Dark Energy Spectroscopic Instrument (DESI)

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Sneaking around with DESI data

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Recovering from the Contreras Fire
Benjamin Weaver, NOIRLab
January 24, 2023

DESI collaborators have previously reported on the Contreras Fire and how it initially affected DESI operations along with an update as personnel were able to return to Kitt Peak National Observatory (KPNO). Even as we were cleaning up KPNO and the DESI instrument in August 2022, we knew that it would possibly be several months before the Tohono O’odham Utility Authority (TOUA) could replace the infrastructure bringing electrical power and line internet (a fiber-optic link) to KPNO. In fact, line internet was not restored until December 7, 2022, almost exactly six months after the start of the fire.

The story of restoring electrical power and the generators used while restoration was in progress is definitely worth telling, and far more lengthy and detailed than the story I’d like to tell now, recovering from the missing internet connection.

The DESI Data Management team, working closely with the on-site NOIRLab personnel, developed a plan to work around the missing internet, even as DESI returned to full-night operations. It wasn’t a new plan; in fact, it was a very old plan: “Sneakernet”. The idea is that a human being (presumably wearing sneakers) can hand-carry data on physical media from one location to another, and in certain circumstances, the effective bandwidth of that transfer can be much, much higher than the bandwidth of an internet connection or other communication channel. This is actually even more true now than it was in the past, because the storage density of physical media has experienced exponential growth similar to that of Moore’s law for CPU performance. However, while Moore’s law has been showing signs of reaching fundamental physical limits, storage density trends continue to rise.

Old school data transfer station at NOIRLab’s Tucson facility

With that in mind, we purchased six 2 TB SanDisk solid-state drives (SSDs), and a few fast USB cables. There is nothing exotic about the hardware; these items would be available in many consumer electronics or department stores. Then with the 6 disks initially carried to KPNO, we would place an entire night of data (or sometimes several nights) on a SSD. One of the personnel working at KPNO would be designated to carry the SSD to Tucson at the end of the work day. Then we would pick up the disk either in person or from a drop box and copy the data to NERSC where it could be processed with the usual DESI data pipeline. Since we purchased six SSDs, that meant we always had several SSDs on standby at KPNO in case an already-transferred SSD could not be brought back from Tucson to KPNO the next day. On weekends, no one would be available to carry the SSDs, so we simply had several nights on Monday evening’s SSD.  From the point of view of the pipeline, the only difference was that an entire night (or several nights) would arrive all at once, after a delay of 1 to 3 days, instead of the normal operations procedure of one exposure at a time, almost immediately after the exposure is completed.

Transfer disk plugged in

The DESI instrument resumed taking data starting with night 20220825 (August 25, 2022). The first SSD was brought to Tucson on September 2, 2022. This process continued until line internet was restored on December 7, 2022. The original data transfer system was ready to go at that point and was reactivated almost immediately after line internet was restored. Thus the last night of data transferred by sneakernet was 20221206 (December 6, 2022). In total, 16,726 exposures over 100(!) nights were hand-carried to Tucson. This amounts to 6.1 TB of data.

Tucson to NERSC data transfer submitted successfully

The bandwidth of hand-carried SSDs is impressive, but so is the bandwidth of modern USB cables. After some experimentation, we discovered that we could transfer data directly from the external SSD to NERSC via Globus Online, with bandwidth performance indistinguishable from the case where the data were first copied to a workstation, then transferred via Globus. In other words, the limiting bandwidth was the internet itself between Tucson and NERSC, not the internal systems of the receiving workstation.

While we were transferring DESI data with a very old system (but with the latest SSDs), we were also experimenting with a new one: Starlink. In fact there were at least two separate Starlink installations active at KPNO during the recovery.  One, which we’ll call the “DESI Starlink”, was a strictly limited command and control channel for use by instrument experts. The DESI Starlink was not used for any significant data transfer. The other, which we’ll call the “NOIRLab Starlink” was used for more general access to all of KPNO. We did use the NOIRLab Starlink for some data transfer, with mixed results.

At least currently, Starlink works best with a point-to-point style networking connection, optimized for individuals or residences, much like a residential cable modem. Since the uplink bandwidth is currently a factor of 10-20 times slower than the downlink bandwidth, it is less practically suited as a two-way connection between two substantially-sized networks, such as between KPNO and the Tucson base facilities. This manifested as a significant, recurring routing problem. Although bandwidth was adequate for interactive use and for the transfer of small data files, the connection was frequently not working and the routing systems had to be monitored constantly. Nevertheless, we were able to use the NOIRLab Starlink to maintain an off-site copy of the DESI operations database. This copy would have been impractical to maintain using daily SSD transfers, because the copy is a real-time mirror of a live database; it is more of a stream than a set of files. There were occasions where the database mirror fell behind due to interruptions in the Starlink connection. In these cases files created by the database were transferred manually on the SSDs and then once in Tucson the data files were manually added to the database mirror.

It was interesting and a good experience to transfer data over sneakernet. It was also a lot of work and hours. I want to give credit to a big team who all contributed to this effort: Keith Blaine, Paul Demmer, Matthew Evatt, Steve Grandi, Bob Marshall, Rod Rutland, David Sprayberry, Christopher Stone, Bob Stupak (NOIRLab), Martin Landriau (LBNL), and Klaus Honscheid (OSU). Here’s one job we hope we never have to do again!

Lightning and a lunar eclipse over Kitt Peak

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January 18, 2023

The all-sky camera on the Mayall telescope were the DESI instrument is located provides a view of the entire sky above Kitt Peak National Observatory. This camera helps observers monitor weather conditions and visibility, and confirm when it is safe to open the dome above the telescope. While the skies above Kitt Peak are very often clear, that is not always the case, as shown below in a series of camera images from the night of September 23, 2022. The night began with clouds and rain, and for over an hour flashes of lightning lit up both the sky and the camera view. Finally, toward the end of the night the storm gave way to thin clouds that allowed some stars to peek through. The final frame shows the Milky Way stretched across a mostly clear sky before the series starts over.

Credit: Luke Tyas

The all-sky camera can also capture celestial events, like the lunar eclipse that occurred on the night of November 8, 2022. DESI Lead Observer Luke Tyas created the timelapse below of the eclipse from 108 individual images from the all-sky camera. At first the full moon shines brightly just right of center, reflecting so much light into the sky that only a few bright stars are visible. As the eclipse begins the moon dims to a red dot scarcely brighter than the brightest stars, and the Milky Way can be seen following the eclipsed moon across the sky. As it nears the western horizon the moon emerges from the Earth’s eclipsing shadow, re-illuminating the telescope domes visible on the horizon and washing out the Milky Way.

Credit: Luke Tyas

Untangling the Cosmic Web

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Even the smallest of correlations between galaxies and their environment can impact measurements on the largest scales.
Claire Lamman, Harvard University
September 9, 2022

Understanding the largest structures in the Universe and how they evolve is more convoluted than just mapping out the positions of galaxies. This is because galaxy characteristics are not random; they are tangled both with how we choose targets and the underlying cosmic structure that we’re measuring. Interpreting DESI data involves carefully separating these threads, making sure to leave no loose ends.

An important idea to keep in mind is that DESI is not directly measuring the large-scale structure, because it’s mostly dark matter. Instead, we measure “tracers”: galaxies that trace the underlying web. It’s difficult to choose a sample of tracers which perfectly represent the true density. Certain properties of galaxies make them more likely to be observed, and these properties are often correlated with the structure itself. For instance, we’re more likely to observe big, luminous galaxies, which are more likely to be found in dense clusters.

