and the active optics system, 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. 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 was initially comprised of 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 judgement 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 re-assembly 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

Figure 23: 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 14 June 2018. 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 28 November 2018.

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 §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
April 2017. 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 February
2018 to March 2019 and for the focal plane system installation from May
through September 2019. During both of these periods, the telescope was locked
into position. The interlude between these two phases corresponds to the
Commissioning Instrument campaign, which was described in §11.2. Besuner et
al. (2020) present a more detailed description of the entire installation
process.

####  12.2.1 Top End and Corrector

Preparations for the removal of the top end began with the removal of the
KOSMOS (Martini et al., 2014) instrument from the Cassegrain focus so that we
could center an alignment telescope on the rotator axis with the aid of a
cross-hair marked on the $f/8$ secondary mirror. The next steps were the
removal of 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 locked
the telescope into position at the zenith. The next step was the disassembly
of the bottom end, including 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 re-install 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 23) 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 years. On June 14, 2018 the exterior crane lifted the old top
end, maneuvered it through the dome slit, and placed it on a flatbed truck
that carried it to its retirement home in the parking lot by the Kitt Peak
Visitor Center. The crane then lifted 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 April 19, 2018,
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 pre-tensioned upper
ring. We lifted the ring with the pre-tensioning 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. The ring was then
lowered 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 July 13, 2018
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 November 2018 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 November 28, 2018 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 mid-May
2018 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 Western Environmental Corporation representative verified
that the Shack met temperature, humidity, and cleanliness requirements in mid-
October 2018. The certification tests showed that the temperature stability
was better than $0.1^{\circ}$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 mid-December 2018 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 §12.3).

####  12.2.3 Focal Plane and Fiber System

Figure 24: Installation of the first petal of the focal plane system on 25
June 2019. 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 ten petal positions
are temporarily covered by ten 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 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 §12.2.3 for more details.

We began preparations for the assembly of the focal plane system at the Mayall
during the Commissioning Instrument campaign in April and May of 2019. As
described in §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 ten 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 $36^{\circ}$ about
the corrector axis) relative to its neighbors. In addition, the insertion
direction was tilted by $\sim 7.2^{\circ}$ 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 24). 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 $\sim 10$ 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 $100\,\mu$m, and better than $50\mu$m was achieved
under good conditions. Shourt et al. (2020) provide a more extensive
description of the alignment and integration of the FPA with a laser tracker.

While the Commissioning Instrument 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 $\sim 7.2^{\circ}$ 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 Commissioning Instrument 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 May 8, 2019, 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
June 25, 2019. The installation of the remaining petals proceeded smoothly,
and we installed the final petal on July 23, 2019. Figure 24 is a photo of the
first petal ready for installation.

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 declination 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 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 Commissioning
Instrument, 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, 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 curent, and CCD cosmetics. Once the focal
plane and calibration systems were installed, we measured light leaks, image
quality, spectral resolution, cross talk, 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 (December 2019). 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 25 shows an example of
the data from one (SP4) of the ten 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
Commissioning Instrument, both it and the FVC went through FV. The main step
was illumination of the fidicuals on the Commissioning Instrument, and imaging
those spots with the FVC. This produced the first recorded images through the
new corrector. On-sky observations, described in §11.2 and shown in Figure 3,
demonstrated the superb performance of the corrector optics.

Figure 25: 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 900s exposure in dark conditions obtained on 19
December 2020. Note the greater sensitivity of the thicker CCDs in the red and
NIR channels to cosmic rays.

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 $\geq 99$% 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 October 2019
upon completion of FV for the focal plane and fiber system.

##  13 On-Sky Performance Results

Commissioning began in October 2019 as soon as a sufficient fraction of the
instrument was ready for on-sky observations. The purpose of this phase was to
test and optimize the instrument and demonstrate that it met all performance
requirements from $0^{\circ}$ to $60^{\circ}$ 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 §11.2 as part of the Commissioning Instrument 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
uncovered several hardware issues that impacted performance. In the last
subsection, we describe those issues and how we addressed them. Commissioning
was interrupted from March through November 2020 due to the global COVID-19
pandemic and formally ended in December 2020 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 §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 26 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 $0.7^{\prime\prime}$
and $0.9^{\prime\prime}$ FWHM). We determined the 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 of about 40% for our typical seeing of $1.1^{\prime\prime}$
and increases as the seeing degrades. The total throughput shown in Figure 26
is corrected for fiber aperture loss. 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.

