astrometric solution is used to offset the telescope to center on the target field as well as to identified pixel locations on the guide GFAs for the set of predetermined guide stars specified in the exposure request. Because the fiber assignment code assumes the focal plane maintains a fixed position angle in celestial coordinates, any field rotation determined by the astrometric solution is compensated for by a rotation of the hexapod. In order to minimize residual field rotation effects, we update the hexapod rotation once per minute during each exposure.

In the second phase, we turn on the fiducials and back-illumination system and take an FVC image. We use the predicted pixel coordinates for the fiducials and fibers to aid in matching the spots in the FVC image to the positioners and fiducials. The singlet lens of the FVC can be treated as a pinhole camera such that the sky is assumed to map linearly onto the FVC CCD with a tangent plane projection. We calculate the astrometric coordinates of the GIFs and use their spot locations to astrometrically calibrate the FVC CCD. We then use the tangent plane projection and corresponding FVC coordinates of each fiber target to compare the desired and actual coordinates of the positioners and calculate correction moves. The fiducials are used to determine the transformation from the FVC pixels coordinates to the focal plane physical coordinates. This transformation relies on knowledge of the corrector distortion independent of any distortion from the FVC lens; the latter was intentionally minimized in the FVC lens design. The distortion model includes both radial and nonaxisymmetric terms, the latter changing as a function of ADC lens angle. During commissioning, we used special dither sequences (see Section 13.3) to quantify the accuracy of the coordinate transformation from FVC pixels to celestial coordinates. We also identified small, static patterns in the residuals due to errors in the distortion model that are applied as corrections to observing sequences.

Two iterations of the fiber positioning loop are sufficient to meet the required fiber positioning accuracy of better than 10 µm (or 0.′15) positioning accuracy in focal plane coordinates. The largest residual contribution to the positioning error comes from dome seeing and turbulence effects. We mitigate this effect by using the known and measured positions of all stationary devices in the focal plane (fiducials, nonfunctional fiber positioners) to fit large-scale turbulence modes in every FVC image and to derive corrections at each fiber position. Under good observing conditions, this approach has yielded fiber positioning accuracies as good as 2.7 µm rms between the coordinates determined by PlateMaker and the fiber position measured by the FVC after the correction move.

### 8.5.4. Guider

The DESI guider system provides correction signals to the Mayall TCS to maintain a stable and accurate position during science exposures. Based on the acquisition image taken with the six guide GFAs at the start of every DESI exposure, the PlateMaker application finds the astrometric solution for the current pointing and determines the location of guide stars taken from the GAIA catalog on each of the guide GFAs. For each GFA, we select two guide stars based on S/N and isolation criteria and load their locations in GFA CCD pixel coordinates to the guider software. Once initialized, the ICS starts the guider loop, and 5 s GFA exposures are taken in $a \approx 9$ s cadence. For each guide star and every GFA frame,

50 × 50 pixel regions of interest are extracted from the full-frame images. The Guider application receives these postage stamps and uses star finding and centroiding algorithms to measure the star locations relative to the preassigned positions. ICS combines the results of these algorithms for all guide stars to compute a correction signal that is sent to the TCS. The TCS employs a filtering algorithm to process these correction signals.

The DESI guider algorithm is robust against small disturbances, low-S/N stars, and occasional lost signals, for example, due to passing clouds. Studies during commissioning showed that a relatively long exposure time of 5 s is needed to obtain guide star images of sufficient S/N when observing conditions are poor. While a higher cadence is preferable, we determined that this rate is sufficient to maintain the required level of telescope tracking accuracy.

For testing and performance evaluation, the DESI guider application supports a special mode that automatically locates suitable guide stars in the full-frame GFA images and then reconfigures the software to cut out the same 50 × 50 pixel regions of interest around these guide stars. When operated in this mode, no star catalogs are required, and the guider runs independently of other ICS components such as PlateMaker. Early guiding and tracking studies during DESI commissioning relied heavily on this mode.

### 8.5.5. Active Optics System

The DESI AOS maintains the alignment of the prime-focus instrumentation with the primary mirror through corrections applied with the hexapod. The AOS software calculates the alignment corrections with intra- and extra-focal images obtained with the four GFA cameras that are configured for wave front sensing, as described in Section 5.6. We fit the out-of-focus stars with a pupil plane Zernike expansion and use an optical model of the corrector to calculate hexapod corrections for five degrees of freedom: tip and tilt of the focal surface, x- and y-decenter of the focal surface, and defocus. The focus varies with temperature changes and telescope pointing. We use a static look-up table derived with a laser tracker system to correct for changes with telescope position and use the AOS system to correct for temperature changes. The focus accuracy is about 50 µm. The AOS does not correct for rotation. We use a separate, empirical model to correct for field rotation during exposures. This model is a polynomial that depends on hour angle and decl. that we derived from measurements of the rotation rate from guide star observations.

The AOS is an adaptation of the similar system for DECam that was developed by Roodman et al. (2014). The key difference is that the DESI GFA wave front sensors are not behind the shutter used for science exposures, and therefore may operate independent of the science exposures. The advantage of this design is that we may use the AOS to check the alignment prior to the start of each science exposure. In practice, we have found that a static look-up table provides sufficiently good values for the correction to adjust the hexapod decenter, tip, and tilt settings as a function of telescope position. We use the results of the wave front calculation to set the focus and to compensate for temperature effects. The AOS system is in routine operation and works reliably.
### 8.5.6. Dynamic Exposure Time Calculator

The purpose of the Dynamic ETC is to achieve uniform depth in all spectroscopic exposures and therefore uniform redshift success rates. Dynamical determination of the exposure time for each exposure also produces an improvement of 25%-50% in survey efficiency relative to use of an exposure time that is independent of nighttime conditions. The Dynamic ETC achieves this with real-time estimates of the accumulated effective exposure time of each science exposure while it is in progress and provides feedback via the ICS to adjust the length of that exposure if observing conditions change. The inputs to the Dynamic ETC calculation are the images from the sky monitor cameras and the same images from the guider GFAs that are used for guiding. Both sky monitor and guider GFA frames are collected continuously during each spectrograph exposure. The Dynamic ETC uses these image data to determine the seeing, the fraction of the flux that enters the fibers, the transparency, and the night sky continuum brightness. This summary information is also provided to the observers and archived in the telemetry database.

The Dynamic ETC algorithm uses the full-frame guider GFA images to measure both the seeing from stellar profiles and the transparency with stellar photometry relative to their known brightness from Legacy Survey photometry. We calibrated the photometric performance of the GFAs with observations obtained under the best conditions, and regularly verify the stability of this calibration. The guide exposures have a typical duration of 5 s, and there are a little over 3 s between exposures. The noise in the spectrograph data is dominated by Poisson fluctuations in the sky level. The sky monitor (Section 7.7) measures the sky continuum flux in real time through dedicated broadband filters. We have used numerous spectroscopic observations from fibers that observed “blank sky” under a wide range of conditions to calibrate the relationship between the continuum observed by the sky monitor and the sky recorded by the spectrographs. This calibration includes smaller S/N contributions due to finite galaxy sizes and the spectrograph CCD read noise. The sky monitor measures the sky flux every 60 s, as this time is needed to obtain a sufficient detection of the sky continuum brightness under the darkest conditions (new moon with the telescope pointed at the zenith).

The Dynamic ETC combines these data to determine if sufficient effective exposure time has been accumulated to end the exposure. In some cases, the observing conditions require exposure times that would accumulate too many cosmic rays or incur a loss of throughput due to changes in atmospheric refraction with airmass. In these cases, the Dynamic ETC sends a trigger to the ICS to read out the spectrographs and start a new exposure at the same location. The only overhead associated with this split is the 60 s spectrograph readout time and about 4 s to prepare the CCDs and the spectrograph controllers. The Dynamic ETC can also automatically stop an exposure based on twilight or airmass limits, or when conditions deteriorate sufficiently that the accumulated S/N starts to decrease. D. Kirkby et al. (2022, in preparation) will provide a more complete description of the Dynamic ETC.

## 8.6. Integration, Testing, and Performance

The ICS is connected to every component of the instrument as well as the Mayall Telescope. In order to manage the large

number of interfaces, we developed ICDs that detail the hardware connection and define deliverables and responsibilities. These documents were reviewed by the project teams and were placed under change control. While the hardware components were under development, we created software simulators for each interface to allow early testing and validation. We adopted a spiral development approach with increasingly complex systems and included a number of functional verification tests throughout the construction project. Some of these tests were coordinated with the project to support other activities, for example, spectrograph testing and evaluation. The four key DOS system tests were: ICS-Mayall TCS integration; single spectrograph support; ProtoDESI, described in Section 11.1; and a full system test using the emulated hardware components.

The ICS has been in routine operation since the start of the DESI precommissioning activities described in Section 11. During DESI commissioning, a steady stream of improvements, bug fixes, and feature enhancements was applied to the ICS. The requirements to provide telemetry storage, data mining, and alarm and error handling infrastructure were demonstrated before the start of commissioning. Since the start of DESI operations, the robustness and ICS availability have surpassed the required uptime of 97%, and the typical inter-exposure times have achieved the 2 minute design goal. For reference, the average exposure time is ~1100 s for the dark program and ~800 s for the bright program; so, for example, a 10 s change in the inter-exposure time has a ~1% impact on the survey margin.

## 9. Data Systems

Data systems provide the software and infrastructure to plan the DESI survey, code to implement that survey plan within the ICS environment, and software to process the resulting data into calibrated spectra and redshifts for analysis by the DESI collaboration. Its key components are: (1) Target Selection; (2) Survey Planning; (3) Fiber Assignment; (4) Data Transfer; (5) Spectroscopic Pipeline; and (6) Nightwatch. Target Selection uses data from the Legacy Imaging Surveys (Dey et al. 2019), the Wide-field Infrared Survey Explorer (Wright et al. 2010), and Gaia (Gaia Collaboration et al. 2016, 2018) to select which astronomical objects should be observed by DESI. Survey planning selects a strategy for observing a tiling pattern of overlapping telescope pointings that can be completed within the 5 yr DESI program while achieving the required density of target observations. Fiber Assignment selects which targets should be assigned to which fibers for each telescope pointing. Data Transfer copies raw data from KPNO to the National Energy Sciences Computing Center (NERSC)<fnref n="109" /> in near-real time for processing and makes backups to tape at NERSC and a geographically separated mirror at NOIRLab.<fnref n="110" /> The Spectroscopic Pipeline processes those raw data into sky-subtracted flux-calibrated spectra, classifications, and redshifts. Lastly, Nightwatch provides a simplified and faster processing of the data, capable of running on a single 24-core computer at KPNO, in order to provide data quality feedback to observers within minutes of each exposure. The next subsections briefly describe each of these components. Further details are provided in the paper on the Target Selection Pipeline (Myers et al. 2022) and in future

<footnote n="109">¹⁰⁹ https://nersc.gov</footnote>

<footnote n="110">¹¹⁰ https://noirlab.edu</footnote>
papers on Survey Operations and Simulations (E. Schlafly et al. 2022, in preparation), Fiber Assignment (A. Raichoor et al. 2022, in preparation), the Spectroscopic Pipeline, Data Transfer, and Nightwatch (J. Guy et al. 2022, in preparation), and Spectral Classification and Redshift Fitting (S. Bailey et al. 2022, in preparation).

## 9.1. Technical Requirements

Data systems developed and implemented the target selection, survey plan, fiber assignment, and spectroscopic data processing capable of producing a data set meeting the science requirements in Tables 4 and 5 given the raw data from the DESI instrument. The derived technical requirements for data systems include the assignment of 10 standard stars and 40 sky fibers per petal for calibration, the need to transfer and back up data within 12 hr of sunrise, and to provide fully processed data to the collaboration within 48 hr of observation. The detailed performance of data systems relative to the requirements will be provided in the papers cited above.

## 9.2. Major Components

### 9.2.1. Target Selection

Algorithms to select science targets from imaging catalogs were developed by the Target Selection Working group of the DESI science collaboration. Data systems provided a parallelized implementation of those selections, including quality assurance (QA) and infrastructure to track input provenance and ensure repeatability. Target Selection also provided selection algorithms for: standard stars for flux calibration; identification of blank sky locations for modeling and subtracting the sky; GFA targets for guiding, focus, and dynamic exposure time decisions; and coordination of targets for commissioning, survey validation, and secondary programs. Most selection algorithms are simple cuts in observed parameters (e.g., colors of objects), though QSO selection used a random forest algorithm. Targeting bit masks track which objects pass which selection criteria, thus providing the flexibility for the same object to pass multiple selections.