These ideas are not new. Any survey must be careful when their measurement is correlated with the way they select their sample. Here’s a simple example:

Incredible. My research finds that 76% of people enjoy filling out polls!

The purpose of DESI is to measure out structure very accurately. This means it’s more important than ever to account for even tiny sources of bias, and it can get complicated. Another example is that denser regions have more active galaxies, which can have more distinct spectral lines that make them easier to get redshifts for. Or we can undersample galaxies in dense regions because they are so close together that DESI’s fibers may collide if we observe some at the same time. Yet another example is that we correct for the dust in our own galaxy, but the dust maps we use include a little bit of dust from beyond the Milky Way! This extragalactic dust is more likely to be found in dense regions, so we can end up over-correcting for dust in denser regions and over-estimate their mass.

Each bias must be carefully accounted for. This can be done by comparing our sample to similar surveys or simulations to check for systematic incompleteness. However, many effects must be carefully studied on their own, and there’s enough examples to fill many papers!

I recently finished one such study, about a bias created by the alignment of galaxy shapes. Elliptical galaxies tend to align themselves with density filaments. That is, their shapes stretch along the direction of strands in the cosmic web. This effect is very small, but becomes noticeable when sampling millions of galaxies. This is tied to target selection because DESI is slightly more likely to choose a galaxy that is pointed at Earth; this is the orientation where a galaxy’s light is the most concentrated on the sky. Therefore, we are more likely to observe galaxies which lie in strands that are parallel to our line of sight, and undersample galaxies in density strands stretching across our line of sight.

The contrast in galaxy clustering we measure in one direction versus another tells us how fast the cosmic web forms (Redshift Space Distortions). Galaxy alignments can bias our measurements of this by about 0.5%. Although this seems tiny, it could make the difference between competing models of dark energy. It’s called “precision cosmology” for a reason! Here is a version of my paper written for the general public.

Even small knots like galaxy alignment can tangle up our measurements, which is one reason you need a large team like DESI to unravel the cosmic web.

Recovery effort update two months after the Contreras Fire

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David Sprayberry, NOIRLab
August 25, 2022

After the Contreras Fire swept over part of Kitt Peak in June, July was devoted to restoring electrical power and water service to the mountaintop. We also carefully assessed every structure for safety, possible fire damage, and the extent of cleaning required. Currently electricity at the summit is provided solely by back-up generators (more on that later). Because the water system lost pressure during the power outage, it was necessary to flush it with chlorine and then fresh water from the storage tanks, and then have the water tested to ensure it meets EPA standards for safe drinking water (it passed!). Work at the telescopes during July was limited to skeleton crews going up for specific tasks to prevent further issues with sensitive scientific equipment. At the Mayall, this included restarting the building’s cooling system, changing filters in the purified dry air system that purges the focal plane enclosure, and changing filters in the HVAC system serving the spectrograph clean room (fondly known as “the Shack”). We also performed regular inspections in all the NOIRLab domes for leaks and electrical faults.

Beginning August 1, telescope staff were allowed to resume almost normal on-site work. At the Mayall we spent the first week cleaning everything we could find a way to reach inside the dome, and any other areas potentially affected by ash or smoke intrusion. The interior of the dome wasn’t particularly dirty, but we cleaned everything thoroughly anyway. We also removed the protective tarp and plastic sheeting from the primary mirror and prime focus corrector, and made a preliminary inspection of the optics, which look fine. During the second week of August we caught up on overdue preventive maintenance of the telescope and dome, and worked to provide a low-bandwidth internet connection to the Mayall.

Road Conditions

Throughout August, road and weather conditions have significantly limited the time our team can spend on-site. The fire damaged guardrails along the road, and there has been a lot of erosion from burned areas washing across the road during heavy rains. This extraordinary erosion has clogged many of the drainages that cross the road, sometimes causing impassible mudflows. Additionally, the fire damaged several drainage culverts under the road. The Arizona Department of Transportation (ADOT) is making progress on repairs, but they are hampered by frequent heavy summer “monsoon” thunderstorms since late July. Finally, erosion is worsening the normal amount of rockfall along the road during storms. Combined with this year’s strong, frequent storms, we are encountering more rocks and boulders in the road.

Damaged guardrail along the mountain road.
Mudflow across road during heavy rain.

Road restrictions are still in place. Repair work frequently requires closing both lanes, and so that we interfere as little as possible ADOT requires us to go up and down the mountain in one convoy at the same times every morning and afternoon, weather permitting. NOIRLab is also concerned for site safety due to the possibility of road closures caused by mudflows or rockfall. Whenever rain begins or is threatening, everyone on the site ceases work, makes their work site safe, and assembles for departure in an early convoy.

Boulder found Monday morning, August 22, after a weekend of heavy rain.
Electrical Power and Internet

The fire damaged between 10 and 20 utility poles that bring electricity and fiber optic data to the summit. Our utility provider, the Tohono O’odam Utility Authority (TOUA), is working hard to replace the damaged poles, but they face a number of obstacles. Many of the poles are in very remote locations. Other lines and poles elsewhere on Tribal lands were also damaged by storms. And like everyone else TOUA is grappling with supply chain disruptions. It will be at least several weeks until power is restored, and then fiber optic lines will be replaced.

The observatory is currently powered by on-site backup generators. One serves the Mayall and adjacent University of Arizona buildings, and another serves the rest of the mountaintop. This is expensive in terms of refueling needs, and leaves the site without back-up power sources if one of the generators were to fail. We are in the process of renting additional generators and connecting them to use as primary power sources. Once this is done the permanent back-up generators can return to back-up status.

Keeping the generators fueled is a challenge. Not all vendors will deliver diesel fuel to our remote site. Recently our normal supplier was unable to make our regular delivery when their truck broke down, and the Mayall generator ran out of fuel before an alternate supplier could reach the summit. Storms and road conditions further complicate the situation.

For temporary internet service DESI has obtained a Starlink system and service for the Mayall. The equipment is installed and connected to one network switch in the Mayall building, but that connection requires a standalone DNS server in the Mayall to allow normal access to the many other computers and switches in the building. DNS within NOIRLab is normally provided by one central server in Tucson, but that is obviously not an option right now, so work is underway to construct this standalone DNS system.

Optics Cleaning

We want to clean both the primary mirror and front surface of the prime focus corrector with carbon dioxide “snow”, and do a wet wash of the primary mirror. However, these operations require a relative humidity of < 50%, sustained through the day, to prevent condensation after the “snow” cleanings and promote drying after the wet wash. We haven’t seen a relative humidity reading that low since the August 1 return to the summit. DESI very much wants the optics cleaned before going back on-sky, as relative throughput measurements using the guide cameras are the first tests called for. We’re waiting for the first relatively dry day to seize an opportunity for optics cleaning.

Restarting DESI

Restart of the DESI equipment can’t really begin until some internet connectivity is restored and remote monitoring of the equipment’s health is possible. The expected order of events is to re-establish a fully-functioning network with internet connectivity; then bring the DESI computer cluster back on line; and then change filters inside the focal plane cooling system, restart focal plane cooling, and test basic focal plane and fiber view camera functions.

