blog

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 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.

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!

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!”

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)!

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.

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.

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.

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!

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

12. Draco. Star locations and art of the constellation Draco. You can’t have an old map without a serpent!
13. 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.
14. 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.
15. DESI Dog. Our beloved mascot, the DESI coyote, with the constellation Canis major, or “the big dog”.

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.

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.

Bela Abolfathi, University of California Irvine

June 1, 2021

On May 17th,  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.

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. 

Michael Levi, DESI Director

May 19, 2021

On the eve of the official start of the DESI science survey, I wanted to take a minute to reflect on how we arrived at this exciting moment.   We’ve now proven that we can obtain 100,000 science-quality spectra in a single night, so we are ready!  It has been quite a long and at times bumpy road to get here, but it has always been an honor to be part of this team.  Of course, the ground was prepared by SDSS-BOSS, which established BAO as a precision probe of dark energy and created a desire to go beyond plug-plates.  The seeds of this project germinated in 2008 out of the DOE/NASA JDEM space mission competition, when Saul Perlmutter and I were co-PI’s of the SNAP/JDEM proposal.  The competing JDEM concepts led to the realization that a lot of the proposed BAO science could be performed from the ground, and this in turn led very quickly to a new concept for a ground-based spectroscopic survey.  A rapid-fire set of ad hoc proposals and public presentations to DOE and NSF agency committees ensued as a core formed around this idea.

 At the 2009 Particle Astrophysics Scientific Assessment Group (a subpanel of the NSF/DOE HEPAP Federal Advisory Committee), Nikhil Padmanabhan (then an LBNL Chamberlain Fellow), gave a presentation, which resulted in a recommendation to support the R&D.  David Schlegel presented the BigBOSS concept to the 2010 Decadal Survey, which recommended mid-scale projects, explicitly calling out ground-based spectroscopic surveys as a compelling area of interest.  In early 2011, two unsolicited proposals were submitted to DOE, for BigBOSS led by LBNL, and for DESpec a competing concept led by FNAL.  This prompted the formation in mid-2011 of a dark energy community planning panel, chaired by Rocky Kolb, to advise the agency.  The panel identified a wide-field spectroscopic survey as a key project that would meet the needs of the dark energy community.  DOE agreed and, in the year following, approved “Mission Need” (aka CD-0) for a Mid-Scale Dark Energy Spectroscopic Instrument (MS-DESI) on Sept 18, 2012. 

DOE didn’t make a down-select between BigBOSS and DESpec.  Instead, they assigned the management of the new MS-DESI project to Berkeley Lab on Dec 12, 2012 and charged us to form a new collaboration.  I was appointed Project Director the next week and given the task to figure it out and come back with a plan.  Fortunately, we had just received a $2.1M grant from the Gordon and Betty Moore Foundation for a spectroscopic survey (followed a bit later by a similar grant from the Heising-Simons Foundation) and this provided the financial resources for us to get started building the first spectrograph and initiating the long-lead acquisition of the corrector lenses.   At the same time, we began to hold collaboration meetings and created a “big tent” where all were welcome, and the DESI collaboration grew to include most of the institutions of the original proposals, plus many additional recruits.

 A year later, in 2013, I presented the new plan and the newly formed science goals to the Particle Physics Project Prioritization Panel (P5), a subpanel of the NSF/DOE HEPAP Federal Advisory Committee.  The P5 report, issued in mid-2014, recommended MS-DESI only in an optimistic budget scenario.  Fortunately, budgets did align sufficiently for us to eke out a start in 2015 (albeit financially constrained) with CD-1 approval (site selection, and establishment of cost range) in March 2015 and with Congressional authorization in the appropriations language that year.  DESI was then baselined in Sept 2015, construction started, and the rest is history as they say.  Although there were plenty of remaining bumps in the road during construction, I will save that for another time.

 It is a privilege for me to represent the collaboration and contribute to its continuing success, and I hope all our collaborators feel equally privileged.  The DESI collaboration has come together in this common quest to understand dark energy and the mysteries of the accelerating Universe.  Now the telescope and instrument are humming, twilight and science awaits.