### 9.2.2. Survey Plan

The survey plan selected a 14,000 deg² footprint observable from the Mayall Telescope and designed a tiling pattern of about 15,600 overlapping tiles with an average of 5.2 overlapping visits per location on the sky for the dark program, and 3.0 for the bright program. In practice, these values are somewhat lower by an amount that scales with the percentage of nonfunctional positioners (85.5% in use as of 2021 December). The survey strategy prioritizes what tiles should be observed when given the current observing conditions, tile airmass, galactic dust extinction, and the distribution of remaining observable hours versus LST. This survey strategy is algorithmically encoded in the NTS described in Section 8.5.1. A survey simulation using 10 yr of Mayall weather history and a model of instrument overheads and downtime tested the survey plan to ensure that it is viable to complete the full footprint within a 5 yr survey.

### 9.2.3. Fiber Assignment

Fiber Assignment selects which targets should be assigned to which fibers for each tile, while also assigning fibers to sky

locations and standard stars for calibrations. Lower-density targets such as QSOs are given higher priority and assigned first, followed by higher-density lower-priority targets such as LRGs and ELGs. Standard stars and sky locations are initially assigned to positioners that cannot reach any science targets. Any disabled positioners that are serendipitously pointing at blank sky locations are also used as sky calibration fibers. If needed, low-priority science targets are replaced with standard star and sky assignments if required to achieve the minimum calibration target density (10 standard stars and 40 sky fibers per petal). Sky fibers are also distributed such that groups of 50 fibers on the focal plane will have at least one sky assignment to ensure uniform coverage over both the FOV and across the spectrograph CCDs. Figure 22 shows an illustration of fiber assignment for a very small subset of the focal surface.

Since neighboring positioners have overlapping reach, fiber assignment also checks for and excludes target positions in which the mechanical components of neighbors would interfere. Since preplanned fiber assignments cannot perfectly know the exact transforms between sky coordinates and focal plane coordinates (e.g., due to the exact airmass of observation), fiber assignment includes a buffer zone of 200 µm in total reach and an extra 50 µm around each positioner (in addition to the anticollision safety buffer zone used in operations) so that small variations between the expected and actual transforms do not introduce new collisions. Although fiber assignment does not purposefully make assignments that would collide, this is additionally checked by the online code using the final observed conditions, and colliding assignment requests are not fulfilled.

### 9.2.4. Data Transfer, Archive, and Distribution

Data are transferred from KPNO to NERSC<fnref n="111" /> every 10 minutes using rsync. A long running process on a data transfer node at NERSC initiates the transfer. This design ensures that data are transferred frequently, while also being robust to individual transfers taking longer than the cadence for checking for new data. A cronjob checks every minute and restarts the process if needed, providing rapid recovery robustness to unexpected events such as a reboot of the transfer node. Once a day, the previous night's raw data are archived to tape storage at NERSC (also leaving a copy on disk), and copies of the raw and processed data are synched to NOIRlab in Tucson to provide a geographically separated backup. Although other technologies such as Globus<fnref n="112" /> could have provided better throughput, they also come with additional configuration and maintenance burdens, and the simple rsync method is sufficient to keep up with the data in real time to meet the requirements.

### 9.2.5. Spectroscopic Pipeline

The Spectroscopic Pipeline converts the raw data into sky-subtracted flux-calibrated spectra, classifications (star, galaxy, QSO), and redshifts. Similar to the raw data transfers, a long running process monitors the NERSC disk for the arrival of new data and launches jobs into a real-time queue for immediate processing (Kremin et al. 2020). This job launcher process is purposefully separated from data transfer so that either can be paused or restarted as needed without requiring

<footnote n="111">¹¹¹ https://nersc.gov</footnote>

<footnote n="112">¹¹² https://globus.org</footnote>
<figure><img src="image_27.png" /><figcaption>**Figure 22.** A view of ~1% of the DESI focal surface for a particular set of fiber assignments. The coverage regions of each positioner are shown in overlapping circles with red for those assigned to science targets, green for standard stars, blue for sky locations, cyan for disabled positioners serendipitously pointing at a blank sky location, and gray for disabled positioners not assigned to anything. The irregular polygons show the keepout regions for the individual positioners. The smaller polygon is the outer *φ* arm, and the larger polygon is the inner *θ* arm that spins about the central axis of the positioner. Targets that are reachable by nondisabled positioners are shown in small dots with science targets in red, sky locations in blue, and standard stars in green. Location 160 in the upper right shows a case where it was necessary to avoid a collision with a neighboring positioner (174) even though the assigned targets are well away from each other.</figcaption></figure>

direct coordination. For example, if the compute queue is down for maintenance, the data transfers can still continue and the processing jobs will catch up when they can.

The spectroscopic pipeline uses the “spectroperfectionism” formalism from Bolton & Schlegel (2010) to extract spectra using a 2D model of the spectrograph PSF as measured from arc lamp spectra each afternoon. This method provides spectra with uncorrelated flux bins on a uniform wavelength grid plus a model of the non-Gaussian line-spread function for each fiber at each wavelength (the “Resolution Matrix”). These components are used throughout the data processing steps, for example, to model how an input sky spectrum appears slightly differently on each of the different fibers with their unique resolutions.

The DESI spectroscopic pipeline keeps up in real time with DESI exposures with 10 nodes (32 Intel Haswell cores/node)

of the NERSC real-time queue such that it delivers fully calibrated spectra and redshifts each morning after observations. These are used to make decisions on the quality of the data for individual targets, and these decisions inform the observing plan and fiber assignments for the following night. The spectroscopic pipeline meets all science requirements in Tables 4 and 5. Further performance details will be provided in J. Guy et al. (2022, in preparation).

### 9.2.6. Nightwatch

Nightwatch provides a simpler, faster version of the spectroscopic pipeline for the purposes of semi-real-time data QA. It uses row-by-row boxcar spectral extractions instead of the full 2D PSF modeling of spectroperfectionism; it does not use the per-fiber resolution information when performing sky
subtraction; it only applies an average flux calibration corrected for airmass and galactic dust extinction instead of performing a per-exposure fit to standard stars; and it does not fit classifications and redshifts.

A summary web page per-exposure shows the CCD read noise per amplifier (30 cameras × 4 amplifiers/camera) and their expected ranges, the integrated sky-subtracted flux per fiber in each of the 30 cameras, and five randomly selected spectra. This page alone is sufficient to identify many problems, e.g., with CCD performance or configuration, stuck spectrograph shutters, or light contamination in the dome or spectrograph room. Additional pages provide details on per-CCD amplifier QA metrics (overscan levels, cosmic-ray rate); raw data CCD images; offsets of the spectral traces compared to expected locations; integrated fluxes, signal-to-noise, and positioning accuracy versus fiber number and versus (x,y) location on the focal plane; guiding offsets versus time during the exposure; and the ability to view individual spectra.

One copy of Nightwatch runs on a dedicated 24-core machine at KPNO, providing results within about 3 minutes of the end of the spectrograph readout. This copy provides the fastest turnaround, but is only available to observers at KPNO or with VPN access, so a second copy runs at NERSC. This has additional data transfer delays of up to 10 minutes, but provides a semi-real-time view of the data QA to any collaborator without requiring VPN access to the KPNO network.

## 9.3. Parallelism and Code Design

The data systems code was designed from the beginning to run in parallel at a high-performance computing (HPC) center such as NERSC. This ability has been critical for rapid turnaround of nightly data for survey operations and timely processing of internal data assemblies that process all data using a tagged software release. DESI uses message passing interface (MPI) as its primary mode of parallelism due to its performance and ubiquity at HPC centers; some algorithmic steps also use Python multiprocessing. The largest production runs so far have used 40,800 Intel KNL cores simultaneously. Work is in progress to adapt the slowest pieces of the DESI pipeline to GPUs (PSF fitting, spectral extractions, and redshifts) for the next-generation Perlmutter machine at NERSC, which will have dramatic performance improvements. For example, the speed up in the spectral extraction code should be of the order of 20 × per-node throughput relative to the previous generation CPU-only system (Margala et al. 2021).

While designing for HPC deployment from the start, data systems also made some pragmatic design choices to maximize the efficiency of human contributors over raw computing performance. Most DESI code is written in Python, leveraging compiled code “under the hood” via ` numpy`,<fnref n="113" /> `scipy`,<fnref n="114" /> and `numba`.<fnref n="115" /> All algorithmic steps can be run on a Linux or Mac laptop using files for input and output, without requiring MPI, centralized databases, GPUs, or other HPC or pipeline infrastructure. This use of Python and separation of algorithms from pipelining and parallelism has enabled non-computing-expert collaborators to make significant contributions, and

greatly facilitated the speed with which new algorithm ideas can be implemented and tested.

## 9.4. Testing and Integration

All data systems code repositories are open source at https://github.com/desihub, with significant contributions from both the DESI project staff and from members of the science collaboration. Developers make branches of the code, implement their features, push the branch to GitHub, and open a pull request for review before integration into the main branch. Each repository includes unit tests that run automatically when pull requests are opened.

Additionally, each night a cronjob updates the main copy of each repository at NERSC and re-performs the unit tests to ensure cross-repository consistency. This cronjob also performs an integration test by simulating a small amount of pixel-level raw data, processes it with the spectroscopic pipeline as if it were real data, and verifies the resulting redshifts. Any integration test failures generate email alerts.

Quarterly software releases of version-tagged repositories include additional testing on real data and end-to-end tests of target selection, fiber assignment, survey operations (via simulation), and the spectroscopic pipeline. These software releases are used for interim internal data releases, processing all available data to date.

During the DESI construction project, extensive simulations guided the code development. These included pixel-level simulations of raw data, higher-level (faster but less detailed) simulations of raw extracted spectra (including sky and not flux calibrated), and rapid simulations of fully sky-subtracted flux-calibrated spectra with added noise based upon the instrument model. Any of these levels of simulation could be inserted into the spectroscopic pipeline for processing as if it was real data.

This extensive simulation and testing program enabled the code to work well from the very beginning of operations. Although we continue to make improvements, the core pipeline met the science requirements and scaling performance needs from the start of commissioning.

# 10. Mayall Upgrades

We performed numerous upgrades to the Mayall Telescope infrastructure to support the installation and operation of DESI. The following subsections summarize the technical requirements on the upgrades, improvements to the telescope control system, infrastructure upgrades, and other facility improvements.

## 10.1. Technical Requirements

The key technical requirement on the telescope upgrades was that they do not degrade the delivered image quality relative to the baseline adopted to plan the DESI survey. That baseline was obtained from the study by Dey & Valdes (2014), who measured the delivered image quality for the Mayall from thousands of images obtained with the MOSAIC prime-focus imager between 2006 and 2011. Those data cover a range of wavelength, airmass, temperature differences between the mirror and ambient environment, and exposure time. Dey & Valdes (2014) reported values of 1"/2–1"/3 for the median of the nightly median values in images obtained in the reddest filters (i, I, z), and that the median of the best values from each night ranges from 0"/93 to 1".

<footnote n="113">¹¹³ https://numpy.org</footnote>

<footnote n="114">¹¹⁴ https://scipy.org</footnote>

<footnote n="115">¹¹⁵ https://numba.pydata.org</footnote>
There were also requirements on improvements to electrical, cooling, and communications infrastructure, as well as facility improvements to support installation. The electrical upgrades included additional capacity in the prime-focus cage for new electronics, in the computer room for additional computing and storage capacity, in the Large Coudé Room for the thermal enclosure and spectrographs, at the top end to support the calibration lamps, and in the facility uninterruptible power supply. Cooling capacity was added to both the Large Coudé Room and the top end for the spectrographs and focal plane system, respectively. The TCS upgrades included the capability to interface with the DESI ICS. Other facility improvements and additions included a new, larger control room on the “utility” or U floor.

## 10.2. Telescope Control System

The Mayall TCS was modernized as part of the preparations for DESI installation (Abareshi et al. 2016; Sprayberry et al. 2016). Key motivations for this project were to minimize the time between successive exposures and to guide accurately during the ~20 minutes planned for the longest science exposures. The guiding enhancements included an interface between the DESI guider system and the TCS.

The modernization project drew substantially from the TCS modernization project at the 4 m Blanco Telescope that prepared for the DES (Warner et al. 2012). The main changes were the replacement of the old VxWorks TCS computer, new tape encoders on the hour angle and decl. axes, and new digital servo controllers for the telescope drives. We also replaced the old 2D polynomial pointing model with the commercial TPOINT software. These updates occurred during 2014, and regular operations shifted over to the new TCS at the start of the 2015B semester.

Pointing map measurements with the new TCS showed rms errors of 2"-3" at that time, which is substantially better than the 10"-20" errors typical of the previous pointing map and the old, mechanical encoders. The unguided or open-loop tracking errors after the upgrades also exhibited substantial improvement. The best model yielded equivalent open-loop tracking errors of 1".6 hr⁻¹ in R.A. and 0".1 hr⁻¹ in decl., far better than the 30" hr⁻¹ performance typical of the previous polynomial pointing model with the old mechanical encoders and analog servo controllers.