Figure 26: 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, 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.

###  13.2 Guiding and Fiber Flux

Figure 27: (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 January and April 2021. The requirement is 100
milliarcseconds rms on each axis (vertical, dotted line). The contribution in
right ascension is larger than the contribution in declination, which is
expected for an equatorial mount telescope.

The previous subsection demonstrated how DESI provides exceptional throughput.
The next subsections show how well the telescope, corrector, and focal plane
delivers 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 1-D. 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 right ascension and declination, 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 27 shows
the distribution of measured seeing values from guider images obtained between
October 23, 2019 and March 15, 2020. These FWHM measurements were obtained
from the measured radial profile of stars on a guider chip. The best FWHM
measurements are between $0.6-0.65^{\prime\prime}$ and demonstrate the
excellent performance of the corrector system. The median is
$1.11^{\prime\prime}$, which is superior to the median of
$1.2-1.3^{\prime\prime}$ measured by Dey & Valdes (2014). For typical seeing
of $1.1^{\prime\prime}$, about 60% of the flux from a point source enters the
fibers. Figure 28 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 §10.2). The latest model from April 1,
2021 yielded excellent pointing and tracking performance. The median
performance was about 100 mas from January through April of 2021 on each axis
and this is shown in the right panel of Figure 27. The performance is slightly
better along the declination 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.

Figure 28: FWHM as a function of zenith distance. This figure is based on
measurements of the FWHM from $\sim 480,000$ 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 $\sim 50^{\circ}$ deg, and the
behavior with the DESI corrector and the previous MOSAIC corrector are very
similar. This is a modified version of a figure shown in Meisner et al.
(2020).

The overall performance of the GFAs has been excellent. We regularly monitor
the dark current and readnoise, 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 $10-11^{\circ}$ C. There is a
modest amount of variation in the readnoise that we attribute to low-level
pattern noise. The readnoise has consistently remained below the requirement
of $25\,e^{-}$ rms per pixel.

###  13.3 Fiber Positioning

Figure 29: Results of a fiber dither test on December 19, 2021. (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 right ascension and declination for all positioners (top left), the
distribution of offsets in declination (top right) and right ascension (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 $0.16^{\prime\prime}$ and the procedure is sensitive to
offsets smaller than $0.05^{\prime\prime}$. (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 $40\,\mu$m is
indicated by the arrow in the upper-left corner; for context, we note that the
fiber diameter is $107\,\mu$m and the plate scale is approximately 70 $\mu$m
per arcsecond. This field was observed near zenith at an altitude of
$81.4^{\circ}$, 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.

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, 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 two-dimensional 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 $N_{\rm exp}$, with some number of fibers
$N_{\rm fibers}$ 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 ($\sim 5^{\prime\prime}$) 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 §5.8.
Within about two months, these improvements and checks led to improvements in
fiber positioning from initially $\sim 10^{\prime\prime}$ rms to the current
performance of $\sim 0.1^{\prime\prime}$. For reference, offsets of
$0.1^{\prime\prime}$ ($0.2^{\prime\prime}$) correspond to throughput loses of
0.07% (3%) for a galaxy with a half-light radius of $0.35^{\prime\prime}$
observed in $1.1^{\prime\prime}$ seeing. Figure 29 shows one illustrative
result of the dither analysis from December 19, 2021 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 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. Non-moving devices with known stationary positions in the focal
plane – both fiducials and non-functional positioners – 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 $\sim 10$% improvement in positioning.

###  13.4 Spectroscopic PSF Stability and Resolution

Figure 30: Stability of the PSF across three 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.

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 $F=\Sigma p_{i}q_{i}/\Sigma
q_{i}q_{i}-1$ and the two quantities are the intensity distribution of the
true PSF $p_{i}$ and the linearly-interpolated PSF $q_{i}$ based the arc
calibration data. We measure the PSF from arc calibration frames obtained as
part of the daily calibration protocol. 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 30 shows the
variation in the PSF bias measured with calibration data obtain 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 31 shows two examples of the
superb sky subtraction with spectra of emission line galaxies 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 Guy et
al. (2022), in preparation.