Paul Martini, DESI Instrument Scientist

May 17, 2021

The discovery of cosmic acceleration over twenty years ago captured the imagination of the general public and professional scientists alike, and the cause of this acceleration remains one of the greatest, unsolved questions in science. Something mysterious, such as a new particle, a new force of nature, or some property of space itself, is overcoming the gravitational attraction of all of the mass in the universe. While we commonly call the origin of this acceleration ‘dark energy,’ naming it did not solve the mystery, nor have the last two decades of progressively larger and more sophisticated sky surveys. 

Enter the DESI survey. For the last decade, hundreds of scientists and engineers have worked to build a new instrument and design a survey capable of constructing the most precise measurement of cosmic expansion history ever. On Friday, May 14 this work came to fruition with the formal launch of survey observations. And over the next five years, DESI will measure approximately 35 million galaxies and quasars that extend across about 12 billion years of cosmic history. These observations will measure an order of magnitude more targets than the largest survey to date, and complete these observations in a small fraction of the time. 

DESI aims to survey about 14,000 square degrees of the night sky, which corresponds to most of the sky visible from Arizona. We will do this with over 15,000 unique observations that we call tiles. Each of these tiles is a unique configuration of the 5000 robotically-positioned fiber optic cables designed to collect the light of specific galaxies and quasars distributed across the 3.2 degree diameter field of view of the instrument. 

The image below shows a small fraction of this field of view that includes the nearby Coma cluster of galaxies, one of the largest concentrations of galaxies in the local universe. The circles in the bottom panel illustrates the high density of the spectroscopic observations obtained with a single tile. Nearly 10,000 of these tiles are designed for good to great conditions, such as when the Moon does not appreciably affect the darkness of the night sky. In these cases, we observe our faintest targets, including luminous red galaxies, emission-line galaxies, and quasars. The remaining tiles target brighter galaxies and Milky Way stars, sources that can be observed when the moon is brighter and there may be moderate cloud cover.

As the DESI survey will be so much larger than previous surveys, we anticipate measuring cosmic acceleration and other parameters with substantially smaller statistical errors. Yet in order to fully realize these gains, and not be limited by systematic effects, it is especially important for us to carefully plan all aspects of the experimental design and maximize the reproducibility of our work. One aspect of this is that we have built substantial software and hardware systems to help us to achieve the same sensitivity with all of our tiles, in spite of the fact that they will be observed on nights spread over five years, and obtained under nights with varying amounts of atmospheric turbulence or seeing, and varying amounts of cloud cover. Other aspects include superb and repeatable algorithms that handle every aspect of observing, ranging from the selection of potential targets, to the assignment of targets to the individual fibers, to the measurement of their properties. 

We have spent the last many months building, testing, and refining all of these aspects of the survey with increasingly sophisticated and extensive observations. During this Survey Validation phase, we have obtained over 1 million unique redshifts, which has already made DESI the second largest spectroscopic redshift survey in the world before it formally begins. Over the coming months, we expect that the number of new galaxies and quasars we measure will continue to expand, much like the universe itself!

Daniel Eisenstein, Harvard University

April 28, 2021

On April 5, 2021, DESI turned to the second and final phase of its Survey Validation (SV) work, the so-called “1% survey”.  Whereas the first phase of SV was aimed at getting long and definitive observations of a broad superset of the planned spectroscopic targets, the 1% survey aims to demonstrate that we can operate the facility in a model closely matched to that planned for the 5-year survey.

The 1% survey is being conducted with target selection similar to what is planned for the main survey and with exposure times only mildly longer.  Importantly, we aim to observe each region of sky more times than in the main survey so that we achieve a higher fiber-assignment completeness, the fraction of targets that are assigned a fiber.

A single exposure with DESI provides about 600 fibers per square degree, but the survey target lists contain about 3500 targets per square degree for the dark/grey conditions (when the moon is small or set) and about 2000-2500 targets per square degree for bright conditions (when the moon is fuller).  In the main survey, we will return to a point on the sky numerous times to observe about 80% of the targets, but allowing some incompleteness so that we can efficiently move on to new regions.  In the 1% survey, we plan to return to each region at least 10 times for dark-time targets and 8 times for bright-time targets, each with a distinct target list, so as to observe all but a few percent of the targets.