We evaluated the impact of the TCS upgrade on the image quality with 329 nights of data obtained between 2015 December and 2018 February for the Mayall z-band Legacy Survey (Dey et al. 2019), which was the largest program on the Mayall Telescope since the Dey & Valdes (2014) study. The seeing in these images was characterized in a similar manner as the Dey & Valdes (2014) study, namely we first calculated the median seeing and the best seeing for each night, and then calculated the median of the median values and the median of the best values. The median of the median values of each night was 1".03, and the median of the best values per night was 0".84. Both metrics indicate that the upgrades prior to installation somewhat improved the delivered image quality.

## 10.3. Facility Improvements

Some significant improvements for DESI included increases in the capacity of the building’s electrical wiring and battery backup, cooling system enhancements, and bandwidth

upgrades to the building’s ethernet system (Sprayberry et al. 2014). Updates to the facility electrical system and battery backup capability were needed to support the greater power draw from DESI components, and especially to maintain and protect them against interruptions that would impact survey efficiency. The updates to the cooling system were needed to support the stable thermal environment for the spectrographs, the linear pulse tubes for the spectrograph cryostats, and the additional processing and data storage in the computer room. The DESI focal plane system has a separate cooling system described in Section 5.7.

One major change was the surveying and clearing of the Large Coudé Room. This room is located on the “main” or M floor, the bottom floor of the dome enclosure of the telescope. The Large Coudé Room was originally intended for a large spectrograph, and was used for a number of instrument setups, although over the years the room also became a convenient storage area. This room was carefully surveyed in 2014 to support the design of the thermal enclosure or Shack for the 10 spectrographs (see Section 7.5), and then cleared of its venerable equipment collection and the heavy fixtures intended to support the original large Coudé spectrograph in order to prepare for the assembly of the Shack.

The DESI project also needed infrastructure improvements to the computer room, which is located on the Mezzanine level just above the M floor. Mayall staff cleared a number of obsolescent computer racks and tape storage files out of this room and increased the cooling capacity of this climate-controlled room to support the expanded computer processing and storage requirements of the DESI project. Much of this work was completed by 2016 to support the early integration efforts of the ICS with the TCS.

The DESI installation plan included reassembly and testing of the new optical corrector on site prior to integration with the new top ring and subsequent integration with the telescope. The reassembly and testing of the corrector occurred on the ground floor of the telescope building. This area has access to the exterior via a large garage door, high ceilings, and is connected to the telescope level with a pair of cranes (5 ton and 50 ton capacity) that may extend through a hatch in the M floor. Mayall staff cleared the ground floor area to support the delivery of the many large crates that contained the corrector components, as well as numerous other components that moved through this space. They also erected a large clean tent (4 m × 4 m footprint, and high enough to support a crane with a 6 m hook height) to support the reassembly and testing of the corrector.

The operations model for DESI is to have two personnel on site every night: a highly trained expert observer (called the Lead Observer) from the collaboration and an NOIRLab staff Observing Assistant, who is responsible for telescope operations and personnel safety. Observers drawn from the collaboration membership also connect remotely every night and assist with observations (Observing Scientist) and data QA (Data Quality Scientist). Pre-COVID, the operations model had been for the Observing Scientist and Data Quality Scientist to be on site as well.

Many more people were expected to be on site during the installation, functional verification, and commissioning periods. As the original control room on the “console” or C floor could not accommodate more than five people, the Observatory built a new, larger control room on the “utility” or U floor,
which is several floors below the M floor. The new control room accommodates up to 15 people, is more easily and safely accessible than the original control room, and reduces the thermal load on the dome environment.

## 11. Precommissioning Activities

The development of DESI included two technology demonstration and risk reduction activities with on-sky observations prior to the start of commissioning of the complete instrument. The first of these employed an instrument called ProtoDESI whose goal was to reduce technical risks associated with the alignment and stability of the robotic fiber positioners. ProtoDESI was used for observations from 2014 August to 2016 September. The first subsection provides a brief description of ProtoDESI, with an emphasis on how it influenced subsequent development of the project and planning for commissioning. ProtoDESI is described in detail in Fagrelius et al. (2018). The second subsection describes the Commissioning Instrument (CI). We developed the CI to take advantage of a schedule gap between the installation of the corrector system and the start of the installation of the FPA so that we could begin commissioning tasks with the new corrector system. The CI was used for observations in 2019 April and May. Ross et al. (2018) describe the CI, and Coles et al. (2018) describe the critical metrology program for the CI. The description below emphasizes the completion of the CI, the observing campaign, and how this work influenced commissioning of the complete instrument.

### 11.1. ProtoDESI

The main purpose of ProtoDESI was to test the fiber positioners, including both the physical mechanisms and the acquisition procedure. It was also useful to test a GFA prototype and improve the guiding algorithm. The instrument included three fiber positioners, 16 fiducials, one GFA camera, a back-illumination system for the fibers, and a backup finder scope camera in the event of difficulties with the fiber alignment. The three fiber positioners each had a 3 m length of fiber that was reimaged on an SBIG STF-8300M camera. ProtoDESI was mounted to the f/3.2 refractive Mosaic corrector at the prime focus. We operated ProtoDESI with an early version of the FVC and the ICS and therefore we were able to test key software components several years in advance of commissioning.

We used ProtoDESI to test the fiber positioner technology in conjunction with an early test of the fiber positioning software PlateMaker, which calculates the coordinate transforms necessary to place fibers on targets (see Section 8.5.3). The alignment of fibers with targets required that ICS coordinate the activation of the back-illumination system with exposures from the FVC, use the measurements from the FVC to determine the projected location of the fiber tips on the focal plane, and combine these data with astrometry from the GFA to project the fiber tip locations into sky coordinates. The observations with ProtoDESI were extremely valuable to develop and test all of these software tools, as well as to develop tools and procedures for the eventual calibration and commissioning of the complete FPA. In addition to fiber positioning, we used ProtoDESI to characterize the GFA detectors, such as to demonstrate that the GFA CCD met the quantum efficiency requirement. We also used these observations to develop more

optimal GFA exposure times to minimize errors due to atmospheric scintillation, decrease heating of the GFA unit, and improve the guiding algorithms.

The ProtoDESI experience was invaluable to help plan DESI commissioning. One impact was that we more thoroughly defined the purpose, conditions, data requirements, analysis plan, criteria for success, time estimate, and one or more responsible parties for each commissioning task. Another was that we identified a number of new tasks. These included the development of robust tools for automatic focus sequences, use of the GFAs to refine fiducial positions, telescope tracking and guiding tests, and positioner dither tests. We also identified multiple precommissioning tasks that better prepared us to analyze data once we went on sky. These tasks included a more complete development of data models and interfaces, a more thorough plan for the metrology of the FPA, enhanced lab testing to minimize the number of systems that would be tested on sky for the first time, and motivated us to substantially expand the core group of experts who would contribute to commissioning.

### 11.2. Commissioning Instrument

Development of the CI began in 2017 in order to take advantage of a forecast 2 month schedule gap between the installation of the new corrector system and the readiness of the FPA for installation. The CI was designed to realize an early start on commissioning tasks related to the telescope, the performance of the corrector and corrector system, guiding algorithms, the AOS, the FVC, and the ICS. We determined that tasks from these categories could readily fill the expected 2 month period with a fairly simple arrangement of imaging cameras combined with illuminated fiducials. For this reason, and due to the earlier success of ProtoDESI, we decided that the CI would not include fiber positioners, nor a fiber feed to a spectrograph. To obtain good value from the CI, we used the GFA requirements to establish similar requirements on the location, tilt, FOV, noise, pixel size, and readout time on the imaging cameras that would be part of the CI.

The main components of the CI were five commercial SBIG STXL-6303 cameras and 22 illuminated fiducials. These SBIG cameras were chosen because their CCD array and pixel size (3072 × 2048, 9 µm pixels) produced a 28 square arcminute FOV that was comparable to the 25 square arcminute FOV of the GFAs, and with somewhat higher sampling ($0.0'13 \text{ pixel}^{-1}$ versus $0.0'2 \text{ pixel}^{-1}$) and better noise properties, although somewhat lower efficiency. The fiducials were identical to those produced by Yale (Baltay et al. 2019) for the FPA. The cameras and fiducials were supported by a short, steel cylinder called the CI FPA. Four of the five commercial cameras were located at the same off-axis distance as the GFAs on the focal plane system, while the fifth provided unique, on-axis imaging through the corrector. Behind the cameras was an electronics rack that contained a dedicated NUC PC for each of the five SBIG cameras, a simplified DESI petal controller electronics box supplied by the University of Michigan to operate the illuminated fiducials, temperature sensors, an ethernet switch, and a Raritan PDU for remote power control. These electronics are shown in the left panel of Figure 23. Each camera had an SDSS r-band filter mounted immediately in front of its shutter to mimic the bandpass of the GFA cameras. Most of the cameras and their filters are visible in the middle panel of Figure 23. One of the requirements of the CI FPA was that it
<figure><img src="image_28.png" /><figcaption>**Figure 23.** Three views of parts of the Commissioning Instrument (CI). (Left) Top view of the CI FPA. This side shows most of the electronics and faces away from the corrector. The electronics are mounted on a platform above the cameras and illuminated fiducials. (Middle) View of the CI that faces the corrector. The five larger apertures are for the five SBIG cameras, with one at the center of the FOV and the others near the periphery in the four cardinal directions. There are also 22 smaller apertures, each of which houses one of the illuminated fiducials. (Right) View of the CI after installation on the new corrector in late 2019 March.</figcaption></figure>

would match the 815 kg mass and moment expected for the FPA in order to assess the expected pointing and tracking performance of the telescope with the FPA. We met this requirement by surrounding the CI FPA with a larger, more massive steel cylinder. An additional 221 kg of counterweights were attached to the corrector cage to mimic the mass of the FPE.

The greatest challenges in the CI development were the strict alignment and metrology requirements. The five cameras and 22 fiducials needed to be aligned to the aspheric focal surface. The alignment requirements were that the full area of each CCD and each fiducial had to be within 50 µm of the focal surface, and that each CCD had a tilt of no more than 0.05° relative to the best-fit local tangent. The metrology requirements were to measure the x, y positions of each fiducial with ±10 µm accuracy, and the orientation of each CCD and the nearest fiducial with ±5 µm accuracy. We met these requirements with a careful metrology program that was based on a combination of CMM touch probe metrology and a custom microscope with an integrated projection system (Coles et al. 2018).

The 2 month CI campaign was very successful because it retired multiple risks and identified many areas for further work with sufficient time to address them (approximately 6 months) before the start of commissioning. One major accomplishment on the first night was the demonstration that the new corrector system could produce subarcsecond images across the entire FOV, including at the center where the focal plane system does not have an imaging capability (see Figure 3 and 4). One important discovery was that the historical value for the radius of curvature of the primary mirror was incorrect by 18 mm, which corresponds to a shift in the position of best focus by 9 mm. Fortunately the hexapod had a focus range of ±10 mm and was able to compensate for the difference. In the midst of the CI campaign we used this measurement to adjust the position of the cage to place the center of the hexapod range at the best focus. Another measurement with the CI was the amount of field rotation during exposures as a function of position on the sky due to a combination of polar axis

misalignment and variations in atmospheric dispersion. We used these data to develop a model for field rotation that we subsequently refined during commissioning of the full focal plane system. We also made substantial progress on many commissioning tasks, including a new pointing model for the telescope, an initial assessment of the throughput of the corrector system, further development of guiding algorithms, measurement of the performance of the ADC, and development of ICS, PlateMaker, and the AOS, including mapping the hexapod settings to keep the corrector aligned to the primary mirror over the full range of telescope pointing for the survey. Lastly, we obtained substantial experience with scheduling personnel, documentation, and organization of commissioning activities that helped the subsequent commissioning proceed more smoothly.

## 12. Acceptance, Integration, and Testing

The DESI subsystems went through extensive reviews during the design and construction phases. These included an R&D review in 2012, reviews mandated by DOE at CD-1 (2014), CD-2 (2015), CD-3 (2016), an annual review in 2017, two reviews in 2018, and at CD-4 (2020). In advance of each DOE review, LBNL organized a “Director’s Review” to assess readiness for the corresponding DOE review. The project also organized Preliminary Design Reviews of all subsystems in advance of CD-2 (Performance Baseline), Final Design Reviews of all subsystems in advance of CD-3 (Start of Construction), and numerous more focused reviews on individual instrument components, manufacturing readiness for major procurements, safety, installation, commissioning, and operations planning.

The Level 2 managers and cognizant scientists were responsible for representing their subsystems at the relevant reviews, as well as shepherding their subsystems through formal subsystem acceptance by the Project. The subsystem acceptance process is described in the first subsection and usually included a pre-ship review with verification of all requirements that could be met prior to installation, as well as an installation plan. Subsystem acceptance was only complete
upon delivery to the Mayall, demonstration that no damage occurred during shipment, and delivery of an installation plan and the documentation required for long-term support.