Once the focal plane guiders are running we can perform a simple throughput test if the optics have been cleaned and observing conditions are good. The next major phase will be restoring spectrograph operations. This involves restarting the cryostat control systems, pumping down cryostats, turning on the cryocoolers, and waiting for the CCDs to reach working temperature. The spectrograph restarts do not depend on weather conditions, but we do need a back-up electrical power supply or to know that one is on the way, so we don’t risk another unplanned warm-up were the back-up generator to fail.

We don’t yet have a firm timeline for restarting DESI, as so much depends on the weather and other things we have little or no ability to control. We’re doing the best we can, and will continue to post updates as developments warrant.

First batch of Year 1 Key Project 1 papers

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Anand Raichoor and Christophe Yeche
August 22, 2022

We are glad to announce the publication on arXiv on Friday, August 19, 2022 of eight papers from Year 1 Key Project 1 (Y1KP1). These papers describe:

Targets are selected from the Legacy Survey DR9 photometric catalogs, which are derived from three optical imaging surveys in the grz-bands—primarily assembled for that exact purpose, completed with the near-infrared WISE and the Gaia data.

They summarize the long-term effort by many researchers from the DESI collaboration to design a key part of the DESI experiment. All the different algorithms were tested and optimized during the Survey Validation (SV). At the end of the first part of the SV, the team has provided a final version of the target selection for all the tracers. They were finally validated during the One-Percent Survey.

With this target selection work coming to an end, the effort is now focusing on participating in the preparation of the large-scale structure analysis of the first year of data.

A second batch of papers will be published on arXiv in the coming months, prior to the SV data release…so stay tuned!

Example spectra for the DESI five main target classes, illustrating the diversity of observed galactic and extra-galactic objects. Credit: Ting-Wen Lan

Contreras Fire Threatens DESI and Kitt Peak National Observatory

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Angela Berti, University of Utah
June 29, 2022

On Saturday, June 11 a lightning strike started a wildfire in the Arizona mountains less than ten miles from the Kitt Peak National Observatory (KPNO), where DESI observing is done with the Mayall 4-meter telescope. Besides the Mayall, Kitt Peak is home to over 20 telescopes and other buildings that support scientific observations on the mountain, including dormitories where staff sleep. All of these structures were potentially threatened by the wildfire.

By Tuesday, June 14 the wildfire (now called the Contreras fire) had spread to thousands of acres and was less than five miles from Kitt Peak. Around noon local time KPNO safety staff and the fire Incident Commander told everyone on site to immediately prepare to evacuate the mountain. Observing would have to be put on hold. The night of June 13 would be DESI’s final look at the sky for at least several weeks. Within hours a convoy was organized and began to bring those at the summit down the mountain to safety. A skeleton crew of four remained at the summit overnight for full-time fire watch.

On Wednesday, June 15 about 15 firefighters were at KPNO clearing defensive space around observatory buildings. Eight NOIRLab staff were also allowed to return for a few hours to protect critical equipment. This included taping plastic sheets and tarps over DESI’s prime focus corrector and the Mayall telescope’s primary mirror. These sensitive components could be damaged by smoke and ash should the fire get too close.

Plastic sheeting taped over DESI’s prime focus corrector. Credit: Bob Stupak

Nearly 200 firefighters were by then battling the Contreras fire, which was now only three miles away to the south. By the evening at least seven large air tankers were dropping fire retardant near KPNO.

The fire was just two miles away by the morning of June 16. Firefighting teams dropped over 100 loads of fire retardant along the perimeter of the observatory, and local news reported that the Contreras fire was the “#1 priority to wildland fire in the entire United States” due to the value of KPNO.

By early morning on Friday, June 17, the fire swept over the Southwest Ridge section of the observatory, home to MDM Observatory (two optical telescopes), the Arizona Radio Observatory, and the NRAO Very Long Baseline Array radio dish. KPNO webcams mounted on some of the telescopes stopped returning images not long after as the fire disrupted electricity and internet service on the mountain. DESI collaborators around the world could no longer monitor instruments remotely due to the loss of connectivity.

The view of Kitt Peak National Observatory on the morning of June 17 from a camera mounted on the exterior of the Mayall telescope.
Credit: Clara Delabrouille

Good news finally came around midday on June 17 as light rain began to fall in nearby Tucson, Arizona. In the afternoon word came from two employees of NOIRLab who were on the mountain assisting firefighters with KPNO’s water system that no fire had reached the Mayall telescope. By the time the Contreras fire was 100% contained it had spread to nearly 30,000 acres. The fire destroyed four “non-scientific” structures, but none of the more than 20 telescopes atop Kitt Peak were burned!

On Tuesday, June 21, the DESI collaboration gathered in Berkeley, California for its first in-person meeting since 2019 due to the covid-19 pandemic. Many collaboration members who couldn’t be there in person joined remotely, and everyone expressed their gratitude for the incredible firefighters who saved DESI and KPNO from the Contreras fire.

One Year and 12.8 Million Galaxy Redshifts

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David Schlegel, Lawrence Berkeley National Laboratory
May 14, 2022

In its first year of survey operations, DESI has dwarfed all prior redshifts surveys by mapping 12.8 million unique galaxies and quasars. One-quarter of those are at redshifts greater than 1.0.

Exactly one year ago, DESI began its five-year survey with over 100 DESI scientists on Zoom to witness the event. Although the instrument had been demonstrated to deliver science-quality data, this was not yet a smooth operation. At least a few scientists and engineers would join the observing staff each night to trouble-shoot problems with the telescope, the robotic focal plane, the guide cameras, the spectrographs, the CCD detectors, the cooling systems, or the control systems.

As the nights and weeks progressed, problems that would interrupt observations were identified and fixed. A 72-night shutdown starting July 11, 2021, serviced and replaced components of the robotic focal plane. The return-to-survey-operations on September 21 switched to our current nightly staffing model of an on-site Telescope Operator, an on-site Lead Observer, and two remote Support Observing Operators (who split the night). Only rarely are instrument experts still called in the middle of the night to trouble-shoot problems.

The observing efficiency has greatly improved from those first few nights of the survey. We routinely achieve 90% open-shutter time, defined as the time when the spectrograph shutters are open and collecting light from galaxies. The other 10% of the time is spent slewing the telescope to the next field, reading out the CCD detectors and re-configuring the focal plane (with various steps choreographed to occur concurrently). To date, the record open-shutter time was 10 hours 35 minutes on the long winter night of December 7, 2021.

During this first year, survey data has been collected on 242 nights. Of the remaining nights, 72 were for the maintenance shut-down, and only 51 were completely lost to weather or other engineering tasks. 2462 Main Survey DARK tiles and 2073 BRIGHT tiles have been observed. The raw data are transferred to the NERSC supercomputing center as they are collected, and are fully-reduced to calibrated spectra and redshifts by 10 AM the following morning. To date, the galaxy and quasar map consists of 12.8 million unique, reliable redshifts. In addition, 3.6 million unique stars have been observed. The number of redshifts is plotted as a function of time below.

The number of unique galaxies and quasars (top curve) and unique stars (bottom curve) with confidently-determined redshifts as a function of time. The first year of survey operations from May 14, 2021, through May 13, 2022, has delivered 12.8 million and 3.6 million such redshifts, respectively. (Anand Raichoor)

These next two figures show the distribution of observations on the sky. The survey began by observing non-overlapping tiles on the sky. Starting in September 2021, overlapping tiles were being observed that bring the typical number of visits of each patch of the sky to 5 for the DARK survey and 3 for the BRIGHT survey. Currently, prioritization is given to observing near the equator (declination 0), with diversions elsewhere to avoid pointing within 50 degrees of the moon or in the direction of the wind  windy nights. At Kitt Peak, the strongest winds are typically from the south, which has driven some observing to declinations > 32 degrees to avoid telescope wind shake. The DARK survey has observed 9500 square degrees (of a 14,000 square degree footprint) with at least one visit, with approximately 1300 square degrees fully completed with multiple visits.