So far, we have started observations on 16 regions, each 7 square degrees and each including both dark-time and bright-time targets.  We expect to finish this program in May.  With this data set, we will be able to get a first estimate of the 3-dimensional clustering of the galaxy and quasar samples, which will then allow the collaboration to tune the models of simulated galaxy positions in our cosmological mock catalog.

With the observations moving at normal survey pace from one target set to the next, the rate of gathering new redshifts has increased dramatically.  In the first 8 nights, we acquired over 400,000 separate successful redshift measurements of 350,000 unique sources.

The plot shows a visualization of one of the 16 regions, which has 13 dark-time visits, showing a map of the galaxy locations in Mpc on the plane of the sky.  Only galaxies between redshift 0.90 and 0.95 are shown, with the luminous red galaxy targets plotted in red and the emission-line galaxy targets in blue.  One can see the cosmic web of large-scale structure, with walls, filaments, and voids, as well as the tendency of red galaxies to cluster together more than blue galaxies.  This is about 1 part in 20,000 of the final survey size!

Daniel Eisenstein, Harvard University

April 19, 2021

Since mid-December, DESI has been intensively performing its Survey Validation (SV) observations.  With SV, we seek to optimize the 5-year survey design using on-sky observations of our target classes.  We of course based our designs on what was known before DESI, but DESI is a big enough step forward that one can’t be sure until one  verifies the performance with the instrument itself!

One of the challenges for Survey Validation is that we want to verify our survey plans with targets that are fainter and spread over wider areas of the sky than have previously been observed.  So we need to make our own truth tables by observing our targets with much longer exposures than we plan for the survey, so that the correct answers become obvious.  We then can split the observations into portions to see what a survey-length exposure would yield. 

From mid-December through the end of March, DESI observed 161 separate tiles as part of the first phase of its survey validation program, collecting over 1600 separate spectroscopic exposures.  We combined 4.3 million separate observations of 465,000 distinct examples of our target classes, with an average of 95 minutes per target, obtaining redshifts for 412,000 objects.  This is already one of the largest extragalactic spectroscopic data sets ever collected, including 108,000 redshifts above 0.8.

The whole DESI collaboration has mobilized to analyze these on-sky data, because rapid answers are needed to define the main survey.  The results have been very encouraging; we’ll share more examples in future blog posts.

Brian Bauer, Daniel Allspach, and Noah Franz

April 16, 2021

We are a working group of three undergraduate students at Siena College. We started with DESI under Dr. John Moustakas in February 2020. We were intrigued by the opportunity to work with such a large collaboration with so many scientific possibilities. Since then, Noah Franz and Brian Bauer have focused on identifying strong gravitational lenses in DESI spectra by using Python to separate the source and lens galaxy spectra then analyzing the success. Daniel Allspach has been working to fit stellar templates to model DESI spectra continua and determine galactic demographics and outflow classifications. Contributing to these projects has provided us with invaluable insight and experience into working with large collaborations.    

After working with DESI, attending Zoom telecons, and learning how a collaboration functions, what we enjoyed most of all is the welcoming nature of everyone involved. Without this feeling of acceptance we would have found it difficult to integrate into a project effectively. Although receiving emails in the wee hours of the morning can be a little odd at first, we were able to quickly adapt and become accustomed to the practices and methods employed by DESI. Being included in any aspect of the project, especially as an undergraduate, is an honor, yet we were continually challenged to dive deeper and join as much as we can. A welcoming environment provides the perfect jumping off point for new members and that is truly the best part about the DESI collaboration.

With over 700 contributing members, DESI is a large collaboration, and, as undergraduate researchers we found entering the community of mostly graduate students and PhDs intimidating and overwhelming. While some of the DESI updates and zoom meetings can be daunting, after spending time reading the literature and learning the acronyms we became accustomed to the jargon. Once we surpassed this learning curve, we were able to start our own research projects. In many ways this was our introduction into large scale collaborative astronomy and cosmology: many things are new to us.  Having the DESI environment to help us orient and navigate this research environment has been incredibly helpful. DESI has given us a fantastic first experience not just with research but also working with a large collaboration. As a result, we all were inspired to continue research going into the future.