Upon acceptance, each subsystem became the responsibility of the installation team. The installation of each subsystem is described in the second subsection. Upon the completion of installation, each subsystem went through functional verification. This phase included system-level integration and testing activities, although did not re-verify requirements. Functional verification included essentially all testing that could be done with the dome closed, and especially focused on integration with the ICS.

## 12.1. Subsystem Acceptance

The completion of the subsystem acceptance process marked the formal hand off of each subsystem to the installation team. In most cases the subsystems were provided by collaboration member institutions (or partners), including several international collaborators, and not directly by LBNL. As a consequence, the acceptance process often represented the formal delivery of equipment provided as a contribution to membership, and in some cases marked the point when the scientists and engineers who developed a subsystem would complete their work with DESI and move to other projects. As a result, the acceptance process was used to ensure we had sufficient information to complete installation and integration and maintain the subsystem during operations (in practice, a subset of those groups formed our commissioning team and the current operations team, and the remainder are available for consultations as needed). We consequently created a Subsystem Acceptance Board to review the requirements on each subsystem, verify that we had sufficient documentation, and to make a recommendation on acceptance to the Project Manager. The Subsystem Acceptance Board was chaired by the Instrument Scientist and also included the two Project Scientists, the Systems Engineer, and the Acceptance, Integration, & Testing (AI&T) Scientist.

Each hardware subsystem had a review prior to shipping. In practice, this was a dry run for the formal, post-delivery acceptance review. The pre-ship review included documentation for all requirements that could be verified prior to integration with other subsystems. These include laboratory tests such as precision metrology, analyses such as the performance of the as-built lenses in an optical model, and documentation of requirements met by design, for example, that the petals had the correct number of fibers for the sky monitor. The review ensured that the system was in compliance with the relevant ICDs, that the ICD documentation was mature and complete, and that the documentation described the procedures for installation and long-term maintenance in sufficient detail. The pre-ship review frequently helped identify missing documentation well in advance of the final subsystem acceptance review.

We anticipated that not all subsystems would meet all requirements and maintained margin against this eventuality. We also created a Nonconformance Review Board to review any instance where a subsystem failed to meet requirements. This Board initially comprised the AI&T Manager, L2 Manager, Project Manager, System Engineer, and Instrument Scientist, and we typically added one or more other experts from within the collaboration. The Board performed an initial judgment of the scope of any cost and/or schedule impact of

the nonconformance. If the scope was judged to be major, the Board was expanded to include the Project Scientists and Director. Some conditions that led to nonconformances were that a subsystem failed to meet performance, technical, or material requirements or that it failed to meet a requirement in ICDs or other design documents. If the nonconformance could not be easily addressed, the Board conducted an analysis of the potential implications for higher-level requirements and drafted a waiver request. Some examples of waiver requests include that the efficiency of the blue gratings in three spectrographs was somewhat below the requirement and that the ADC rotator exhibited some vibrations at the required speed. In the case of the blue gratings, the throughput of other spectrograph components exceeded their specifications, so the net throughput of the spectrographs were compliant. In the case of the ADC rotator, further study showed that the vibration was minimized at 50% nominal speed, and that this speed was sufficiently fast relative to the telescope and dome that it would not affect the length of the inter-exposure sequence. Waiver requests were submitted to the Subsystem Acceptance Board as part of the documentation for subsystem acceptance.

Subsystem acceptance occurred after post-delivery inspection. The rationale for this timing was to place the responsibility for shipment on the partner who would prepare it for shipment. The partner would also retain responsibility for the subsystem through post-delivery inspection and testing, as the partner would be best equipped to address any issues that arose during shipment. The degree of post-delivery inspection and testing varied substantially between subsystems. In some cases, such as the spectrograph cryostats, it was not practical to conduct on-site testing until after the cryostats were installed on the spectrographs in their final location, so the post-delivery inspection was simply a visual inspection for shipping damage. In other cases, such as the corrector system, there were substantial verification tests during the reassembly of the corrector prior to installation. Once the post-delivery inspection and testing were complete, the Subsystem Acceptance Board reviewed the subsystem. In all cases, the Board recommended acceptance to the Project Manager.

## 12.2. Integration

The planning and execution of DESI integration with the Mayall occurred over many years, starting with the upgrades to the TCS and other facility improvements described in Section 10. In this subsection, we describe the integration stages of the new instrumentation. These include the removal and replacement of the top end and the installation of the new corrector, modifications to the bottom end, construction of the thermal enclosure and support structure for the spectrographs, installation of the new focal plane instrumentation, and routing of the fiber cables and other utilities. In addition to the CD reviews, there was a dedicated DOE review of the installation planning in 2017 April. This planning included careful attention to personal and equipment safety procedures, as well as the detailed installation and lift procedures for all components. The peak of the installation activity occurred during shutdowns of the telescope for the corrector system installation from 2018 February to 2019 March and for the focal plane system installation from 2019 May through 2019 September. During both of these periods, the telescope was locked into position. The interlude between these two phases corresponds to the CI campaign, which was described in
<figure><img src="image_29.png" /><figcaption>**Figure 24.** Installation phases of the new top ring. (Left) Transfer of the new top ring from the ground level to the dome floor via the dome slit with a 450 ton crane on 2018 June 14. The same crane was used to remove the old top end. (Right) Installation of the fully assembled, new top end with cage, hexapod, and corrector barrel to the Mayall's Serrurier truss on 2018 November 28.</figcaption></figure>

Section 11.2. See Besuner et al. (2020) for a more detailed description of the entire installation process.

### 12.2.1. Top End and Corrector

We began preparations for the removal of the top end with the removal of the KOSMOS (Martini et al. 2014) instrument from the Cassegrain focus in order to center an alignment telescope on the rotator axis with the aid of a cross-hair marked on the f/8 secondary mirror. Our next steps were to remove the Mosaic prime-focus imager (Dey et al. 2016) and corrector optics (Sawyer et al. 2010), the f/8 secondary mirror, and associated utilities. We then disconnected or severed the air, data, and electrical connections running to the top ring assembly and added temporary counterweights to account for the removal of these items. At this point we locked the telescope into position at the zenith and started to disassemble the bottom end, which includes the Cassegrain guider and rotator and the central baffle assembly. We then removed the primary mirror, which was re-aluminized and stored safely off the telescope until the completion of the work on the top end; however, we did reinstall the empty primary mirror cell and the Cassegrain rotator to facilitate later alignment of the new DESI top ring. As the Serrurier truss would be unstable without the upper ring in place, we braced the truss to prevent any relaxation or rebounding of the truss upon removal of the old ring. We also constructed a two-level work platform (visible in Figure 24) to provide personnel access to the upper assembly. These access platforms were useful for the removal of the remaining hardware on the top rings in preparation for the removal of the ring assembly. We designed and built a temporary hydraulic jack system to aid in the separation of the ring from the truss.

We hired a 450 ton, 300 foot crane to remove the old Mayall top ring and transfer the new ring from the ground level into the dome. To remove the old ring, we first used the jack system

attached to the truss members to slightly raise the ring off of the truss pads. This was more straightforward than feared, as fortunately the ring had not become rusted in place over the preceding 45 yr. On 2018 June 14, we used an exterior crane to lift the old top end, maneuver it through the dome slit, and place it on a flatbed truck that carried it to its retirement home in the parking lot by the Kitt Peak Visitor Center. We then used the same crane to lift the new top ring from the ground floor into the dome. Both of these operations were completed with an exterior crane, rather than the 50 ton dome crane, because the top ring is too large to fit through the hatch between the dome floor and the ground floor of the Mayall unless the ring were rotated such that the longest dimension was aligned with the diagonal of the hatch. The exterior crane was a less risky and less complex solution.

The new ring, vanes, and cage for the top end arrived on 2018 April 19, before the removal of the old top end assembly; they were stored in the ground floor garage until the old assembly was removed. The first step of their installation was to match-drill the locations of the alignment pins and bolts that would be used to fasten it to the truss. As the ring experiences stress and some deformation with the mass of the corrector installed in the cage, we put the ring into a similar state of stress and tension in order to accurately mark the locations for the pins and bolts. We reproduced the expected deformation (a few millimeters) with an assembly that replicated how the vanes attached to the cage and provided a comparable amount of tension.

The next stage of installation was to match-drill the pretensioned upper ring. We lifted the ring with the pretensioning system to the top of the truss with the 5 ton dome crane, used the alignment telescope previously centered on the Cassegrain rotator axis to align the ring to the axis, and match-marked the holes for the alignment pins and bolts. We then lowered the ring to the M floor, drilled and tapped for the pins and bolts, and lifted to the C floor for integration with the cage and vanes. Prior to the start of
this final assembly, we added strain gauges to the vanes to help
balance the forces on them. We used a laser metrology system to
center the prime-focus cage within the top ring.

The new corrector and ancillary alignment equipment arrived on 2018 July 13 by charter aircraft from the UK. We carefully unpacked each of the three corrector segments, inspected them on a rotary table, and reassembled and tested the complete barrel. We then integrated the corrector with the hexapod, which required the temporary separation of the forward section of the corrector from the other two sections. All of this work occurred on the ground floor of the Mayall. In early 2018 November, we used the 50 ton dome crane to lift the corrector plus hexapod to the dome floor, where we integrated it with the new prime-focus cage in the new top ring. Finally, on 2018 November 28, we used the 50 ton dome crane to install the 10,700 kg top end assembly onto the top of the Serrurier truss.

12.2.2. *Spectrograph System*

The kit of components for the thermal enclosure or Shack
arrived in 2018 mid-May, and we completed most of the
assembly by the end of June, although some residual work on
utilities and other details continued for the following several
months. A WEC representative verified that the Shack met
temperature, humidity, and cleanliness requirements in 2018
mid-October. The certification tests showed that the temper-
ature stability was better than 0.1°C, the humidity was within
range, and the air cleanliness met the requirements of a class
10,000 clean facility. Later in October, we assembled the Rack
inside the Shack to prepare it for the installation of the
spectrographs.

In 2018 mid-December we installed and tested the Shack
controller, spectrograph purge system, and stand-alone fiber
illumination system. This included mounting the Shack
electronics box on the south wall of the Shack, running
electrical lines, ethernet, and air hoses through the rear cable
tray of the Rack, and testing the stand-alone fiber illumination
system and the dry air purge system for the cryostat windows.
We installed the cryostat electronics in early 2019. This
equipment included three computer racks that were placed
outside of the Shack, and numerous electrical and coolant
cables that connect to the spectrographs. Once the Shack and
associated utilities were ready, we moved the first
spectrograph from storage at the Kitt Peak Instrument Handling
Facility into the Shack and onto the Rack. We then attached the
cryostats to the spectrograph, connected various utilities, and
began functional verification tests (see Section 12.3).

12.2.3. *Focal Plane and Fiber System*

We began preparations for the assembly of the focal plane
system at the Mayall during the CI campaign in 2019 April and
May. As described in Section 5, we chose to ship individual
petals from LBNL and integrate the system on the Mayall after
review of the relative merits of integration on the telescope
compared to shipment of the fully integrated focal plane system
to the Mayall. The greatest challenge in the on-site integration
was the insertion of the 10 petals, as each petal is a large,
complex, delicate piece of equipment with only 0.6 mm of
clearance relative to its neighbors. Each petal had to be inserted
at a unique angle (increments of 36° about the corrector axis)
relative to its neighbors. In addition, the insertion direction was

tilted by ~7°2 relative to the horizon, as that corresponds to the
orientation of the telescope when locked into place at the
Southeast Annex maintenance position.

To overcome these challenges, we built a petal insertion
structure called the “Sled” (see Figure 25). The Sled consists of
two large, welded steel frames connected to each other by a set
of six adjustable-length struts that control six degrees of
freedom. The lower frame of the Sled is rigidly mounted to the
Southeast Platform of the Mayall facility. The upper frame
supports a pair of linear motion guide rails and a central lead
screw. Prior to petal insertion, we adjusted the struts to align
the rails parallel to the axis of the telescope when it is parked in
the maintenance position. To install a petal, the Petal Mount
Adapter (PMA) is driven along the Sled’s linear rails, and the
base plate of the PMA has linear bearings and a coupling to the
Sled’s lead screw. Six adjustable-length struts connect this base
plate to a roughly tetragonal steel frame above it, and mounted
to this frame is a steel rotation arm. We attach a petal to one
end of the PMA arm and a counterweight to the other. The
rotation axis of the arm is nominally colinear with the corrector
axis. This allows us to clock the petal to its designated angular
position in the focal plane. We then use the PMA struts to
finely align the petal to the focal plate adapter ring. At this
point the petal is ready for insertion by driving the PMA down
the Sled rails. All driving torques during the petal insertion are
provided via handwheels, with significant gear ratios between
the operator and the load. Each strut has an inline load cell, so
that we can quantitatively monitor the net reaction forces
between the telescope and the petal during insertion and load
transfer. The cells have a resolution better than ~10 N.