DESI on a T-shirt (and stickers of course!)

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Claire Lamman, Harvard University
February 28, 2022

We all know the recipe for a great scientific collaboration: novel technology, topical science goals, and cool stickers.

In November 2021 we achieved the final ingredient with the launch of our official DESI Swag Shop.

The DESI shop is hosted on the third-party site Redbubble. This is a place for independent artists to sell their designs on a variety of products—everything from t-shirts to shower curtains. While not specifically designed for organizations to distribute swag, it is a convenient way to make a large selection of items easily available to individuals around the world. We set our shop settings to 0% profit margin—which means DESI makes no money and the prices are as low as possible.

In honor of the opening of our online shop, I made a special DESI-gn that highlights both the science and instrumentation parts of our collaboration. “Dark Energy” is represented by a map of large-scale structure, and “Spectroscopic Instrument” is represented by our focal plane and its 5,000 robotic positioners.

Four months later, 486 items have been purchased from our shop. The most popular are:

  1. DESI Logo sticker
  2. DESI Plane sticker
  3. DESI Logo magnet
  4. DESI Plane t-shirt
  5. M31 Focal Plane sticker

Beyond stickers and t-shirts the shop also has masks, mugs, puzzles, posters, and even socks. Here’s a look at how the popularity of these items compare:

Stats like this may be helpful for conference organizers deciding what types of merch products to use. The key takeaway here: never forget the stickers!

Selecting Targets for the DESI Survey

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Edmond Chaussidon, CEA Saclay
January 12, 2022

Although DESI will be able to collect 5,000 spectra simultaneously, galaxies are still too numerous to be all observed during the next five years of observation. Target Selection (TS) is a crucial step to identify which objects to observe during the spectroscopic survey.

To constrain the cosmic expansion history through measurements of Baryon Acoustic Oscillations, DESI probes the matter in the Universe with four different tracers:

  • Bright galaxies (BGS) in the redshift (z) range 0.05 < z < 0.4
  • Luminous Red Galaxies (LRG) for 0.4 < z < 1.0
  • Emission Line Galaxies (ELG) for 0.6 < z < 1.6
  • Quasars (QSO) for 0.9 < z < 2.1

Quasars will also probe the Universe at higher redshift (z > 2.1) through Lyman-alpha forest studies measuring the light absorption by the gas in front of quasars.

Easy to say that we want to select different galaxy types, but how? The main information we need are the galaxy colors, which we can determine thanks to the measurements provided by photometric surveys. We used the catalogs of the DESI Legacy Imaging Surveys, a program conducted over more than 14,000 square degrees of sky from the Northern hemisphere, in three optical bands: g (in the blue-green), r (in the red), and z (in the red/near-infrared, and not to be confused with redshift!). The data were collected via three independent programs:

  • The Beijing–Arizona Sky Survey (BASS) observed ∼5,100 square degrees of the North Galactic Cap (NGC) in g and r using the 2.3-meter Bok Telescope.
  • The Mayall z-band Legacy Survey (MzLS) provided z-band observations over the same footprint as BASS using the 4-meter Mayall Telescope.
  • The Dark Energy Camera Legacy Survey (DECaLS) was performed with the Dark Energy Camera (DECam) on the 4-meter Blanco Telescope. DECaLS observed the bulk of the Legacy Imaging Surveys footprint in g, r, and z.

These optical data were complemented by two infrared bands from the all-sky data of the WISE satellite, namely: W1 (3.4 μm) and W2 (4.6 μm).

The selection of the BGSs, LRGs and ELGs was mostly done based on conditions on the source colors. For the QSOs, however, which are harder to select because their colors are quite similar to those of the overwhelming stellar background, we developed a more complex approach based on machine learning. To further reduce the contamination of the sample from unwanted stars or galaxies, we applied an additional constraint on source magnitude. All these conditions were tuned on previous spectroscopic samples as those from the BOSS/eBOSS program. But of course, we also intensively tested them during the Survey Validation (SV) phases emulating a nominal observation with DESI. Voilà! Lo and behold, they all give very satisfying results.

DESI is now on sky, observing every night the quasars and the galaxies that we selected using these algorithms, on its way to build the largest 3D map of the Universe to date!

The Target Selection and Survey Validation steps are described in great detail in the upcoming eight scientific papers (one Overview, two for the SV, four for the TS and one for the Milky Way Survey). Pipeline and algorithms for data reductions and operations will be described in another set of five scientific papers later. So, stay tuned!

For the curious, below are more details on how we proceed for the four classes of targets, and some information about the density of objects we thus select.

Bright Galaxy Target Selection

Star–galaxy separation in BGS is performed using a GGaia – rraw cut. This criterion exploits the fact that the Gaia magnitude is measured with an aperture of a space-based point-spread function (PSF) while the Legacy Imaging Surveys magnitude captures the light from the entire source. This cut separates point sources (stars) from extended sources (galaxies). This selection will observe ~850 targets per square degree. (See Hahn et al., in prep.)

Luminous Red Galaxy Target Selection

LRG selection is done by the cut (red line) in the (r – z) – (z – W1) space. This selection uses the W1 infrared band to separate galaxies (color points) from stars (grey points). The different colors show the redshift of the galaxies in the color–color space. This selection will observe ~615 targets per square degree. (See Zou et al., in prep.)

Emission Line Galaxy Target Selection

To avoid stellar contamination (black smooth line) in the target selection, ELG selection uses a cut in (r – z) – (g – r) space. The color histogram is the redshift distribution of ELGs in the color space. This selection will observe ~2,387 targets per square degree. (See Raichoor et al., in prep.)

Quasar Target Selection

Only point sources are considered during the selection and the target selection aims to separate quasars from stars. However, the separation between these two classes is less obvious than in the previous cases. A more sophisticated selection has to be applied. Quasars are selected via a Random Forest classification instead of the classical color cuts selection done in previous spectroscopic surveys. The idea of the selection is to separate quasars from stars based on the “infrared excess” of QSOs. The stellar locus is illustrated by the red line and the quasars by the blue/green/yellow points. This selection will observe ~308 targets per square degree (see Chaussidon et al., in prep.).

The Old is New Again: Social Distancing While Mapping the Universe

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David Lee Summers, Kitt Peak National Observatory
November 17, 2021

Many people who hear the word “astronomer” imagine a scientist spending long nights in an observatory dome peering through a telescope’s eyepiece. In fact, when the Nicholas U. Mayall telescope was commissioned in 1973, astronomers could sit at the telescope’s focus to operate the cameras. In those early days, the telescope operator worked at an analog console in a control room inside the dome. Over the years those telescope controls migrated to computers and the astronomers joined the telescope operators in the control room.

The original Mayall control room.

The control room is a cozy space. It’s a long, narrow room. The old analog console dominates one wall. Desks and shelves line the other walls. The closest restroom is two floors below the control room and the control room didn’t even have a sink until one was installed circa 2010. A small microwave allowed the operator and observers to heat their night lunches. When planning for the DESI survey began, people soon realized this space was far too small for the number of scientists who would need to collaborate on a given night while commissioning the instrument. Even during regular observing, it was expected that three or four people would need to be in the control room at the same time.