Figure 31: 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 30 November 2021.

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 §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 32 shows a collection of resolution measurements for a subset of fibers
in all ten spectrographs from January 2022. 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.

Figure 32: Spectral resolution as a function of wavelength for all ten
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 correspond to all measurements from
January 2022 for all ten spectrographs and a subset of fibers. The resolution
exceeds the requirement 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.

###  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 §8.5
and summarized in Figure 20. Our requirement is that the inter-exposure
sequence be less than two 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 Declination 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 pre-established guide stars for every
field, and we have pre-established 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 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 slews, 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 fiducuals, 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 118s 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 to 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 which 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). As of January 2022, 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. 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.

##  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 expertise that was
developed by the earlier Stage III and II dark energy projects, most notably
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
requirements. 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. 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. 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 underscored 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 vendors 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 ten petals, the involvement of a
production engineer in the fiber system work starting with the transition from
the research and development 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 instrument control system 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, inventory tracking, and precision
tooling for assembly. We would have benefited from more resources for the
quality assurance/quality control of the positioner production as early as the
development and planning stage. We would also have benefited from a large-
scale, pre-production 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 underestimated 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 pre-commissioning
activities with ProtoDESI and the Commissioning Instrument 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 §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

The Dark Energy Spectroscopic Instrument (DESI) is the largest multi-object
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 five years, 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 Dark Energy Task Force.

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 §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
$3.2^{\circ}$ diameter field of view. Four of the lenses are constructed from
fused silica, two of which have an aspheric surface, and the remaining two
form an atmospheric dispersion corrector constructed from borosilicate glass.
The lenses are approximately a meter in diameter and are coated with extremely
high-performance, broadband anti-reflection 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
$<0.4^{\prime\prime}$ 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 twelve 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
$20\,\mu$m for lateral alignment and $20\,\mu$rad 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 is 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 lookup
table and the analysis of intra- and extrafocal 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 ten identical, $36^{\circ}$ wedge-shaped petals that place the
fiber tips on the aspheric focal surface of the corrector. Each of the ten
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 are the two 4 mm diameter brushless DC motors in each
fiber positioner. Placing the fiber tips on astronomical targets with an
accuracy of $\sim 10\,\mu$m ($0.2^{\prime\prime}$) 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 ten
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 volume phase holographic
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
$<0.1^{\circ}$C provided by the environmental enclosure, and it is well
determined due to the careful design of the calibration lamp system and dome
screen.

Instrument control system (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 ten GFA
cameras, movement of all fiber positioners, operation of the 123 illuminated
fiducials, activation of the 20 shutters and their pneumatic seals,
illumination of ten 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 quality assurance
(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-realtime 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 declination axes, new servo controllers for the
telescope drives, and a new control computer. These changes led to
improvements in the pointing from $10-20^{\prime\prime}$ to
$2-3^{\prime\prime}$ rms and an improvement in the open loop tracking errors
from $30^{\prime\prime}$/hour to $1.6^{\prime\prime}$/hour in RA and
$0.1^{\prime\prime}$/hour in declination.

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 February 2018 through September
2019 (see §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 are described in
§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 §14 and are intended to be a resource for
other, similar projects. After commissioning, DESI started Survey Validation
on 14 December 2020. 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 14 May 2021 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-
Gaussianty 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 (MICINN), and by the DESI Member
Institutions: https://www.desi.lbl.gov/collaborating-institutions. 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.

## Data Availability

All data points shown in the published graphs are available in a machine-
readable form on the following website: https://zenodo.org/record/6397426.

Mayall (DESI)