One key tool for the alignment process was a Faro VantageE
Laser Tracker, combined with Spherical Mount Retroreflectors
(SMRs) that were attached to both the focal plane adapter ring
on the telescope and to each petal. The SMRs are mounted in
"nests" that magnetically hold them against repeatable contact
points, and the locations of the contact points were measured at
LBNL with a CMM. We measured the relative locations of the
SMRs on the focal plane adapter ring and on each petal during
installation. Typically these measurements had uncertainties of
less than 100 µm, and better than 50µm was achieved under
good conditions. Shout et al. (2020) provide a more extensive
description of the alignment and integration of the FPA with a
laser tracker.

While the CI campaign was in progress, we conducted a
mock installation exercise on the ground floor of the Mayall
building. This included a custom corrector simulator with an
identical interface flange to the end of the corrector, oriented at
the same ~7°2 angle relative to the ground, as well as used
dummy petals with the same envelope, mounting features,
mass, and center of mass. This mock installation led to a
number of improvements in the installation procedure.

As soon as the CI was removed from the telescope, we
moved the installation fixtures for the focal plane system from
the ground floor to the Southeast Annex. The first petal arrived
on 2019 May 8, and the remainder followed at intervals
throughout June and July. Before installation, each petal was
briefly powered on, back illuminated, and tested to determine if
any damage had occurred during shipment. We installed the
first petal on 2019 June 25. The installation of the remaining
petals proceeded smoothly, and we installed the final petal on
2019 July 23. Figure 25 is a photo of the first petal ready for
installation.
<figure><img src="image_30.png" /><figcaption>**Figure 25.** Installation of the first petal of the focal plane system on 2019 June 25. This photo shows the telescope (left) parked at the Southeast Annex maintenance position. The focal plane adapter assembly is the black steel cone with multiple side port holes that is bolted to the rear flange of the corrector barrel and extends from the end of the cage. The 10 petal positions are temporarily covered by 10 triangular plates that protect the C4 lens from accidental contact. The red structure in the center of the photo is the Petal Mount Adapter (PMA), which holds the petal at the end of its rotation arm. The hand wheel at the right drives the arm via a right-angle gearbox. Several struts are visible connecting the PMA to its base plate. One of the Sled's linear rails is visible at the bottom of the photo. See Section 12.2.3 for more details.</figcaption></figure>

One unforeseen challenge in this process was a small drift between the telescope and the Southeast Annex platform with time. This drift occurs because the telescope is mounted to a rigid, concrete pier that is mechanically isolated from the rest of the Mayall building in order to maximize its stability. During the petal installation, we observed differential motion as large as a few tenths of a millimeter in tens of minutes, which we attribute to differential thermal effects on the Mayall building and the pier. We adjusted our procedure to minimize the time between the final measurement and adjustment and the insertion of the petal. Throughout this process we enclosed the workspace as much as practical with plastic sheeting and applied a slight overpressure to maintain a clean and moth-free environment.

Once the FPA was complete, we installed the FPE and routed the fiber cables to the spectrographs. The FPE is mounted directly to the cage, rather than the corrector, to reduce the total mass and moment attached to the hexapod. The FPE thermally isolates the FPA at a constant temperature, as well as provides a clean and low-humidity environment. The frame of the FPE includes four rigid "tower" assemblies that are parallel to the corrector axis and extend the full length of the FPE. The fiber cables, electrical cables, and hoses route through these towers. From the FPE, the fiber cables route to the upper ring along the Southeast and Northeast upper spider vanes, down the telescope truss, and through the hour angle

and decl. axes. NOIRLab staff designed and built a series of cable carriers for the fibers that avoid twists and enforce their minimum bend radius of 200 mm. The fiber cables then extend through custom penetrations in the walls of the Large Coudé Room and into the Shack. Once we completed the fiber cable installation, we installed the four calibration lamp boxes on the Mayall's top ring.

### 12.3. *Functional Verification*

The two goals of the Functional Verification (FV) phase were to verify that subsystems were not damaged during installation and to integrate them with the other subsystems. The main focus of this integration was software, as hardware integration occurred during installation. We chose to not spend time on the re-verification of requirements during this phase. Instead, we emphasized the start of on-sky commissioning as soon as practical, as that phase provides more and better opportunities for the verification of the instrumentation.

The emphasis on software integration meant in practice that FV for each subsystem was complete upon the successful integration of that subsystem with the ICS. As a consequence, ICS was the first subsystem to begin FV and the last to finish. Functional verification for ICS began early in the project with the TCS integration in advance of ProtoDESI. FV for the other subsystems occurred as soon as they were installed. In practice this meant that the Shack was the next subsystem to go through
<figure><img src="image_31.png" /><figcaption>**Figure 26.** Example science images from spectrograph SP4. The three panels show inverted, grayscale images from the blue, red, and NIR channels after removal of the overscan and bias level, and before additional processing. Wavelength increases with pixel number along the vertical axis. Spectra of the same 500 fibers are parallel to the columns on each detector. The vertical, dark stripes correspond to the spectra of the brightest sources in this exposure, while the horizontal arcs correspond to night sky emission lines. There is a small gap between each of the 20 fiber blocks in the slithead, and these gaps appear as vertical bands with no flux between each set of 25 spectra. This is exposure ID 68879, which is a 900 s exposure in dark conditions obtained on 2020 December 19. Note the greater sensitivity of the thicker CCDs in the red and NIR channels to cosmic rays.</figcaption></figure>

the FV process, as it was needed to install the spectrographs and cryostats. The Shack was followed by the corrector, as it was needed before the installation of the CI, and then the FPA. Spectrograph FV occurred after each spectrograph arrived and was installed in the Shack.

The main step for the Shack FV was the ingestion of telemetry data (e.g., temperature and humidity) from the WEC control computer to the ICS, which we achieved with a dedicated communications link from the telemetry available from the control computer to our Shack controller electronics box. The Shack controller also reads the pressures of the air supplies for the spectrograph seals and the spectrograph purge system, and it has the capability to power cycle any of the spectrograph controller electronics boxes. During FV of the focal plane, the Shack controller was also used to control the stand-alone fiber illumination system.

Spectrograph and cryostat FV proceeded as each spectrograph and its cryostats were installed in the Shack. The tests for each spectrograph included both a repeat of all pre-ship functional tests and a subset of the pre-ship qualification tests. The functional tests included the operation of the mechanisms, operation of the fiber illumination system, and readout of the detections. The qualification tests were measurements of the read noise, well capacity, gain, non-linearity, dark current, and CCD cosmetics. Once the focal plane and calibration systems were installed, we measured light leaks, image quality, spectral resolution, crosstalk, and stability. We also used the Hartmann doors to check the focus and make minor adjustments to each cryostat. As the last spectrograph was delivered after the start of commissioning, spectrograph FV was not complete until several months into the commissioning period (2019 December). By that time, we had gained sufficient experience with the system that we established a daily calibration protocol. This includes zeros, darks, flat fields, and arc calibration exposures that are typically obtained in the late afternoon. The flats and arc calibrations use the calibration lamp system mounted to the top ring. Figure 26 shows an example of the data from one (SP4) of the 10 spectrographs after the removal of the overscan and bias subtraction steps.

FV of the corrector system included the two ADC motors and the hexapod. Both of these units were extensively tested with ICS in advance of installation and this integration was consequently straightforward. Upon installation of the CI, both it and the FVC went through FV. The main step was illumination of the fiducials on the CI, and imaging those spots with the FVC. This produced the first recorded images through the new corrector. On-sky observations, described in Section 11.2 and shown in Figure 3, demonstrated the superb performance of the corrector optics.

Functional verification of the focal plane and fiber system occurred in three phases. The first phase was a basic checkout of the cooling services. This included leak and pressure checks, the operation of the liquid chiller, the cooling fans, temperature sensors, and humidity sensors, and their integration with ICS. The second phase was a basic functional test of the positioners and GFAs. The positioner operation was first checked with visual inspection, and then the illuminated positioners were imaged with the FVC. At this time, we verified that ≥99% of the fibers were intact after installation. The GFAs were also turned on and operated under ICS control. Finally, the third phase combined the calibration of the positioner parameters with the mapping of the coordinate transformations from the pre-ship metrology of each petal to the FVC. This process included accounting for the distortion caused by both the corrector optics and the FVC optics. The positioner calibration included precise measurements of the six parameters per unit, which are required to accurately transform between motor rotation angles and fiber tip coordinates on the focal surface. This mapping is critical for accurate alignment of the fiber tips with astrophysical sources and to avoid collisions between positioners. Larger uncertainties in these parameters would require larger tolerance regions around the moving components, which would impact fiber assignment. The instrument was ready for on-sky observations in 2019 October upon completion of FV for the focal plane and fiber system.

## 13. On-sky Performance Results

Commissioning began in 2019 October as soon as a sufficient fraction of the instrument was ready for on-sky
<figure><img src="image_32.png" /><figcaption>**Figure 27.** Average measured total throughput compared to the engineering model. The average throughput is measured with standard stars on photometric nights. All measurements have been corrected for losses due to the fiber acceptance fraction and therefore do not include the contribution of seeing. The darker lines (blue, red, and black) show the throughput of the three spectrograph channels with a correction that removed atmospheric extinction, while the lighter lines show the throughput at the zenith. The points are the predicted throughput of the engineering model for both cases. The agreement between the measurement and the model is excellent for the blue channel and very good for the red and NIR channels. The minor differences between the model and the measurement are likely due to many small differences between estimates and the final performance. The dip at 440 nm is due to an issue with the spectrograph collimator coating. The average throughput without atmospheric extinction has been smoothed to reduce the impact of individual telluric absorption features on the measured throughput.</figcaption></figure>

observations. The purpose of this phase was to test and
optimize the instrument and demonstrate that it met all
performance requirements from 0°–60° from zenith. The key
instrument properties that could only be tested with on-sky
observations were the image quality, total throughput and
noise, fiber positioning accuracy, and guiding performance.
During the commissioning period, we also worked to decrease
the length of the inter-exposure sequence, measured the
stability of the spectrograph PSF, and improved the telemetry
data, access to these data, and alarm and error handling. In the
following subsections, we describe the measurement of the
total throughput and noise, characterization of the guiding
performance, validation of the fiber positioning accuracy,
report on the stability of the spectrograph PSF and resolution,
and demonstrate the superb data quality with observations of
some faint emission-line galaxies. We also describe efforts to
decrease the inter-exposure sequence in order to maximize our
observational efficiency. The validation of the image quality
was described in Section 11.2 as part of the CI campaign (see
also Figure 6). The commissioning period led to improvements
in nearly every aspect of instrument operations, including
calibration procedures, data analysis, and hardware upgrades to
improve performance. During commissioning we also uncov-
ered several hardware issues that impacted performance. In the
last subsection, we describe those issues and how we addressed
them. Commissioning was interrupted from 2020 March

through November due to the global COVID-19 pandemic
and formally ended in 2020 December with the start of the
Survey Validation phase.

13.1. *Throughput*

The total system throughput was one of the most important
parameters throughout the project. As described in Section 2.1,
we developed a detailed total throughput budget early in the
development of DESI to set requirements on the instrument
subsystems. We updated this budget throughout the project as
we obtained empirical data for many contributions (e.g., optical
coatings, FRD, detector response). We obtained our first
opportunity to measure the total system throughput with
observations of the standard stars that we include in all petals
on every field we observe, and we gradually refined these
throughput measurements as we acquired more data and
improved the fiber positioning accuracy. Figure 27 shows the
total measured throughput from standard stars observed over
many photometric nights, both with and without corrections for
atmospheric absorption based on the extinction curve for Kitt
Peak. The measurement with atmospheric extinction has been
corrected to airmass X = 1.

The measurements shown in the figure are based on
observations over nights that exhibited excellent image quality
(seeing between 0.7 and 0.9 FWHM). We determined the
<figure><img src="image_33.png" /><figcaption>**Figure 28.** (Left) Image quality measured in guider images during commissioning in early 2020. These are measurements on individual 5 s guider images over many nights and at a range of airmass. (Right) Guiding performance from observations between 2021 January and April. The requirement is 100 mas rms on each axis (vertical, dotted line). The contribution in R.A. is larger than the contribution in decl., which is expected for an equatorial mount telescope.</figcaption></figure>

fractional flux in a fiber for each observation based on stacked images of the guide frames that were obtained in conjunction with the spectroscopic observations. This fiber aperture loss is about 40% for our typical seeing of $1''1$ and increases as the seeing degrades. The total throughput shown in Figure 27 is corrected for fiber aperture loss, and this correction takes into account the variation in the projected fiber size with position on the focal surface. We assessed that we obtain a consistent throughput using data observed with worse seeing, which imply a higher correction. The figure also shows the predicted throughput from the system engineering model, which is the total throughput budget based on the best empirical data or analytic estimate for individual contributions. The agreement between the data and the engineering model is extraordinary for the blue channel and very good, within 10%, for both the red and NIR channels. The mild disagreement between the prediction and model for the two redder channels is likely due to many small discrepancies in the engineering model.

