://arxiv.org/pdf/1611.00036.pdf

5) DESI Collaboration: Amir Aghamousa, Jessica Aguilar, Steve Ahlen, Shadab Alam,Lori E. Allen, Carlos Allende Prieto, James Annis, Stephen Bailey, Christophe Balland,Otger Ballester, Charles Baltay, Lucas Beaufore, Chris Bebek, Timothy C. Beers, EricF. Bell, Jos Luis Berna, Robert Besuner, Florian Beutler, Chris Blake, HannesBleuler, Michael Blomqvist, Robert Blum, Adam S. Bolton, Cesar Briceno, DavidBrooks, Joel R. Brownstein, Elizabeth Buckley-Geer, Angela Burden, Etienne Burtin,Nicolas G. Busca, Robert N. Cahn, Yan-Chuan Cai, Laia Cardiel-Sas, Raymond G.Carlberg, Pierre-Henri Carton, Ricard Casas, Francisco J. Castander, Jorge L.Cervantes-Cota, Todd M. Claybaugh, Madeline Close, Carl T. Coker, Shaun Cole, JohanComparat, Andrew P. Cooper, M.-C. Cousinou, Martin Crocce, Jean-Gabriel Cuby,Daniel P. Cunningham, Tamara M. Davis, Kyle S. Dawson, Axel de la Macorra, Juan DeVicente, Timothée Delubac, Mark Derwent, Arjun Dey, Govinda Dhungana, ZhejieDing, Peter Doel, Yutong T. Duan, Anne Ealet, Jerry Edelstein, Sarah Eftekharzadeh, et al., "The DESI Experiment Part II: Instrument Design," Instrumentation and Methods for Astrophysics, December 2016, URL: https://arxiv.org/pdf/1611.00037.pdf

6) "The Dark Energy Spectroscopic Instrument (DESI)," URL: https://www.desi.lbl.gov/

7) Glenn Roberts, "DESI Opens Its 5,000 Eyes to Capture the Colors of the Cosmos," LBL, 28 October 2019, URL: https://newscenter.lbl.gov/2019/10/28/desi-opens-its
-5000-eyes-to-capture-the-colors-of-the-cosmos/

8) "VIDEO: The Making of the Largest 3D Map of the Universe," LBL (Lawrence Berkeley Laboratory) News Release, 12 April 2019, URL: https://newscenter.lbl.gov/2019/04/12/video-the-making-of-the-largest-3d-map-of-the-universe/
 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).