Dark Energy Spectroscopic Instrument (DESI)

Hee-Jong Seo

by

aberti

What is your position or role in the DESI project?
I am currently a co-convener of the DESI Year 1 BAO Key Project.

Where were you born?
Busan, South Korea

Where do you live now?
I live in Athens, Ohio, a small college town. This year, though, I am living in Berkeley, California, for my sabbatical leave. I am super excited.

What do you do as part of DESI?
My postdocs, students, and I are members of the galaxy—quasar clustering working group and our focus is on how to better reconstruct the cosmological information in the distribution of the galaxies, part of which has been lost during the evolution of the Universe for the last 13.8 billion years. We are also working on how to best clean up the observational systematics from the data using a deep learning method.

As a co-convener of the Year 1 BAO Key project (aka Key Project 4), I and many KP4 participants together are working on robustly deriving the DESI Year 1 BAO measurement. We expect the first year DESI data alone to produce the best BAO measurement to date, and therefore it requires a much more stringent systematic control than has been ever done.

What is the most interesting or exciting thing about your job?
What can be more exciting than learning about our Universe? 🙂

Any advice for an aspiring scientist?
Do what you like the most (but I am 99.99% sure that whoever reading this already like astrophysics very much). And the postdoc time is the best time (so, keep swimming).

What do you do for fun?
Learning new things such as piano, kick-boxing, and ballet.

If you weren’t a scientist, what would be your dream job?
This job was my dream job. My alternative dream job was a cartoonist (I had good talent on this, except that I could not redraw the same character more than once).

What excites/interests you most about DESI?
This is the best galaxy survey data today and also I very much appreciate and enjoy the collaborative environment of DESI. Working with people across the world to solve a common question brings you some exciting moments from time to time.

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.

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.

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

What is your position or role in the DESI project?
After co-chairing the Lyman alpha WG for several years, I am now co-convener of Key Project 6. We plan to measure BAO above redshift 2 from the correlations of the Lyman alpha forest, and its cross-correlation with the quasars themselves.

Where were you born?
I was born in Barcelona.

Where do you live now?
Finally, I am back to Barcelona after 9 years of wandering around the globe.

What do you do as part of DESI?
Besides coordinating the BAO measurement of KP6, I like to study the distribution of matter on Megaparsec scales using the Lyman alpha forest, and compare the measurements to hydrodynamical simulations.

What is the most interesting or exciting thing about your job?
I like working on puzzles, like a difficult Sudoku or the accelerated expansion of the Universe.

Any advice for an aspiring scientist?
Being a scientist is fun, but there are other cool jobs out there. So give it a good shot, but don’t get too obsessed about it.

What do you do for fun?
I like playing sports (climbing, tennis, football) and I love trying new food and beers.

If you weren’t a scientist, what would be your dream job?
I’d like to understand the chemistry of cooking. I know, I’d still be a scientist, but a very different one!

What excites/interests you most about DESI?
I like being part of a large collaboration, seeing lots of creative people working together to achieve these very complex and exciting measurements.

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.

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.

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!

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.)

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.

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.
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).

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.