## 13.2. Guiding and Fiber Flux

The previous subsection demonstrated how DESI provides exceptional throughput. The next subsections show how well the telescope, corrector, and focal plane deliver the light from our targets to the fibers. The top-level requirement on the telescope guiding performance (L3.3.3) is the assumption that the telescope guiding accuracy is 100 mas rms 1D. Multiple factors contribute to the guiding accuracy, including the telescope pointing model and tracking, the performance of the algorithm that identifies and measures sources on the guider chips, the algorithm that combines the measurements to create correction signals in R.A. and decl., and the correction for field rotation with the hexapod.

We used the guider cameras to validate the performance of the corrector and telescope after the installation of the focal plane system. Figure 28 shows the distribution of measured seeing values from guider images obtained between 2019 October 23 and 2020 March 15. These FWHM measurements were obtained from the measured radial profile of stars on a

guider chip. The best FWHM measurements are between $0''6$ and $0''65$ and demonstrate the excellent performance of the corrector system. The median is $1''11$, which is superior to the median of $1''2-1''3$ measured by Dey & Valdes (2014). For typical seeing of $1''1$, about 60% of the flux from a point source enters the fibers. Figure 29 shows the FWHM as a function of zenith distance, which demonstrates the excellent performance of the ADC and the overall stability of the top end and hexapod control of the corrector system.

We obtained several new data sets to update the pointing model before optimizing the guiding performance (see Section 10.2). The latest model from 2021 April 1 yielded excellent pointing and tracking performance. The median performance was about 100 mas from 2021 January through April on each axis, and this is shown in the right panel of Figure 28. The performance is slightly better along the decl. axis, as expected for an equatorial mount telescope. The measurement is after a correction for field rotation during the spectroscopic exposure. We still plan to implement a range of filters on the raw guide signal to improve this performance.

The overall performance of the GFAs has been excellent. We regularly monitor the dark current and read noise, and these studies have shown very good overall stability. The dark current tracks the CCD temperature in a consistent manner over the typical operational temperature of $10°-11°$ C. There is a modest amount of variation in the read noise that we attribute to low-level pattern noise. The read noise has consistently remained below the requirement of $25~e^-$ rms per pixel.

## 13.3. Fiber Positioning

Our method to ensure the $107~\mu$m diameter fibers are centered on astronomical targets combines many complex elements. These include the alignment of the fiber tips with the aspheric focal surface of the corrector, the tilts of positioners relative to the focal surface, precision metrology of illuminated fiducials and GFA pixels, optical distortion models of both the corrector and the FVC camera lens, and a model for atmospheric refraction. To verify all of these individual steps,
<figure><img src="image_34.png" /><figcaption>**Figure 29.** FWHM as a function of zenith distance. This figure is based on measurements of the FWHM from ~480,000 guider images obtained during DESI commissioning, where the median nightly seeing has been subtracted from each measurement. The grayscale shows the number of points in each bin in zenith distance, and each bin in zenith distance is normalized to unity as there are many more observations near the zenith than at higher airmass. The FWHM has no dependence on zenith distance until ~50°, and the behavior with the DESI corrector and the previous MOSAIC corrector are very similar. During the first year of observations, <1% of dark tiles and <2% of bright tiles were observed more than 50° from the zenith. This is a modified version of a figure shown in Meisner et al. (2020).</figcaption></figure>

we developed an end-to-end test based on a series of small, random offsets of each fiber that we refer to as fiber dithering.

Fiber dithering treats the calculation of fiber positioning errors as a nonlinear optimization problem. The model parameters are the flux of each star assigned to a fiber, the 2D misalignment of each fiber relative to the nominal location, the seeing, the transparency, and any telescope offsets between each exposure of a dither sequence. The basic scheme is that we take some number of exposures $N_{\text{exp}}$, with some number of fibers $N_{\text{fibers}}$ assigned to stars. Between each exposure, we dither each fiber in a separate, random direction by some known amount. We then measure the integrated flux of the source in each fiber in each exposure with the spectrographs, as well as measure the precise, relative motion of the fiber positioners with the FVC. The dither pattern needs to match the PSF size to get good centroid measurements for the positioners. If the positioning errors are much larger than the PSF size, then positioners may have no dither positions that collect the light of the target star, and we would not measure their centroids. In this case, we still dither the fibers by the PSF size between exposures, but we also add larger (~5") dither steps onto each target to attempt to align an occasional positioner with the target star on different parts of the focal plane.

We used this dithering analysis on multiple occasions during commissioning as an integrated check of all of the components of fiber positioning. The analysis is especially sensitive to large-scale features, such as shifts between the GFA pixels and other focal plane metrology or errors in the distortion models. Based on the results of several fiber dither sequences at a range

of zenith distances, we rechecked the raw focal plane metrology data and replaced the original FVC lens with the simpler design described in Section 5.8. Within about 2 months, these improvements and checks led to improvements in fiber positioning from initially ~10" rms to the current performance of ~0.1". For reference, offsets of 0.1" (0.2") correspond to throughput losses of 0.07% (3%) for a galaxy with a half-light radius of 0.35" observed in 1" seeing. Figure 30 shows one illustrative result of the dither analysis from 2021 December 19 with data from the red channel of the spectrographs. The dithering analysis also provides a direct way to measure the total throughput independent of fiber positioning errors.

We attribute the remaining radial pattern to residual errors in the distortion model. These residual errors vary between fields and therefore have not been removed. Both of these variations between fields, and variations between the blue, red, and NIR channels, are of the same order of magnitude as the variations shown in the figure. In addition to this distortion pattern, dome seeing and turbulence effects also contribute to fiber positioning inaccuracies. Nonmoving devices with known stationary positions in the focal plane —both fiducials (123) and nonfunctional positioners (of the order of 700)—provide a measure of the turbulence field at each iteration of the positioning loop. We fit this field and correct all measured positions for the turbulence field. Based on a dither analysis with and without these corrections, we find that the turbulence correction produces a ~10% improvement in positioning.
<figure><img src="image_35.png" /><figcaption>**Figure 30.** Results of a fiber dither test on 2021 December 19. (Left) The four, smaller panels show the offset of each fiber positioner from the best-fit position determined by the dither test. The four panels show the offsets in R.A. and decl. for all positioners (top left), the distribution of offsets in decl. (right) and R.A. (bottom left), and the total offset (bottom right). The total offset corresponds to the amplitude of the decenter of each positioner, and in this case the width of the distribution is 0.′′16, and the procedure is sensitive to offsets smaller than 0.′′05. (Right) Amplitude and direction of the offsets in focal plane coordinates. The primarily radial pattern may indicate a residual error in the distortion model. An offset of 40 µm is indicated by the arrow in the upper-left corner; for context, we note that the fiber diameter is 107 µm and the plate scale is approximately 70 µm per arcsecond. This field was observed near zenith at an altitude of 81°4, and the direction of zenith is indicated in the lower-right corner. The fluxes in the fibers at each dither position are from the red channel of the spectrographs.</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>**Figure 31.** Stability of the PSF across 3 days of operations for the three spectrograph channels. The fractional change in the PSF is measured from three sets of arc calibrations obtained on three consecutive days, and the fractional change $\delta F/F$ is the change in the PSF relative to the average computed for a selection of fibers at a selection of wavelengths. Each point color corresponds to a different spectrograph, and the distribution of points for each spectrograph corresponds to the variation in the PSF for a range of fibers. There was a night of observations between each calibration set, and the fiber positioners are in different locations for each calibration. The variation is consequently indicative of the level of variation per night.</figcaption></figure>

### 13.4. Spectroscopic PSF Stability and Resolution

The spectral extraction code employs a model of the spectrograph PSF to accurately measure the flux, noise, and spectral resolution of every spectrum and subtract night sky lines. Requirement L3.1.6 states that the PSF bias shall not exceed 1%, where the PSF bias is $\delta F/F = \Sigma_{pi}q_i/\Sigma_i q_i - 1$; the two quantities are the intensity distribution of the true PSF $p_i$ and the linearly interpolated PSF $q_i$ based on the arc calibration data, and the sum is over all $i$ pixels that contribute to the PSF. This bias captures the fractional difference between the true PSF shape and the linearly interpolated model, and determines the sky subtraction accuracy. We measure the PSF from arc calibration frames obtained as part of the daily calibration protocol (typically afternoon calibrations). The main factors that are expected to produce changes in the PSF are changes in

forces on the fiber cables and environment-induced changes in the spectrographs. The forces on the fiber cables change when the telescope moves to different positions on the sky and when the positioners move the fiber tips within their patrol regions. Changes in the spectrographs could be produced by changes in the temperature and humidity of the Shack environment.

We have measured the size of the PSF bias with numerous, dedicated tests to determine if daily (afternoon) measurements of the PSF from arc calibration data are sufficient. For one test, we fixed the telescope at the altitude of the dome flat screen, moved the telescope to different azimuth locations in the dome, rotated the dome to place the flat field screen in front of the telescope, and took a sequence of arc calibrations. For a second test, we kept the telescope in the same position and took a sequence of arc calibrations with the fiber positioners in different configurations. Figure 31 shows the variation in the
<figure><img src="image_37.png" /><figcaption>**Figure 32.** Example spectra of two emission-line galaxies that illustrate both the sensitivity of the instrument and the quality of the sky subtraction. These two galaxies are at sufficiently high redshift that the [O II] doublet falls in a wavelength region with a substantial number of atmospheric emission lines. The noise spectrum in both panels illustrates the increase at discrete wavelengths due to the Poisson noise from these lines. The noise is shifted by $-10^{-17}$ in these units to make it more visible. These galaxies were both observed on 2021 November 30.</figcaption></figure>

PSF bias measured with calibration data obtained on three consecutive afternoons after nighttime observations. After each night of observing, the fiber positioners are in different locations, as they are left in the configuration for the last observation of the night. The PSF bias is <1% for nearly all fibers, and it is sufficiently stable to obtain a sky subtraction precision better than 1%. Figure 32 shows two examples of the superb sky subtraction with spectra of ELGs at $z > 1$. These are particularly challenging objects because redshift measurements require detection of the faint, marginally resolved [O II] doublet, which is in the midst of significant atmospheric emission at these redshifts. The sky subtraction algorithm and PSF stability will be described further in J. Guy et al. (2022, in preparation).

In addition to this superb stability, we assessed that the spectrographs delivered the expected spectral resolution. We measure the spectroscopic resolution with the afternoon arc calibration described in Section 12.3. Specifically, we measured the 2D PSF of each fiber in each spectrograph as a function of wavelength, convolved this model with the CCD pixels, projected the model onto the dispersion axis, and then measured the FWHM of the profile. Figure 33 shows a collection of resolution measurements for a subset of fibers in all 10 spectrographs from 2022 January. This figure demonstrates that the spectral resolution exceeds the requirement. In the red and NIR channels, this resolution is about twice that of SDSS and BOSS, and this enables DESI to observe the faint continuum between the OH sky lines and resolve the [O II] doublet from high-redshift star-forming galaxies.

### 13.5. Inter-exposure Sequence

During commissioning and early in the early operations phase, we worked to minimize the length of the inter-exposure sequence, which is described in Section 8.5 and summarized in

Figure 21. Our requirement is that the inter-exposure sequence be less than 2 minutes, which is a Level 4 requirement that flows down from the requirement to maximize the survey efficiency.

The times for the spectrograph CCD readout and the setup of the telescope are as short as practical, and both begin as soon as the shutter closes. The spectrograph readout time is 60 s. This readout occurs in parallel with other tasks during the inter-exposure sequence, and those take sufficiently long that the inter-exposure sequence is not limited by the spectrograph readout time. The time to move the telescope, dome, and ADC depend on the relative locations of the current and next field. Early in the project, we used the move parameters of the telescope’s hour angle and decl. drives, the dome motors, and the ADC motors to simulate the distribution of times for each of these motions to complete for simulated survey observations. These actions happen in parallel, and the telescope dominates 88% of the time, while the dome dominates the remaining 12%. The mean (median) time for these moves to complete is 41 s (27 s). Based on our operations experience to date, moves complete somewhat more quickly, possibly because we have continued to optimize the survey software to minimize slew distances. While the telescope is in motion, we adjust the location of the hexapod for the new position based on a static look-up table and perform the blind of the fiber positioners to set up on the new field. The hexapod adjustment takes approximately 5 s, and the execution of the blind move takes 35 s.