A new, larger space within the Mayall building was identified to serve as the control room. A former recreation room on the so-called Utility floor was renovated into a modern control room in 2017. Even before DESI operations began, we moved into the new control room to finish the Mosaic Z-band Legacy Survey. The new control room allowed several scientists to collaborate during the night. There was plenty of desk space for computers and monitors to watch all the telescope and DESI functions. An adjoining room held a small kitchenette and a meeting room. What’s more, restrooms were only one room further on. The U-floor control room seemed a vast improvement as a practical workspace—at least until the unexpected happened and operations had to be stopped in March 2020 for the COVID-19 pandemic.

The utility floor Mayall control room, circa 2018.

Although we paused on-sky DESI commissioning, much thought was given to safe operations. Clearly we couldn’t have several people working together in one enclosed room, especially in the days before a vaccine had been developed and before the effectiveness of masks had been demonstrated.

Before the DESI installation began, some astronomers who used Kitt Peak facilities already observed remotely from their home institutions. They could log in and control an instrument through the internet and communicate with the telescope operator through video conferencing software. This idea became the core for new, safe DESI operations. The telescope operator would return to the old control room next to the telescope. A lead DESI observer would work in the new U-floor control room. All other collaborators would work either from home or their home institutions and communicate with the team at the telescope over Zoom.

As an operator who has worked at Kitt Peak since the 1990s, it’s been gratifying to watch DESI’s development and see this next step in the Mayall Telescope’s illustrious history. It’s also been gratifying to see how elements of the telescope’s legacy along with new technology have kept us operating through these challenging times.

Congratulations to Frank Valdes

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Adam Bolton, NOIRLab
November 1, 2021

DESI collaborator Dr. Frank Valdes, a scientist at NSF’s NOIRLab, has been honored with the 2021 ADASS Prize for an Outstanding Contribution to Astronomical Software. The award recognizes Frank and two other colleagues for their foundational roles in creating the Image Reduction and Analysis Facility (IRAF) software system. IRAF was launched over three decades ago as a pioneering project to create general-purpose software tools that would allow astronomers to translate the images taken by telescopes into calibrated data products suitable for quantitative scientific analysis. In the years since its initial release, IRAF has been used in nearly 25,000 scientific publications. Most significantly for DESI, all the images collected by the Legacy Surveys (DECaLS, MzLS, and BASS) to enable DESI’s spectroscopic target selection were processed through an IRAF-based pipeline created by Frank and operated by him and other NOIRLab staff members. The DESI Collaboration congratulates Frank on this well-deserved recognition!

This mosaic of the standard IRAF test image (M51, the Whirlpool Galaxy), found in every IRAF installation, is composed of plots and images taken from the more than 25,000 refereed papers published since 1986 that mention using IRAF.

DESI Breaking Records

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David Schlegel, Lawrence Berkeley National Laboratory
October 13, 2021

DESI has broken its own records, mapping the three-dimensional location of 100,000 distant galaxies and quasars in the single night of October 8, 2021.

The DESI Main Survey started on the evening of May 14, 2021. Nearly 100 scientists joined the operations team on Zoom that night to witness the beginning of the survey. It wasn’t a perfect night, as the “seeing” (blurring of the sky from atmospheric turbulence) was worse than typical, and some time was lost to instrument communication errors. Even still, eight dark-time tiles and ten bright-time tiles were observed, which was enough to set a record (for any telescope) of mapping 60,000 galaxies in a single night.

The operational efficiency of DESI has been improved since that first night. One important aspect of this is the “robustness” of the complex instrument and control systems. There are 5000 fiber robots on the top end of the telescope commanded to move every 15 minutes, 6 guide cameras, 4 optical wavefront sensors, 30 cryogenic cameras taking data in the spectrograph room, a large assortment of sensors, and dozens of computers to control all these systems. It’s not uncommon to have minor, non-critical faults somewhere in the system, and those faults have to be trapped rather than bring the whole system to a halt.

Operational efficiency also relies upon a careful choreography of reconfiguring the instrument between observations: the telescope is slewed to its next sky location, the focal plane is re-focused as the 375 tons of moving weight slightly sags under gravity, the 5000 fiber robots are re-positioned to point to the locations of unobserved galaxies, and the previous spectroscopic exposure is read out. The inter-exposure time for this dance of activities that was typically 3 minutes during the first night of the survey has been reduced to 2 min 15 sec. For this particular record-setting night, the open-shutter efficiency was 87%, meaning that’s the fraction of time that the shutters on the spectrographs were open and integrating on light from distant galaxies.

Finally, the weather needs to cooperate! The night of October 8th was a fully “dark” night, meaning that the moon was below the horizon. The skies were clear (“photometric”) for all but a few minutes when a thin cloud layer blew through. And the “seeing” was excellent, such that the blurring of the atmosphere was less than 1 arcsecond for most of the night. The sky wasn’t quite as dark as the best nights, partly because we were observing near the ecliptic plane where dust particles in the solar system reflect small amounts of light from the sun. In combination, the weather was sufficiently good that we were running at a survey speed of 120%—meaning 20% faster than expected under nominal, good conditions.

The night sky as seen from the Kitt Peak all-sky camera on the night of October 8, 2021. It was a dark, clear night, with the Milky Way passing overhead early in the night.

How many galaxies were mapped in this one night? We completed observations on 29 dark-time tiles (with one tile completing observations from a previous night). Each of those tiles included approximately 4200 high-redshift targets (galaxies or quasars), with the remaining 800 fibers assigned to calibration targets. Most of those galaxies were “redshifted” (mapped) in a single observation. These data were analyzed by 10 am the following morning on NERSC (the National Energy Research Scientific Computing Center) to confirm reliable redshifts for 98,954 galaxies and quasars. Additionally, two bright-time tiles observed during evening twilight measured redshifts for 4225 low-redshift galaxies, bringing the total haul to 103,179 galaxies and quasars.

Will this record be broken in the future? Most assuredly, if only because the nights are getting longer. This record-setting night had 9 hours 22 min of time between astronomical twilight (when the sky is darkest with the sun at least 18 degrees below the horizon). The best nights for setting records will be the new moons closest to the winter solstice. This year, that will be the nights of December 3 (10 hours, 57 minutes) and January 1, 2022 (10 hours, 59 minutes).

Congratulations to the observing team for the night of October 8, 2021: Liz Buckley-Geer (Fermilab, Lead Observer), Abby Bault (University of California Irvine, Support Observer), Mike Wang (University of Edinburgh, Support Observer) and Thaxton Smith (NOIRLab, Observing Assistant).

The DESI peculiar velocity survey

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Christoph Saulder, Postdoc at KASI
October 11, 2021

While the main goal of DESI is to obtain spectroscopic redshifts for millions of galaxies to map the expansion history of the universe, there are also secondary targeting programs that supplement the main DESI survey and its science goals. The DESI peculiar velocity survey is one of these programs and it aims to improve the measurements of cosmological parameters in the local universe.