## References

  * Abareshi et al. (2018) Abareshi, B., Williams, D., & Marshall, R. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10707, Software and Cyberinfrastructure for Astronomy V, ed. J. C. Guzman & J. Ibsen, 107071J, doi: 10.1117/12.2313194
  * Abareshi et al. (2016) Abareshi, B., Marshall, R., Gott, S., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9913, Software and Cyberinfrastructure for Astronomy IV, ed. G. Chiozzi & J. C. Guzman, 99131O, doi: 10.1117/12.2233087
  * Ahumada et al. (2020) Ahumada, R., Allende Prieto, C., Almeida, A., et al. 2020, ApJS, 249, 3, doi: 10.3847/1538-4365/ab929e
  * Alam et al. (2017) Alam, S., Ata, M., Bailey, S., et al. 2017, MNRAS, 470, 2617, doi: 10.1093/mnras/stx721
  * Alam et al. (2021) Alam, S., Aubert, M., Avila, S., et al. 2021, Phys. Rev. D, 103, 083533, doi: 10.1103/PhysRevD.103.083533
  * Albrecht et al. (2006) Albrecht, A., Bernstein, G., Cahn, R., et al. 2006, arXiv e-prints, astro.  https://arxiv.org/abs/astro-ph/0609591
  * Allende Prieto et al. (2020) Allende Prieto, C., Cooper, A. P., Dey, A., et al. 2020, Research Notes of the American Astronomical Society, 4, 188, doi: 10.3847/2515-5172/abc1dc
  * Baltay et al. (2019) Baltay, C., Rabinowitz, D., Besuner, R., et al. 2019, PASP, 131, 065001, doi: 10.1088/1538-3873/ab15c2
  * Bautista et al. (2017) Bautista, J. E., Busca, N. G., Guy, J., et al. 2017, A&A, 603, A12, doi: 10.1051/0004-6361/201730533
  * Bebek et al. (2015) Bebek, C. J., Emes, J. H., Groom, D. E., et al. 2015, Journal of Instrumentation, 10, C05026, doi: 10.1088/1748-0221/10/05/C05026
  * Besuner et al. (2020) Besuner, R., Allen, L., Baltay, C., et al. 2020, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 11447, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 1144710, doi: 10.1117/12.2561507
  * Blake et al. (2011) Blake, C., Kazin, E. A., Beutler, F., et al. 2011, MNRAS, 418, 1707, doi: 10.1111/j.1365-2966.2011.19592.x
  * Bolton & Schlegel (2010) Bolton, A. S., & Schlegel, D. J. 2010, PASP, 122, 248, doi: 10.1086/651008
  * Bond & Efstathiou (1984) Bond, J. R., & Efstathiou, G. 1984, ApJ, 285, L45, doi: 10.1086/184362
  * Coker et al. (2016) Coker, C. T., Pappalardo, D., Pogge, R., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9908, Ground-based and Airborne Instrumentation for Astronomy VI, ed. C. J. Evans, L. Simard, & H. Takami, 99087P, doi: 10.1117/12.2231117
  * Cole et al. (2005) Cole, S., Percival, W. J., Peacock, J. A., et al. 2005, MNRAS, 362, 505, doi: 10.1111/j.1365-2966.2005.09318.x
  * Coles et al. (2018) Coles, R. A., Brooks, D., Derwent, M., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 107061L, doi: 10.1117/12.2312592
  * Dawson et al. (2016) Dawson, K. S., Kneib, J.-P., Percival, W. J., et al. 2016, AJ, 151, 44, doi: 10.3847/0004-6256/151/2/44
  * de Mattia et al. (2020) de Mattia, A., Ruhlmann-Kleider, V., Raichoor, A., et al. 2020, arXiv e-prints, arXiv:2007.09008.  https://arxiv.org/abs/2007.09008
  * Derwent et al. (2016) Derwent, M. A., O’Brien, T. P., Pappalardo, D. P., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9908, Ground-based and Airborne Instrumentation for Astronomy VI, 99087T, doi: 10.1117/12.2231483
  * DESI Collaboration et al. (2016a) DESI Collaboration, Aghamousa, A., Aguilar, J., et al. 2016a, arXiv e-prints, arXiv:1611.00036.  https://arxiv.org/abs/1611.00036
  * DESI Collaboration et al. (2016b) —. 2016b, arXiv e-prints, arXiv:1611.00037.  https://arxiv.org/abs/1611.00037
  * Dey & Valdes (2014) Dey, A., & Valdes, F. 2014, PASP, 126, 296, doi: 10.1086/675808
  * Dey et al. (2016) Dey, A., Rabinowitz, D., Karcher, A., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9908, Ground-based and Airborne Instrumentation for Astronomy VI, ed. C. J. Evans, L. Simard, & H. Takami, 99082C, doi: 10.1117/12.2231488