Once the telescope is in position, we obtain and analyze a 15 s acquisition image with the guider GFAs. We have preestablished guide stars for every field, and we have preestablished where those guide stars need to be located on the GFAs in order to ensure that the fibers are aligned with their targets. It takes 4 s to analyze the GFA images and about 7 s to
<figure><img src="image_38.png" /><figcaption>**Figure 33.** Spectral resolution as a function of wavelength for all 10 spectrographs. The line width $\Delta\lambda$ is the FWHM of the 2D PSF model fit to nightly average calibration data after the PSF is projected along the dispersion axis. The range of lines corresponds to all measurements from 2022 January for all 10 spectrographs and a subset of fibers. The resolution exceeds the requirement (dashed line) over the entire wavelength range. The crossover region for the red pass dichroic is 566–593 nm, and the crossover region for the NIR pass dichroic is 747–772 nm.</figcaption></figure>

adjust the telescope pointing, start the hexapod rotator, and initialize the guider. In total, we typically complete the field acquisition and are ready to begin guiding about 30 s after the end of the telescope slew. For very small slew, we are ready to begin guiding as soon as 40 s after the spectrograph shutters closed to end the previous exposure. Once we begin guiding, we are ready to back-illuminate the fibers, turn on the fiducials, obtain an FVC image, and calculate and execute the correction move. The correction move is typically complete 51 s after guiding begins. Once this step is complete, we take a second FVC image to record the final locations of the fiber positioners, turn off the back illumination and fiducials, and are ready to start the exposure 15 s after the completion of the correction move. At present we have achieved inter-exposure times as short as 118 s between fields that are close together on the sky.

### 13.6. Performance Problems and Solutions

Early in the operations phase, we experienced problems with the CAN communication to positioners. This would manifest as a loss of communication with an entire CAN bus and cause the temporary loss of communication to the devices on that bus. This would result in the loss of 28–78 devices. While we could recover from this error condition, the recovery procedure was sufficiently long that it proved more efficient to observe the rest of the night without these positioners and to perform the recovery procedure the next day. We traced this problem to communications problems with individual positioners that would disrupt an entire bus. To minimize disruption, we redesigned and changed all of the transverse boards to add a relay for each positioner. This provided the flexibility to switch off individual problematic positioners, rather than risk temporary loss of an entire bus. We also increased the number

of CAN channels from 10 to 20, which reduced the maximum number of devices per CAN bus to 28. As the transverse boards are directly behind the petals, this required a major operation: the disassembly of the FPE and the removal of the petals to obtain sufficient access. We successfully conducted this upgrade during the summer 2021 monsoon season, and we are now able to maintain higher observing efficiency.

The other main performance issue that we identified during early testing at the Mayall was that some positioners would occasionally stop moving the commanded amount, which we traced to performance problems with the motor for either the $\phi$ or $\theta$ arm. The most common failure mode was that the motor would start to move a fairly constant fraction of the requested move, and we referred to this as the “linear $\phi$” (or $\theta$) problem. When the petals were removed in the summer of 2021, we extracted several of these problematic positioners that were near the edge of a petal and consequently possible to extract with minimal risk to neighbors. High-resolution CT scans of these positioners showed the pinion gear inside of the motors had cracked radially through the wall thickness, which allowed the gear to slip relative to the motor shaft. The pinion gear had been attached to the motor shaft via a press fit. We hypothesize that some assemblies may have exceeded the (very challenging) tolerance limits that press fits at this small scale can require, and that the stress induced in these cases initiated a slow-growing crack in the affected gears. A particular challenge in the detection of the failures and then the identification of the root cause was that affected gearboxes may spin properly in the horizontal orientation (typical of our lab tests) and only become symptomatic when pointed downward (as they are at the zenith position of the telescope). While the cracked motors cannot be replaced without rebuilding the petals, we are exploring operational changes to
regain the use of the motors that show linear behavior. As of
2022 January, about 14% of positioners are not used on targets
due to mechanical or electrical problems, although most of
these naturally fall on blank sky positions and are therefore still
useful. Some preliminary data suggest that of the order of one
new positioner becomes problematic per week, although we are
also actively engaged in remediation efforts to restore some of
the unused positioners to regular operations.

14. Successes and Lessons Learned

In this section we describe the key management decisions
and actions that were especially helpful, and in some cases
critical, for the completion of DESI ahead of schedule and
under budget. We also take advantage of hindsight to identify
actions that we could have taken, or could have taken earlier,
that would have helped with the schedule and/or budget. These
lessons learned are most relevant within the broader context of
a project that follows the DOE project management structure.
This at minimum includes that the project has clearly defined
requirements, seeks to follow best practices for project
management, has a well-defined systems engineering approach,
develops a risk management plan, and maintains a risk registry.

The most important early action was to assemble a capable
leadership team with substantial experience with previous large
projects. In this respect, and in many others, DESI drew in
particular on the community of expertize that was developed by
the earlier Stage III and II dark energy projects, most notably
the DES and the SDSS BOSS projects. This leadership group
made several important early decisions that played an outsized
role in the success of the project. Potentially the most important
was to define interfaces and requirements early, along with the
verification methods and acceptance criteria for the require-
ments. This was especially valuable given the dispersed nature
of the instrument development, as we used these requirements
and their verification methods to set clear expectations for the
scope of work of various partner institutions, as well as to
negotiate contracts with vendors for critical early procurements.
The requirements that we wrote at the start of the project in
2014 were sufficiently robust that we did not change them
through the end of CD-4, which is an impressive testament to
value of our early R&D and a major success. This R&D work
included extensive tests of fiber positioners with test stands,
work to optimize the fiber FRD with precision cleaves and
other experiments, and the early construction of the first
spectrograph. A second important decision was to seek early
funding to begin the longest-lead procurements, which were the
optics, detectors, and spectrographs. We would not have
completed the project on time and on budget without the early
definition of the requirements for these key components, our
success with early fundraising, and early R&D. Another key
feature of project planning was the allocation of adequate cost
and schedule contingency, especially to accommodate the
tendency of less-experienced control account managers to
underestimate costs. We started construction (CD-2 in 2015
September) with $13.9M of cost contingency and consumed
$12.0M through to the finish of the project (out of an original
total project cost of $56M). We used the contingency funds for
risk mitigation and maturation of cost estimates while
maintaining the schedule. The project consumed only 3 months
of schedule float to the required finish of 2021 September with
an actual finish of 2020 May, or 17 months ahead of the
baseline schedule. Two lessons learned regarding management

were the value of experienced control account managers and
that having fewer control account managers simplifies earned
value management.

Our experience with large, complex procurements under-
scored the importance of best-value assessments of bids to
avoid commitments to substandard, lowest bids. The most
important considerations were minimum standards for vendors
and any subcontractors and the award of contracts to vendors
that had an established track record with similar work (if they
existed). For example, we anticipated the most likely problems
that could arise in a procurement and required that vendors
possess the equipment they would need to solve such problems.
The selection criteria need to be clearly specified in the Request
for Proposals along with a clear description of the tests that
should be conducted to demonstrate that requirements have
been met. We recommend trying to award contracts for
difficult, specialized work to experienced vendors, to consult
with experts and previous customers to assess a vendor's
capabilities and responsiveness, and to try to back-load the
payment schedule as much as practical to maintain a vendor's
engagement.

Some specific successes with the instrumentation beyond
procurement included the detailed finite element analysis of the
corrector system, the decision to modularize the focal plane
system into 10 petals, the involvement of a production engineer
in the fiber system work starting with the transition from the
R&D stage to production, testing the complete spectrographs at
the optics vendor, and the modular and scalable architecture for
the ICS. Lastly, we benefited from the dedication of
experienced teams on all of the subsystems, and especially
note the extremely transferable experience of the teams at
FNAL and UCL with the DECam corrector, CEA Saclay for
Megacam, UCB with optics and spectrographs, NAOC with
LAMOST, the Ohio State team with the ICS for DECam and
numerous spectrograph projects (MODS, OSMOS), and the
experience of the LBNL team with optics, CCDs, fiber
positioners, and the instrument upgrades of the BOSS
spectrographs.

There were also several notable challenges. One of the larger
challenges was the fabrication and testing of the fiber
positioners, largely because of the sheer scale of the number
of components. Each positioner contains dozens of parts that
required careful quality control (QC), inventory tracking, and
precision tooling for assembly. We would have benefited from
more resources for the QA/QC of the positioner production as
early as the development and planning stage. We would also
have benefited from a large-scale, preproduction run of at least
1000 units, as smaller batches are insufficient to uncover all
issues and may produce insufficient disposable units for
integrated tests. This would have helped to fine tune the
manufacturing process and potentially have alleviated some
delays and added costs. We should have conducted more
positioner testing in the same gravity orientation as they
experience in nighttime operations, as we may then have
identified the issue with the press fit of the gears earlier. This
testing should have included more resources to diagnose any
failed positioners. We also overestimated the yield on the
production of science grade CCDs for the spectrographs and
ultimately needed to purchase additional devices. Lastly, we
largely developed the positioner software independent of the
ICS. We would have benefited from more software developer
resources earlier in the project to methodically coordinate the interfaces between multiple subsystems and institutions.

The main successes of the Assembly, Integration, and Test phase included prototypes, thorough planning and test runs of critical/high value installation steps, and proper staffing levels. The precommissioning activities with ProtoDESI and the CI were invaluable for technical demonstrations. The experience we gained with operations was also important, as this helped us more appropriately staff commissioning. We similarly gained valuable knowledge and risk reductions with test stands for numerous other components, most notably the testing of the completed petals and the completed spectrographs prior to shipment. As noted in Section 12.2.3, we built a complete petal installation setup on the ground floor of the Mayall and practiced petal installation with engineering dummy petals. This experience helped us to improve the installation procedures and led to some minor hardware modifications. Lastly, appropriate staffing levels were absolutely critical to the success of this very stressful and labor-intensive phase of the project. One potential area for improvement would have been to more directly address subsystem requirements in design reviews. In some instances, the review documentation did not explicitly address the relevant subsystem requirements, which unnecessarily complicated the subsystem acceptance process. Another area for improvement would have been earlier integration of subsystems. More complete and integrated lab testing and higher-fidelity hardware simulators may have helped to identify integration challenges that we otherwise uncovered during nighttime commissioning.

## 15. Summary

DESI is the largest multiobject spectrograph constructed to date and was designed to efficiently conduct a comprehensive spectroscopic exploration into the nature of dark energy. DESI will survey 40 million galaxies and quasars in just 5 yr, and we forecast it will measure dark energy parameters sufficiently well to become the first completed Stage IV survey according to the definition developed by the DETF.

DESI is able to conduct this unprecedentedly large survey in such a relatively short time due to the substantial instrumentation development described in this paper. The three most fundamental aspects that enable these goals are that DESI can record 5000 spectra in a single exposure, has very high throughput, and maintains excellent operational efficiency. In Section 2 we provided a detailed description of the science and survey requirements, how they were used to produce the technical requirements, and how the instrument development was managed. We then described the main components of the instrumentation. The most noteworthy components, requirements, and technical challenges are:

Corrector (Section 3): The prime-focus corrector has six lenses that produce a 3.2 diameter FOV. Four of the lenses are constructed from fused silica, two of which have an aspheric surface, and the remaining two form an ADC constructed from borosilicate glass. The lenses are approximately 1 m in diameter and are coated with extremely high-performance, broadband AR coatings. Two key achievements with the corrector are superb image quality and extraordinarily high throughput. Based on the as-built optics and alignment precision, the performance meets requirements, including a flux-weighted mean FWHM < 0.4 over the full 360–980 nm bandpass at zenith. The coatings meet their average

requirement of >98.5% transmission over the same bandpass by approximately 0.5% per coating, which corresponds to a net gain of about 5% for the product of all 12 surfaces.

Corrector support system (Section 4): The corrector barrel and lens cells meet very high static and dynamic tolerances, some of which are as challenging as 20 µm for lateral alignment and 20 µrad for tilt. The corrector support system includes a new top ring that was mounted on the existing Serrurier truss. The replacement of the old, massive split ring is the main reason that the total mass and moment of the Mayall top end are relatively unchanged in spite of the new corrector and other DESI instrumentation. The new ring has a set of vanes that support the prime-focus cage, and the corrector is mounted in this cage with an extremely precise hexapod with six degrees of freedom. We use the hexapod in conjunction with a static look-up table and the analysis of intra- and extra-focal images to maintain the optical alignment of the corrector with the Mayall primary mirror.

Focal plane system (Section 5): The focal plane system includes the FPA with the robotic fiber positioners, illuminated fiducials, and GFA cameras, as well as the enclosure and FVC. The FPA is mounted at the prime focus and is subdivided into 10 identical, 36° wedge-shaped petals that place the fiber tips on the aspheric focal surface of the corrector. Each of the 10 petals has 502 robotic fiber positioners with fibers that connect to one of the spectrographs, 12–14 illuminated fiducials, and a GFA camera. The key technology that enables DESI to fit all of the fiber positioners in an 0.812 m diameter focal surface is the two 4 mm diameter brushless DC motors in each fiber positioner. Placing the fiber tips on astronomical targets with an accuracy of ~10 µm (0.2°) requires many interconnected components to work well. These include the FVC, precision metrology of the petals, illuminated fiducials, the GFA cameras, back illumination of the fiber cables, optical models of the FVC and corrector optics, careful calibration of the individual fiber positioners, and precise and accurate transformations from celestial coordinates to focal surface.