And yet it moves…
Even when accounting for the expansion of the universe (“Hubble flow”), galaxies do not sit around idly, but move around due to their mutual gravitational interactions. These proper motions, which are refereed to as peculiar velocities in our context, encode information about distribution of matter. By default, DESI measures the redshifts of galaxies, which are a combination of the cosmological redshifts caused by the expansion of the universe and the redshifts caused by peculiar motions. As the cosmological redshifts correlate with the distances to these galaxies, one can use redshift-independent distance indicators in combination with the redshift measurements to obtain the (radial) peculiar velocities of these galaxies. As we want to measure the peculiar velocities for a large number of sample, we cannot wait for a rare supernova type Ia to go off in each of them and most of them are too far away for us to resolve any Cepheids variable stars. We therefore have to rely on less precise, but more easily accessible redshift-independent distance indicators, such as galaxy scaling relations, in particular the fundamental plane and the Tully-Fisher relation. These allow us to measure the distances, and thereby the peculiar velocities, to a large number of galaxies in the local universe (redshift of less than 0.1).

Still waters run deep…
Which scaling relation a galaxy obeys depends on its morphological type. Galaxies with essentially no cold gas and therefore no ongoing star-formation appear as the relatively boring looking elliptical galaxies. These quiescent galaxies follow the fundamental plane, which relates the internal kinematics of these galaxies express as the central velocity dispersion to their surface brightness and physical size. By comparing the physical size predicted by this relation to its angular size on the sky, we are able to obtain the distance to such a galaxy by using simple trigonometry. While measurements of the angular size and surface brightness can be done on the already collected data of the photometric DESI Legacy Imaging Survey, we will need spectroscopic data for the central velocity dispersion. The central velocity dispersion quantifies the statistical dispersion of the motion of stars in the center of a galaxy.

The impact of the central velocity dispersion σ0 on the shape of two spectral lines. (credit: ChangHoon Hahn)

In DESI, fibers collect spectra from the innermost parts of these galaxies. In these spectra, the central velocity dispersion manifests as broadening of the spectral lines. However, to reliably recover these features, one needs spectroscopic data of excellent quality. Luckily most of our targets are bright galaxies, which are masked areas that within there are no high redshift galaxies targeted in DESI during dark time (best observing conditions). This means our additional observations have only little competition and over the course of the DESI survey, we will be able to gather data we need without interfering with the main DESI observations.

Round and Round it Goes…
The maximum rotation velocity of spiral galaxies is tightly linked to its absolute luminosity via the so-called Tully-Fisher relation. By comparing the predicted absolute luminosity to the observed apparent luminosity from DESI Legacy Imaging Survey, one can derive the distance to these galaxies. The maximum rotation velocity can be measured by placing fibers along the semi-major axis of the galaxy and just comparing the measured redshifts at these locations. As our targets are big and bright galaxies, the geometry of DESI often even requires fibers to placed somewhere on these galaxies, so we made sure that they will end up on the best possible location to maximize our science output.

These figures illustrate how the additional data about peculiar velocities can improve the strength of DESI cosmological measurements at low redshifts. (credit: Cullan Howlett)

The answer to life, the universe, and everything
While it seems to be simple, getting from redshifts and redshift-independent distance measurements to peculiar motions, it requires a lot of careful modeling of all kinds of biases and selection effects. In the end, we want to obtain solid measurements for cosmological parameters from the data. Peculiar velocities can supplement the measurements on the growth of structure that are usually from redshift space distortions and we can thereby can get tighter constraints on the cosmological parameter f σ8 , also refereed to as the growth rate (of structure). Its evolution over time and its scale-dependence can used to test for possible deviations from General Relativity. Additionally, by using galaxy scaling relations as distance indicators, we are able to collect data about the kinematics of a large sample of galaxies that will help us to gain better insights into the dark matter halo relation.

Step by step…
We were able to obtain our first measurements using the survey validation data of DESI, in particular the 1% survey. These first results helped us to test and improve our target selection methods, which are now applied to the ongoing main DESI survey. We are currently preparing several publications, which are to be released along with the first public release of DESI data, that will present the details of our target selection methods and the first results from the fundamental plane and Tully-Fisher relation. While data collected so far is sufficient for the basic calibrations of our methods, it will still take years until the survey is completed and we have sufficient data for strong scientific results and the chance of discovering any new physics beyond our current standard model.

A DIY Guide for Upgrading Your 100 Million Dollar World Class Astronomical Instrument

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John Della Costa, University of Florida
October 2, 2021

Astronomy is fun. But have you ever wanted to make it even better? Then why not try upgrading your 100 million dollar world class astronomical instrument! This may sound like an extremely difficult challenge that would require more than 30 people and many weeks of labor intensive work around the clock, and you’d be right! But with this handy DIY guide, you’ll be able to contribute in no time at all! So, follow along with this step-by-step tutorial and you’ll be observing the universe with your unrivaled, upgraded instrument faster than you can say, “Oh no, I only see 12 positioners but I plugged in 13. Please don’t make me take the whole petal apart again!”

Working carefully with fibers and electronic connectors.

Step 1: Get to the mountain
Remote mountaintops in dry locations are the best places to house telescopes. If you’re lucky, your amazing instrument will be located on the top of one in Southern Arizona. But first, you have to get to it! After your 4 hour plane ride in the lap of luxury, rent a vehicle and drive for 50 minutes until you get to the mountain road gate. Make sure to remember the combination for the lock (you still have the email with it, right?). Next, drive up the winding path to the summit. Try not to break any vertebrae as your neck becomes rubber from the occasional glimpses of the massive telescope your instrument desperately clings to. You’ve done it, you’re on the mountain!

Step 2: Get your safety briefing
Your safety briefing is one of the most important steps required for you to upgrade your instrument. When you’re on the mountain, there will likely be many creatures just waiting to kill you! So, be prepared, and never, ever, leave your food next to your bed as you sleep with the window open.

Step 2.5: Did you get food for your stay?
Did you really forget to get food for your 3 week stay on a mountain? Did you think you were going to subsist off of cacti and spiders? Now, drive back down the mountain, go to the grocery store another hour away, and drive back another hour again. Make sure you bought enough food to completely fill the freezer in the house so that you’ll have extra to throw out when you leave.

Step 3: Start disassembly of the first petal
By the time you get back to the dome after gluing your freezer shut, Petal 4 should be out of the telescope. But, before you start disassembly, make sure to test those pesky positioners with their mysterious ailment that nobody can understand. Now you’re ready to start disassembly! Just kidding, you have to take off the covers first. Make sure you use the correct tool or you might strip the… You stripped the screws, didn’t you? Well, that’s ok! Just make sure you have your hammer and drill close by so you can start going to town on your extremely fragile, multi-million dollar petal! But luckily, no other screws will be stripped on any other petal (foreshadowing). Now that you’ve spent many precious hours on this, you can start to disassemble the petal! Make sure as you pull out each positioner’s connector you really get your thumb and index finger in there. Don’t worry though, this definitely won’t leave you with nubs after doing it 5000 times… Wow, you’re fast! It only took you 3 hours of nonstop work! You should probably go home now and get some rest. Luckily your accommodations are only a 5 minute drive away (on the top of a mountain in complete darkness)!

Step 4: Start petal reassembly
Once the petal is completely taken apart, all 500 positioners are successfully unplugged, and all the transverse boards are out, you can get started on petal reassembly! Make sure not to snap any fibers, it’s not like they’re irreplaceable or anything. Finally, after two to three days of nonstop work, you’ll have your petal completely put back together. Now, just put it back in the telescope with that giant crane attached to the dome that lifts the 200 pound petal 30 feet into the air (seriously though, this part is terrifying to watch).