  * Dey et al. (2019) Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168, doi: 10.3847/1538-3881/ab089d
  * Diehl & Dark Energy Survey Collaboration (2015) Diehl, H. T., & Dark Energy Survey Collaboration. 2015, in APS Meeting Abstracts, Vol. 2015, APS April Meeting Abstracts, Y2.001
  * Doel et al. (2016) Doel, P., Besuner, R., Brooks, D., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9908, Ground-based and Airborne Instrumentation for Astronomy VI, ed. C. J. Evans, L. Simard, & H. Takami, 99088D, doi: 10.1117/12.2232493
  * du Mas des Bourboux et al. (2017) du Mas des Bourboux, H., Le Goff, J.-M., Blomqvist, M., et al. 2017, A&A, 608, A130, doi: 10.1051/0004-6361/201731731
  * Duan et al. (2018) Duan, Y., Silber, J. H., Claybaugh, T. M., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, ed. R. Navarro & R. Geyl, 1070643, doi: 10.1117/12.2309875
  * Edelstein et al. (2018) Edelstein, J., Jelinsky, P., Levi, M., Tarle, G., & Brooks, D. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10702, Proc. SPIE, 107027G, doi: 10.1117/12.2311821
  * Eisenstein et al. (2005) Eisenstein, D. J., Zehavi, I., Hogg, D. W., et al. 2005, ApJ, 633, 560, doi: 10.1086/466512
  * Fagrelius et al. (2016) Fagrelius, P., Poppett, C., & Edelstein, J. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9908, Proc. SPIE, 99087Q, doi: 10.1117/12.2231331
  * Fagrelius et al. (2018) Fagrelius, P., Abareshi, B., Allen, L., et al. 2018, PASP, 130, 025005, doi: 10.1088/1538-3873/aaa225
  * Fagrelius et al. (2020) Fagrelius, P., Duan, Y., Fanning, K., et al. 2020, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 11447, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 114478K, doi: 10.1117/12.2561631
  * Flaugher et al. (2015) Flaugher, B., Diehl, H. T., Honscheid, K., et al. 2015, AJ, 150, 150, doi: 10.1088/0004-6256/150/5/150
  * Gaia Collaboration et al. (2016) Gaia Collaboration, Prusti, T., de Bruijne, J. H. J., et al. 2016, A&A, 595, A1, doi: 10.1051/0004-6361/201629272
  * Gaia Collaboration et al. (2018) Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018, A&A, 616, A1, doi: 10.1051/0004-6361/201833051
  * Gunn et al. (2006) Gunn, J. E., Siegmund, W. A., Mannery, E. J., et al. 2006, AJ, 131, 2332, doi: 10.1086/500975
  * Hardin et al. (2001) Hardin, R. H., Sloane, N. J. A., & Smith, W. D. 2001, Tables of spherical codes with icosahedral symmetry.  http://NeilSloane.com/icosahedral.codes/
  * Hinshaw et al. (2013) Hinshaw, G., Larson, D., Komatsu, E., et al. 2013, ApJS, 208, 19, doi: 10.1088/0067-0049/208/2/19
  * Hinton et al. (2017) Hinton, S. R., Kazin, E., Davis, T. M., et al. 2017, MNRAS, 464, 4807, doi: 10.1093/mnras/stw2725
  * Holland et al. (2014) Holland, S. E., Bebek, C. J., Dion, F., et al. 2014, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9154, High Energy, Optical, and Infrared Detectors for Astronomy VI, ed. A. D. Holland & J. Beletic, 91541E, doi: 10.1117/12.2057219
  * Honscheid et al. (2012) Honscheid, K., Elliott, A., Annis, J., et al. 2012, Proc. SPIE, 8451, 845112, doi: 10.1117/12.925717
  * Honscheid et al. (2014) Honscheid, K., Elliott, A., Bonati, M., et al. 2014, Proc. SPIE, 9152, 91520G, doi: 10.1117/12.2057073
  * Honscheid et al. (2016) Honscheid, K., Elliott, A. E., Beaufore, L., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9913, Software and Cyberinfrastructure for Astronomy IV, ed. G. Chiozzi & J. C. Guzman, 99130P, doi: 10.1117/12.2229835
  * Honscheid et al. (2018) Honscheid, K., Elliott, A. E., Buckley-Geer, E., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10707, Proc. SPIE, 107071D, doi: 10.1117/12.2311927