Fiber system (Section 6): The fiber system achieved its key requirement of transmitting light from the focal surface to the spectrographs with minimal throughput loss. Several factors were critical to this success, including careful control of stresses on the fibers, precision cleaves of the fibers, AR coating of the fiber tips, careful alignment of the fiber tips in the slitheads, and fusion splicing. Fusion splicing connected the short lengths of fibers in the positioners to the remainder of the fiber cables and slitheads once the positioners were assembled into the petals. This resulted in the creation of continuous, high-throughput fiber cables from the focal surface to the spectrographs.

Spectrograph system (Section 7): The spectrograph system includes 10 spectrographs, their environmental enclosure, the calibration system, and the sky monitor system. Each spectrograph has three channels with a distinct grating, spectral resolution, and camera design that optimizes the performance of that channel. Together the channels record the flux of each object from 360–980 nm with extraordinarily high throughput. Specific contributors to the high throughput include very high-efficiency VPH gratings and superb coatings on the collimator mirror, dichroics, optical elements in each camera, and very sensitive CCDs. The spectroscopic calibration is extraordinarily stable due to the tight temperature control of <0.1°C provided by the environmental enclosure, and it is well
determined due to the careful design of the calibration lamp system and dome screen.

ICS (Section 8): The ICS is responsible for the operation and coordination of all instrument components and the telescope. These include the selection of each field, telescope pointing and guiding, adjustment of the hexapod's six degrees of freedom, readout of the 10 GFA cameras, movement of all fiber positioners, operation of the 123 illuminated fiducials, activation of the 20 shutters and their pneumatic seals, illumination of 10 LEDs on the exposure shutters to back-illuminate the focal plane, readout of 30 spectrograph CCDs, operation of the two cameras in the sky monitor system, control of the FVC, and recording of myriad temperature sensors and other telemetry data. The ICS coordinates the operation of all of components to maximize survey efficiency. The ICS also provides a warning and alert system for potential problems, as well as a database to track performance over time.

Data systems (Section 9): Data systems includes software for target selection, survey planning, fiber assignment, nighttime data QA (Nightwatch), data transfer, and the spectroscopic pipeline. Some of the key achievements of data systems are data quality feedback to observers within minutes of each exposure, the near-real-time transfer of raw data from KPNO to NERSC, and processing of the raw data into sky-subtracted, flux-calibrated spectra with classifications and redshifts by the morning after observations are complete. Survey planning software includes tools to monitor progress relative to expectations and to evaluate alternative strategies. The target selection and fiber assignment modules include the tools to convert astronomical targets into focal plane configurations at each telescope pointing.

Mayall Upgrades (Section 10): We upgraded numerous components of the 4 m Mayall Telescope to prepare for the installation of DESI. An especially important upgrade was the modernization of the TCS, including new encoders for both the hour angle and decl. axes, new servo controllers for the telescope drives, and a new control computer. These changes led to improvements in the pointing from 10"-20" to 2"-3" rms and an improvement in the open-loop tracking errors from 30" hr⁻¹ to 1"·6 hr⁻¹ in R.A. and 0′/1/hr in decl. The size of typical guide corrections are tens of milliarcseconds, which are downscaled by 30% by the TCS control loop.

The acceptance, integration, and functional verification of DESI largely occurred in 2018 and 2019, with the peak of installation during a dedicated shutdown of the Mayall Telescope for most of 2018 February through 2019 September (see Section 12). We then proceeded with commissioning to both test and optimize the complete instrument. During commissioning, we verified a number of requirements that could only be tested with on-sky observations, such as the total throughput. The full list of key commissioning tests is described in Section 13. Throughout the instrument development process, we tracked many key decisions and actions that kept the project on schedule and on budget. We also conducted a retrospective exercise to identify other actions and decisions that could have improved the schedule and/or budget. These successes and lessons learned are described in Section 14 and are intended to be a resource for other, similar projects. Some of the highlights from this section are the importance of the people, the use of best-value assessments for large, complex procurements, and the value of early R&D and prototypes, although in retrospect, we would have benefited from even

greater use of integrated lab tests and simulations. After commissioning, DESI started Survey Validation on 2020 December 14. During this phase, we validated the scientific requirements and specifically the target selection algorithms. The target selection for Survey Validation is by design more expansive than will be employed for the main survey, and includes longer spectroscopic exposures to better characterize redshift measurements with nominal survey exposures. The target selection algorithms were then frozen before the start of the main survey.

The main survey began on 2021 May 14, and we are routinely observing 80,000 galaxies and quasars on every clear night with minimal technical downtime and very high on-sky efficiency. DESI measured over 15 million extragalactic redshifts in the first year of main survey operations and is already the largest spectroscopic survey ever conducted. Data from DESI will lead to improved measurements of cosmic acceleration, more sensitive searches for time evolution of the dark energy component, stronger constraints on modified gravity models, and significantly improved upper limits on the sum of the neutrino masses. Upon the completion of the survey, DESI will have measured redshifts for approximately 14 million galaxies in the BGS, 8 million LRGs, 17 million ERGs, and 2.8 million QSOs. With these data, we expect to measure the cosmic distance scale with at least 0.22% aggregate precision from 0 < z < 1.1, 0.31% from 1.1 < z < 1.9, and the Hubble parameter to 0.84% precision from 1.9 < z < 3.7. DESI may distinguish between the inverted and normal neutrino mass hierarchy, should provide extraordinary new information about the nature of gravity through measurements of anisotropic clustering, and probe the nature of inflation through the amplitude of primordial non-Gaussianity in the matter power spectrum.

The DESI collaboration is indebted to the late John Donaldson, Robin Lafever, Tammie Lavoie, and Glenn Roberts for their many contributions to the success of this project and mourns their passing.

This research is supported by the Director, Office of Science, Office of High Energy Physics of the U.S. Department of Energy under contract No. DE-AC02-05CH11231, and by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility under the same contract. Additional support for DESI is provided by the U.S. National Science Foundation, Division of Astronomical Sciences under contract No. AST-0950945 to the NSF's National Optical-Infrared Astronomy Research Laboratory; the Science and Technologies Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies and Atomic Energy Commission (CEA); the National Council of Science and Technology of Mexico (CONACYT); the Ministry of Science and Innovation of Spain, and by the DESI Member Institutions: Aix-Marseille University; Argonne National Laboratory; Barcelona-Madrid Regional Participation Group; Brookhaven National Laboratory; Boston University; Brazil Regional Participation Group; Carnegie Mellon University; CEA-IRFU, Saclay; China Participation Group; Cornell University; Durham University; École Polytechnique Fédérale de Lausanne; Eidgenössische Technische Hochschule, Zürich; Fermi National Accelerator Laboratory; Granada-Madrid-Tenerife Regional Participation Group; Harvard University;
Kansas State University; Korea Astronomy and Space Science Institute; Korea Institute for Advanced Study; Lawrence Berkeley National Laboratory; Laboratoire de Physique Nucléaire et de Hautes Energies; Ludwig Maximilians University; Max Planck Institute; Mexico Regional Participation Group; New York University; NSF's National Optical-Infrared Astronomy Research Laboratory; Ohio University; Perimeter Institute; Shanghai Jiao Tong University; Siena College; SLAC National Accelerator Laboratory; Southern Methodist University; Swinburne University; The Ohio State University; Universidad de los Andes; University of Arizona; University of Barcelona; University of California, Berkeley; University of California, Irvine; University of California, Santa Cruz; University College London; University of Florida; University of Michigan at Ann Arbor; University of Pennsylvania; University of Pittsburgh; University of Portsmouth; University of Queensland; University of Rochester; University of Toronto; University of Utah; University of Waterloo; University of Wyoming; University of Zurich; UK Regional Participation Group; and Yale University.

The authors are honored to be permitted to conduct scientific research on Iolkam Du'ag (Kitt Peak), a mountain with particular significance to the Tohono O'odham Nation.

For more information, visit https://desi.lbl.gov.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

Facility: Mayall (DESI).

## Data Availability

All data points shown in the published graphs are available in machine-readable form in Zenodo at https://doi.org/10.5281/zenodo.6397426.

## ORCID iDs

J. Aguilar https://orcid.org/0000-0003-0822-452X

S. Ahlen https://orcid.org/0000-0001-6098-7247

Shadab Alam https://orcid.org/0000-0002-3757-6359

David M. Alexander https://orcid.org/0000-0002-5896-6313

L. Allen https://orcid.org/0000-0002-7789-5119

C. Allende Prieto https://orcid.org/0000-0002-0084-572X

J. Ameel https://orcid.org/0000-0003-3638-2584

J. Asorey https://orcid.org/0000-0002-6211-499X

S. Bailey https://orcid.org/0000-0003-4162-6619

A. Balaguera-Antolínez https://orcid.org/0000-0001-5028-3035

O. Ballester https://orcid.org/0000-0002-7126-5300

A. Bault https://orcid.org/0000-0002-9964-1005

S. F. Beltran https://orcid.org/0000-0001-6324-4019

A. Berti https://orcid.org/0000-0003-3582-6649

Florian Beutler https://orcid.org/0000-0003-0467-5438

S. Bose https://orcid.org/0000-0002-0974-5266

S. Brieden https://orcid.org/0000-0003-3896-9215

A. Brodzeller https://orcid.org/0000-0002-8934-0954

D. Brooks https://orcid.org/0000-0002-8458-5047

R. N. Cahn https://orcid.org/0000-0003-2748-0641

Z. Cai https://orcid.org/0000-0001-8467-6478

R. Capasso https://orcid.org/0000-0002-3083-6840

A. Carnero Rosell https://orcid.org/0000-0003-3044-5150

F. J. Castander https://orcid.org/0000-0001-7316-4573

J. L. Cervantes-Cota https://orcid.org/0000-0002-3057-6786

S. Chabanier https://orcid.org/0000-0002-5692-5243

E. Chaussidon https://orcid.org/0000-0001-8996-4874

C. Chuang https://orcid.org/0000-0002-3882-078X

S. Cole https://orcid.org/0000-0002-5954-7903

A. P. Cooper https://orcid.org/0000-0001-8274-158X

A. Cuceu https://orcid.org/0000-0002-2169-0595

T. M. Davis https://orcid.org/0000-0002-4213-8783

K. Dawson https://orcid.org/0000-0002-0553-3805

J. Della Costa https://orcid.org/0000-0003-0928-2000

A. Dey https://orcid.org/0000-0002-4928-4003

B. Dey https://orcid.org/0000-0002-5665-7912

G. Dhungana https://orcid.org/0000-0002-5402-1216

Z. Ding https://orcid.org/0000-0002-3369-3718

K. Douglass https://orcid.org/0000-0002-9540-546X

Y. Duan https://orcid.org/0000-0002-2611-0895

D. J. Eisenstein https://orcid.org/0000-0002-2929-3121

S. Escoffier https://orcid.org/0000-0002-2847-7498

X. Fan https://orcid.org/0000-0003-3310-0131

V. A. Fawcett https://orcid.org/0000-0003-1251-532X

J. Erezza https://orcid.org/0000-0002-0194-4017

C. S. Frenk https://orcid.org/0000-0002-2338-716X

B. T. Gänsicke https://orcid.org/0000-0002-2761-3005

L. Garrison https://orcid.org/0000-0002-9853-5673

E. Gaztañaga https://orcid.org/0000-0001-9632-0815

Alma X. Gonzalez-Morales https://orcid.org/0000-0003-4089-6924

G. Gonzalez-de-Rivera https://orcid.org/0000-0003-4452-743X

V. Gonzalez-Perez https://orcid.org/0000-0001-9938-2755

O. Graur https://orcid.org/0000-0002-4391-6137

G. Green https://orcid.org/0000-0002-0676-3661

D. Gruen https://orcid.org/0000-0003-3270-7644

C. Hahn https://orcid.org/0000-0003-1197-0902

D. Herrera https://orcid.org/0000-0003-2092-6727

Hiram K. Herrera-Alcantar https://orcid.org/0000-0002-9136-9609

D. Huterer https://orcid.org/0000-0001-6558-0112

V. Iršič https://orcid.org/0000-0002-5445-461X

M. Ishak https://orcid.org/0000-0002-6024-466X

L. Jiang https://orcid.org/0000-0003-4176-6486

Y. P. Jing https://orcid.org/0000-0002-4534-3125

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S. E. Koposov https://orcid.org/0000-0002-2644-135X

A. Kovács https://orcid.org/0000-0002-5825-579X

A. Kremin https://orcid.org/0000-0001-6356-7424

B. L'Huillier https://orcid.org/0000-0003-2934-6243

M. Landriau https://orcid.org/0000-0003-1838-8528
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J. Lasker https://orcid.org/0000-0003-2999-4873

L. Le Guillou https://orcid.org/0000-0001-7178-8868

A. Leauthaud https://orcid.org/0000-0002-3677-3617

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N. Palanque-Delabrouille https://orcid.org/0000-0003-3188-784

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