Step 5: Rinse and repeat!
Now all you have to do is repeat steps 3 and 4 seven more times (you’ll probably only be able to get 8 petals done during your 3 weeks on the mountain)!

Step 5.5: Take some time off
Make sure to take a day off here and there. Go for a hike, see some cacti (make sure to pick up their fruit with no hesitation, they can’t hurt you!), drive down the mountain and back up again, and enjoy the 110 degree weather (it’s a dry heat)!

John on the mountain with the Mayall Telescope in the background.

Step 6: Make sure it’s not too easy
Be sure that Petal 6 has a lot of issues so that you have to take it apart and put it back together. Twice! But don’t worry, it’s great practice, and your thumbs will thank you later.

Step 7: Bask in the glory of an upgraded instrument
Now that you’ve completed the disassembly and reassembly of your world class multi-object spectrograph, you can start observing, again, and get back to deciphering the mysteries of the universe!

Step 7.5:
Wait, you’re telling me we have to test the instrument for weeks after reassembling it? Will we ever get to start observing again?

Well, while you wait, you can look back at the over 5000 pictures you took while you were on the mountain and think back at the amazing, once in a lifetime experience you had while helping to upgrade one of the best astronomical instruments on planet Earth, and remember all the skills you learned and all of the amazing people you met along the way.

An Upgraded DESI Returns to the Sky

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Claire Poppett, Lawrence Berkeley National Laboratory
October 1, 2021

Anybody who has observed with the DESI instrument has experienced uncommunicable positioners on the focal plane that lead to frustrating pauses in the observing schedule whilst the lead observer or a focal plane expert work to bring these devices back. During the Tuscon summer monsoon season in 2021 a team of DESI collaboration members worked with NOIRLab staff to upgrade the DESI instrument and make these experiences a problem of the past.

A technician carefully removes fibers (blue) and electronic connectors (rainbow) from electrical and mechanical supports.

To understand why these positioners become uncommunicable, we must first understand how they communicate.

The DESI positioners communicate via a Controller Area Network (CAN bus) and are grouped into patches of 25-75 positioners per CAN bus. Unfortunately, we have discovered that if a single positioner has a faulty communication signal it can affect the other positioners in that patch. The bigger issue here is that this loss in communications leads to a loss in their telemetry and the possibility that they may exceed our temperature limits without our knowledge. The collaboration agreed that this problem needed to be fixed and so a remediation plan was developed.

It was determined that the simplest way to be able to turn off a faulty positioner was to add a relay switch into the hardware but this required a major upgrade to the architecture of the focal plane. This was not possible without removing the focal plane from the telescope and so at the end of July, NOIRLab staff began the arduous task of getting the telescope into a state that made it possible to remove focal plane petals and install them onto work carts staged in temporary clean tents on the C-floor of the Mayall.

One of 10 petals being removed from the back of the focal plane with the telescope secured at the South-East platform.

Once the petals were on the floor every single fiber and every single positioner wire had to be handled in order to remove the old communications boards from the petals in order to allow new boards with relay switches to be installed. These upgrades were performed on all 10 petals and included extensive testing to ensure that all devices now communicated as reliably as possible. The final petal was inserted back into the focal plane on August 25th and by lunchtime the next day we were able to declare that we had a healthy focal plane.

After only 10 weeks of downtime, the instrument was returned to service during the week of September 12th. Recalibration of the focal plane. Main Survey observations re-commenced on the evening of September 21. Despite the full moon, 60,000 Bright Galaxy Survey redshifts were measured that first night back!

All signs so far point to a truly upgraded DESI focal plane and we can look forward to many more years of DESI science!

Cosmic Cartography

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Claire Lamman, Harvard University
August 20, 2021

For many, the term “explorers” brings to mind long ocean voyages, adventurers plunging into jungles, and traders discovering new connections between continents. Exploring, as a profession, became obsolete as there were simply no large, unexplored regions left and Earth became thoroughly mapped out.

However, there are still many uncharted regions beyond our planet. Hundreds of modern-day explorers are mapping out the galaxies around us, a craft that I like to think of as Cosmic Cartography. Fortunately for us, there’s not as much scurvy and snakes involved, but there is a massive effort behind today’s surveys and cosmology comes with its own set of adventures. 

DESI is creating the most detailed map yet of nearby galaxies. To celebrate the connection between early terrestrial and early cosmic explorers, I’ve made this DESI art in the style of early world maps. Here is a guide to some of the details:

  1. A history of our universe. Since the big bang, our universe went through many changes as matter formed and eventually made stars, galaxies, and us. To understand how these changes happen, and how fast the universe is growing, we need to understand what it’s made of.
  2. Pie chart of the universe. From our observations, we know that normal matter (the stuff that stars, dust, you, and I are made of) only accounts for about 5% of content in universe. 26% is dark matter- matter we cannot see but can study through how its gravity affects the normal matter and light around it. The other 69% is dark energy. As far as we know, about the only thing it has in common with dark matter is a spooky name. Dark energy is the mysterious force we credit with the rapid expansion of the cosmos. For more information see this blog post.
  3. Time Map. Light takes time to travel. The light we see from nearby galaxies is relatively young, but from older galaxies it comes from a much older universe. Therefore, we can get a glimpse at what the universe looked like at different times by mapping it out near and far away. This is the map made by the Sloan Digital Sky Survey (SDSS) of galaxies around us. Earth is at the center. It looks like an hourglass because those are the galaxies in the part of the sky where SDSS was measuring. If you look closely, you may be able to see some structure – in places with the most data you can pick out a sponge or web pattern.
  4. DESI. This is our logo. Our mission is to make a more detailed map so we can explore some of the remaining questions about dark energy and the evolution of the universe.
  5. The DESI Footprint. This map shows the parts of the sky where DESI is looking. We call it the “footprint”. One chunk is south of the Milky Way (left) and one is north of it (right). DESI has already picked out the 30 million galaxies that we will measure. For more information see this blog post.
  6. DESI’s home. DESI is located on a mountaintop in Arizona, Kitt Peak (left). It’s part of the biggest telescope on the mountain, the Mayall (right).
  7. The Telescope. This is what lies under the dome – a massive structure supporting a 15-ton primary mirror, fine-tuned optics, a focal plane, and a separate room filled with spectrographs. More on all of this below.
  8. How it works. This is a simplified diagram of the telescope. First, light hits the primary mirror. It then reflects up into a series of lenses before hitting the focal plane. The focal plane is a disk of 5,000 robotic positioners, each able to gather the light of their own galaxy. The light from each robot is then sent via fiber optic cables to spectrographs, which measure the light so we can get a spectra for each galaxy (11).
  9. The focal plane. We often plot the values of each positioner, as they’re positioned on the focal plane. This is a drawing of what we saw when the telescope pointed at Andromeda. This galaxy takes up the perfect amount of sky so that we have a “picture” of it just by looking at positioner values. The other galaxies are far enough away that each gets its own positioner.
  10. The positioners. Here is a diagram of what one of the robotic positioners looks like, with a drawing of one in the background.
  11. Spectra. This is the type of measurement we want to get of every galaxy. Light from a galaxy contains many signals, coded in the amount of light that is present for different amounts of energy. When you plot the intensity of light as a function of energy (or wavelength) you see something like this. This is the spectra of a quasar, a brilliant type of galaxy with a very active black hole at its center. We expect to see certain peaks at certain energy levels. These shift based on the galaxy’s movement and allow us to measure its distance. For more information see this blog post.