  * Howlett et al. (2015) Howlett, C., Ross, A. J., Samushia, L., Percival, W. J., & Manera, M. 2015, MNRAS, 449, 848, doi: 10.1093/mnras/stu2693
  * Ishikawa et al. (2018) Ishikawa, Y., Sirk, M. M., Edelstein, J., et al. 2018, ApJ, 869, 24, doi: 10.3847/1538-4357/aaebfb
  * Jones et al. (2009) Jones, D. H., Read, M. A., Saunders, W., et al. 2009, MNRAS, 399, 683, doi: 10.1111/j.1365-2966.2009.15338.x
  * Kaiser (1987) Kaiser, N. 1987, MNRAS, 227, 1, doi: 10.1093/mnras/227.1.1
  * Kazin et al. (2014) Kazin, E. A., Koda, J., Blake, C., et al. 2014, MNRAS, 441, 3524, doi: 10.1093/mnras/stu778
  * Kennemore et al. (2018) Kennemore, C., Archer, D., Barbaria, R., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 107061S, doi: 10.1117/12.2313006
  * Kent et al. (2016) Kent, S., Lampton, M., Doel, A. P., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9908, Ground-based and Airborne Instrumentation for Astronomy VI, ed. C. J. Evans, L. Simard, & H. Takami, 99088F, doi: 10.1117/12.2232689
  * Kent (2018) Kent, S. M. 2018, PASP, 130, 044501, doi: 10.1088/1538-3873/aaa6b8
  * Kremin et al. (2020) Kremin, A., Bailey, S., Guy, J., Kisner, T., & Zhang, K. 2020, in IEEE/ACM HPC for Urgent Decision Making, UrgentHPC@SC 2020, Atlanta, GA, USA, November 13, 2020 (IEEE), 1–9, doi: 10.1109/UrgentHPC51945.2020.00006
  * Leitner et al. (2018) Leitner, D., Aguilar, J., Ameel, J., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 1070669, doi: 10.1117/12.2312228
  * Levi et al. (2013) Levi, M., Bebek, C., Beers, T., et al. 2013, arXiv e-prints, arXiv:1308.0847.  https://arxiv.org/abs/1308.0847
  * Margala et al. (2021) Margala, D., Stephey, L., Thomas, R., & Bailey, S. 2021, in Proceedings of the 20th Python in Science Conference, ed. Meghann Agarwal, Chris Calloway, Dillon Niederhut, & David Shupe, 33 – 39, doi: 10.25080/majora-1b6fd038-004
  * Martini et al. (2014) Martini, P., Elias, J., Points, S., et al. 2014, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9147, Ground-based and Airborne Instrumentation for Astronomy V, 91470Z, doi: 10.1117/12.2056834
  * Meisner et al. (2020) Meisner, A. M., Abareshi, B., Dey, A., et al. 2020, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 11447, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 1144794, doi: 10.1117/12.2574776
  * Miller et al. (2018) Miller, T. N., Besuner, R. W., Levi, M. E., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10706, Proc. SPIE, 107060X, doi: 10.1117/12.2312949
  * O’Brien et al. (2014) O’Brien, T. P., Derwent, M., Martini, P., & Poczulp, G. 2014, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9151, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 915141, doi: 10.1117/12.2053594
  * Peebles & Yu (1970) Peebles, P. J. E., & Yu, J. T. 1970, ApJ, 162, 815, doi: 10.1086/150713
  * Perruchot et al. (2020) Perruchot, S., Blanc, P.-É., Guy, J., et al. 2020, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 11447, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 1144786, doi: 10.1117/12.2561275
  * Planck Collaboration et al. (2016) Planck Collaboration, Ade, P. A. R., Aghanim, N., et al. 2016, A&A, 594, A13, doi: 10.1051/0004-6361/201525830
  * Poppett et al. (2018) Poppett, C., Sharples, R., Edelstein, J., Schmoll, J., & Bramall, D. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10702, Proc. SPIE, 107027O, doi: 10.1117/12.2312176
  * Poppett et al. (2020) Poppett, C., Jelinsky, P., Guy, J., et al. 2020, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 11447, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 1144711, doi: 10.1117/12.2562565
  * Raichoor et al. (2020) Raichoor, A., Eisenstein, D. J., Karim, T., et al. 2020, Research Notes of the American Astronomical Society, 4, 180, doi: 10.3847/2515-5172/abc078