John Moustakas, Siena College

February  3, 2021

Jeremy Tinker and Zheng Zheng

January 19, 2021

DESI member and noted rapper David Weinberg, from Ohio State University, was awarded the Dannie Heineman Prize for Astrophysics. He shares the award with Robert Lupton of Princeton University. The award celebrates the massive contributions both researchers have made to ushering in the era of large-scale three-dimensional mapping of the universe through the spatial distribution of the galaxies within it, primarily through their work with the Sloan Digital Sky Survey (SDSS).

As a project, DESI owes a debt of gratitude to SDSS. SDSS was the first truly large-scale galaxy redshift survey, using a CCD camera and fiber-fed spectrographs. With first light in 2000, its goal— realized in 2007 with the completion of SDSS-II— was to map nearly a quarter of sky by obtaining distances (redshifts) of the million brightest galaxies and quasars in the universe. In late 1980s, when the SDSS was merely an idea being batted around the Paris Conference Room of the Chicago O’Hare Hilton, David was a graduate student at Princeton, working with SDSS’s visionary founder Jim Gunn. He both literally and figuratively got started on the ground floor of large-scale spectroscopic surveys.

David became a full SDSS member in 1992, and went on to serve as the Scientific Spokesperson for SDSS-II, a position that demands a myriad of critical tasks related to the design, organization, promotion, and execution of the project. When the SDSS-III collaboration was created in 2008, David was chosen to be the Project Scientist. In recent years David has brought his expertise to the DESI collaboration, being one of the chief architects of the Bright Galaxy Survey component of the project as well as serving as the inaugural BGS working group co-chair. 

While a member of SDSS, David made invaluable contributions to survey design, galaxy target selection, and eventual analysis of the maps of cosmic structure. David was an early adopter and developer of the halo occupation model to describe the distribution of galaxies in space, who introduced the now commonly used term Halo Occupation Distribution (HOD). The model relates galaxies to clumps (halos) in the matter distribution in the universe. The SDSS data led to the pioneering application of the HOD framework to interpret the clustering of galaxies in space, with clustering trend naturally explained and galaxy-halo connection informatively inferred. The HOD model has ever since been widely adopted to analyze galaxy clustering data, becoming a powerful tool in learning about galaxy formation and cosmology and in creating simulated galaxy catalogs for various purposes in large galaxy surveys. 

In addition to mapping the universe with galaxies, David was also a pioneer in a novel method of determining the matter distribution of the universe: the Lyman-alpha forest. This method uses bright quasars as cosmological flashlights. Cool gas along the pathway to our telescopes absorbs some of the quasars’ light, leaving wiggles (Lyman-alpha forest) in the quasar spectra. Such wiggles reveal the spatial distribution of cool gas, which tracks dark matter. Thus, the spectra of quasars taken in SDSS and in DESI provide complementary maps of cosmic structure. 

As a scientist, David wears many hats in addition to survey astronomy. He has produced highly influential work on hydrodynamical simulations of galaxy formation, he has worked on the life cycles of active galactic nuclei, and more recently he has used SDSS’s spectra of stars to perform “chemical cartography” within our own Milky Way Galaxy. His 169-page review article on “Observational Probes of Cosmic Acceleration”, with nearly 900 citations, has become the standard reference in guiding our observational efforts toward revealing the nature of cosmic acceleration.

In addition to his research-oriented scientific pursuits, David has a long-standing collaboration with the MacArthur Award winning sculptor Josiah McElheny. Together, they have created cosmologically-inspired glass sculptures that have been exhibited all over the world. David’s role is in making the designs representative of the physics of the universe. For example, the piece An End to Modernity depicts the history of the universe from the Big Bang to the present day, emphasizing the evolution of cosmic structure and the epoch of galaxy formation.

Since starting at Ohio State in 1995, David has been advisor and mentor to 17 graduate students. Many of his former students, including the two of us, are themselves members of SDSS and DESI, and are using these data to mentor and train their own students, drawing on the lessons learned while working with David. Needless to say, David’s profound influence on cosmological redshift surveys will be felt for many academic generations. Congratulations to David for this well-deserved recognition of his continuing impact on astronomy, cosmology, and its presentation to the public.