More details

  1. Draco. Star locations and art of the constellation Draco. You can’t have an old map without a serpent!
  2. Tomog. Tomog is the Oʼodham word for the Milky Way. The Tohono O’odham people are native to Kitt Peak and the land surrounding it.
  3. Cosmic Linguistics. These equations help astronomers describe and study cosmology. They express everything from how further objects look dimmer to how different components of the universe affect its evolution.
  4. DESI Dog. Our beloved mascot, the DESI coyote, with the constellation Canis major, or “the big dog”.

Diversity of DESI SV Quasars

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Victoria Fawcett, Durham University
July 22, 2021

As the main DESI survey gets truly underway, I want to reflect on the amazing quality of the DESI quasar spectra observed so far. Quasars are incredibly luminous types of Active Galactic Nuclei (AGN), which consist of a supermassive black hole at the centre of galaxies, surrounded by a disc of matter that often outshines the entire galaxy. Quasars are one of the brightest objects in the Universe and are really important in all areas of Astronomy, especially Cosmology.

With over 30,000 quasar spectra observed so far, DESI is already pushing beyond what previous spectroscopic surveys have achieved. The image shows these ~30,000 spectra stacked in different bins of redshift, with the lowest redshift bin (z<0.4) corresponding to the objects closest to us. The dashed lines indicate the main emission lines in the spectra, each of which provide important information about the nature of the object: for example, the Oxygen lines can be used to study outflows close to the supermassive black hole. The clarity of the spectra and different lines really highlights the amazing quality of the data—the two red dotted lines show galaxy absorption lines, so we can even clearly see the affect of the surrounding galaxy!

Other exotic objects such as Broad Absorption Line Quasars (BALQSOs), systems known to host powerful outflows, have also been found within DESI (see image below). The dips to the left of the CIV, SiIV and Lya line are called BAL “troughs”, which deepen with increasing “balnicity index (BI)”; a measure of the strength of the trough. Studying these systems may be really important for understanding the processes that connect AGNs and their host galaxies.

DESI also pushes to fainter and more obscured systems which have been difficult to observe with shallower spectroscopic surveys. Quasars enshrouded by dust (“red quasars”) may represent an important phase in galaxy evolution so understanding their properties is crucial – these are the objects I am most interested in!

With millions of more quasar spectra to come, the future of quasar physics looks bright. I personally look forward to analysing the data and exploring all the weird and wonderful objects DESI has to offer.

The Beginnings of the 3-Dimensional Map

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David Schlegel, DESI Project Scientist
July 22, 2021

Only eight weeks into DESI’s five-year mission, a 3-dimensional map of the universe is starting to take shape.

Plotted below is a “pie diagram” slice through the universe, with earth in the lower left, looking out in the directions of the constellations Virgo, Serpens and Hercules to distances beyond 5 billion light years. As this video progresses, the vantage point sweeps through 20 degrees towards Bootes and Corona Borealis. Each point represents a DESI target. The nearest are the bright galaxy sample (BGS as white points), then the luminous red galaxies (LRGs in red), emission line galaxies (ELGs in green), and finally quasi-stellar objects (QSOs in blue). Each of these targets is composed of 100 billion to 1 trillion stars, although we plot each target only as a single point. Gravity has clustered the galaxies into structures called the “cosmic web”, with dense clusters, filaments and voids.

DESI shuts down today for summer maintenance and upgrades, timed to coincide with the monsoon season in Arizona. When observations re-start in September, five times more galaxies will be observed at each sky location, gaps in this map will be filled in, and the area surveyed will eventually grow to span most of the sky visible from the northern hemisphere.

Siena Galaxy Atlas 2020 publicly released by DESI Legacy Imaging Surveys team

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The DESI Legacy Imaging Surveys team is pleased to announce the public release of the Siena Galaxy Atlas 2020 (SGA-2020), a value-added supplemental catalog to imaging Data Release 9 (DR9).

The SGA-2020 is a multiwavelength atlas of 383,620 nearby galaxies selected over the 20,000 square-degree imaging footprint based on their (large) apparent angular diameter. It has been used in Survey Validation and the Main Survey to improve the selection of BGS targets, for improved masking of faint, dark-time (ELG, LRG, QSO) targets, in secondary target programs (e.g., to facilitate studies of the peculiar velocity field in the local volume via the Tully-Fisher and Faber-Jackson relations), and more. Read more about the SGA-2020 on the AAS poster.

For each galaxy in the SGA, we carry out the following measurements:

  • We generate custom grz and W1W2 image stacks using all the available imaging (upper-left panel);
  • We use The Tractor, in the context of the legacypipe photometric pipeline to build a model of all the sources in the field (middle-left panel);
  • We subtract all the sources in the field except the object of interest from the data and measure the surface-brightness profile using nested, fixed-geometry elliptical isophotes (bottom-left panel).

This procedure yields the non-parametric grz surface-brightness profiles, g-r and r-z color-radius profiles, and grz curves of growth shown in the right-hand panels of the figure above, and more.

Documentation of the SGA-2020 can be found here. A queryable web-application is also available here.

What do DESI’s 5000 eyes see?

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Bela Abolfathi, University of California Irvine
June 1, 2021

On May 17, DESI officially began a 5-year survey of the cosmos to study dark energy and its role in the accelerated expansion of the universe. Over the course of its survey, DESI will collect spectra from over 30 million galaxies across 11 billion years, a feat that will result in the largest 3D map of the universe ever created.

Prof. David Kirkby at UC Irvine created an interactive 3D visualization that helps put into perspective exactly how ambitious of an undertaking this is. The visualization linked to the image below shows around 30,000 galaxies observed during the survey validation phase—less than 0.1% of the galaxies DESI will eventually catalog—jam-packed into the space behind your palm held at arm’s length.

Try the interactive version here (credit: David Kirkby/DESI collaboration)

Each of the four types of galaxies specifically targeted by DESI are denoted by a separate color and correspond to bright galaxies (BGS), luminous red galaxies (LRG), emission-line galaxies (ELG), and quasars (QSO). These types populate different redshift regimes, as seen from the progression of colors over the course of the movie. That’s not to say that types are confined to select regions of redshift-space. Rather, they are targeted in this way because, once they’ve been redshifted to our telescope, their unique signatures fall into the 360-980 nanometer window registered by our detectors. In the case of high-redshift quasars, these objects are selected because they help reveal the distribution of matter in the intergalactic medium via the shadows they cast along their journey.

Like the opening crawl at the beginning of every Star Wars movie, the visualization starts in the nearby universe and takes us back in time to distances far, far away. Exactly how do we determine these distances? We start with each galaxy’s redshift, which we can measure from its observed spectrum. Both the composition and curvature of the universe will determine how fast it’s expanding, and by extension any notion of distance. We therefore need to assume a particular cosmological model in order to then transform from redshift to comoving distance.

The visualization defaults to using the fiducial values of 31% matter, 69% dark energy, and zero curvature. Changing these parameters will have a noticeable effect on the distance traveled as well as the age of the universe.

One of DESI’s main cosmological probes is baryon acoustic oscillations (BAO), the imprints of relic sound waves from the early universe. BAO is a standard ruler used to measure the scale at which galaxies tend to cluster. The concentric circles emanating from the center of the 3D visualization show the comoving 150 Mpc BAO scale at which we expect galaxies to cluster slightly more often compared to a random distribution of galaxies.