  * Roodman et al. (2014) Roodman, A., Reil, K., & Davis, C. 2014, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9145, Ground-based and Airborne Telescopes V, 914516, doi: 10.1117/12.2056904
  * Ross et al. (2018) Ross, A. J., Martini, P., Coles, R., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10702, Proc. SPIE, 1070280, doi: 10.1117/12.2312885
  * Ruiz-Macias et al. (2020) Ruiz-Macias, O., Zarrouk, P., Cole, S., et al. 2020, Research Notes of the American Astronomical Society, 4, 187, doi: 10.3847/2515-5172/abc25a
  * Sawyer et al. (2010) Sawyer, D. G., Daly, P. N., Howell, S. B., Hunten, M. R., & Schweiker, H. 2010, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 7735, Ground-based and Airborne Instrumentation for Astronomy III, ed. I. S. McLean, S. K. Ramsay, & H. Takami, 77353A, doi: 10.1117/12.857619
  * Schmoll et al. (2018) Schmoll, J., Besuner, R., Bramall, D., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10702, Ground-based and Airborne Instrumentation for Astronomy VII, ed. C. J. Evans, L. Simard, & H. Takami, 107027N, doi: 10.1117/12.2312140
  * Schubnell et al. (2016) Schubnell, M., Ameel, J., Besuner, R. W., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9908, Ground-based and Airborne Instrumentation for Astronomy VI, 990892, doi: 10.1117/12.2233370
  * Sholl et al. (2012) Sholl, M. J., Ackerman, M. R., Bebek, C., et al. 2012, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 8446, Ground-based and Airborne Instrumentation for Astronomy IV, ed. I. S. McLean, S. K. Ramsay, & H. Takami, 844667, doi: 10.1117/12.926179
  * Shourt et al. (2020) Shourt, W., Besuner, R., Silber, J., et al. 2020, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 11445, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 114456J, doi: 10.1117/12.2562687
  * Smee et al. (2013) Smee, S. A., Gunn, J. E., Uomoto, A., et al. 2013, AJ, 146, 32, doi: 10.1088/0004-6256/146/2/32
  * Sprayberry et al. (2014) Sprayberry, D., Goble, W., Allen, L., et al. 2014, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9145, Ground-based and Airborne Telescopes V, 91453Y, doi: 10.1117/12.2055625
  * Sprayberry et al. (2016) Sprayberry, D., Dunlop, P., Evatt, M., et al. 2016, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9906, Ground-based and Airborne Telescopes VI, 990608, doi: 10.1117/12.2233177
  * Sunyaev & Zeldovich (1970) Sunyaev, R. A., & Zeldovich, Y. B. 1970, Ap&SS, 7, 3, doi: 10.1007/BF00653471
  * Tamone et al. (2020) Tamone, A., Raichoor, A., Zhao, C., et al. 2020, MNRAS, 499, 5527, doi: 10.1093/mnras/staa3050
  * Tie et al. (2020) Tie, S. S., Kirkby, D., Martini, P., et al. 2020, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 11447, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 1144785, doi: 10.1117/12.2561436
  * Tyas et al. (2018) Tyas, L., Besuner, R., Bramall, D., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10702, Ground-based and Airborne Instrumentation for Astronomy VII, ed. C. J. Evans, L. Simard, & H. Takami, 107027L, doi: 10.1117/12.2312034
  * Warner et al. (2012) Warner, M., Cantarutti, R., Schumacher, G., et al. 2012, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 8451, Software and Cyberinfrastructure for Astronomy II, 84510U, doi: 10.1117/12.926193
  * Wright et al. (2010) Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 1868, doi: 10.1088/0004-6256/140/6/1868
  * Yèche et al. (2020) Yèche, C., Palanque-Delabrouille, N., Claveau, C.-A., et al. 2020, Research Notes of the American Astronomical Society, 4, 179, doi: 10.3847/2515-5172/abc01a
  * Zhou et al. (2020) Zhou, R., Newman, J. A., Dawson, K. S., et al. 2020, Research Notes of the American Astronomical Society, 4, 181, doi: 10.3847/2515-5172/abc0f4