arXiv:1907.10688v1 [astro-ph.IM] 24 Jul 2019
Astro2020 APC White Paper
The Dark Energy Spectroscopic Instrument
(DESI)
Thematic Areas:
Planetary Systems Star and Planet Formation
✓Cosmology and Fundamental Physics
Formation and Evolution of Compact Objects
✓Stars and Stellar Evolution
✓Resolved Stellar Populations and their Environments
✓Galaxy Evolution
Multi-Messenger Astronomy and Astrophysics
Principal Authors:
Michael E. Levi (Lawrence Berkeley National Laboratory)
& Lori E. Allen (National Optical Astronomy Observatory)
Email:
[email protected],
[email protected]
Co-authors: Anand Raichoor (EPFL, Switzerland), Charles Baltay (Yale University), Segev
BenZvi (University of Rochester), Florian Beutler (University of Portsmouth, UK), Adam Bolton
(NOAO), Francisco J. Castander (IEEC, Spain), Chia-Hsun Chuang (KIPAC), Andrew Cooper
(National Tsing Hua University, Taiwan), Jean-Gabriel Cuby (Aix-Marseille University, France),
Arjun Dey (NOAO), Daniel Eisenstein (Harvard University), Xiaohui Fan (University of
Arizona), Brenna Flaugher (FNAL), Carlos Frenk (Durham University, UK), Alma X.
González-Morales (Universidad de Guanajuato, México), Or Graur (CfA), Julien Guy (LBNL),
Salman Habib (ANL), Klaus Honscheid (Ohio State University), Stephanie Juneau (NOAO),
Jean-Paul Kneib (EPFL, Switzerland), Ofer Lahav (UCL, UK), Dustin Lang (Perimter Institute,
Canada), Alexie Leauthaud (UC Santa Cruz), Betta Lusso (Durham University, UK), Axel de la
Macorra (UNAM, Mexico), Marc Manera (IFAE, Spain), Paul Martini (Ohio State University),
Shude Mao (Tsinghua University, China), Jeffrey A. Newman (University of Pittsburgh), Nathalie
Palanque-Delabrouille (CEA, France), Will J. Percival (University of Waterloo, Canada), Carlos
Allende Prieto (IAC, Spain), Constance M. Rockosi (UC Santa Cruz), Vanina Ruhlmann-Kleider
(CEA, France), David Schlegel (LBNL), Hee-Jong Seo (Ohio University), Yong-Seon Song
(KASI, South Korea), Greg Tarlé (University of Michigan), Risa Wechsler (Stanford University),
David Weinberg (Ohio State University), (Christophe Yèche (CEA, France), Ying Zu (Shanghai
Jiao Tong University, China)
Abstract: We present the status of the Dark Energy Spectroscopic Instrument (DESI) and its
plans and opportunities for the coming decade. DESI construction and its initial five years of
operations are an approved experiment of the U.S. Department of Energy and is summarized here
as context for the Astro2020 panel. Beyond 2025, DESI will require new funding to continue
operations. We expect that DESI will remain one of the world’s best facilities for wide-field
spectroscopy throughout the decade. More about the DESI instrument and survey can be found at
https://www.desi.lbl.gov.
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1 An Overview of DESI: 2020-2025
DESI is an ambitious multi-fiber optical spectrograph sited on the Kitt Peak National Observatory
Mayall 4m telescope, funded to conduct a Stage IV spectroscopic dark energy experiment. DESI
featuers 5000 robotically positioned fibers in an 8 deg2 focal plane, feeding a bank of 10
triple-arm spectrographs that measure the full bandpass from 360 nm to 980 nm at spectral
resolution of 2000 in the UV and over 4000 in the red and IR (Martini et al. 2018). DESI is
designed for efficient operations and exceptionally high throughput, anticipated to peak at over
50% from the top of the atmosphere to detected photons, not counting obscuration of the
telescope or aperture loss from the 1.5′′ diameter fibers. More information is in Table 1.
As of this writing in July 2019, DESI construction is nearly complete and the instrument is being
installed at the Mayall telescope. The new prime-focus corrector was operated on sky in
April/May 2019 and confirmed to produce sharp images. All ten petals of the robotic positioners
and all fibers have been constructed; these are being installed on the telescope in July (Figure 1).
Six of the ten spectrographs are installed and entering off-sky commissioning (Figure 2 and 3);
the other four should arrive in fall 2019. We anticipate spectroscopic first-light in October 2019,
with commissioning running through January 2020. The collaboration will then operate a
4-month Survey Validation program in spring 2020 and begin the 5-year survey in summer 2020.
Key Science Goals: The DESI Collaboration will use this facility to conduct a 5-year survey of
galaxies and quasars, covering 14,000 deg2 and yielding 34 million redshifts. The mission-need
science of this survey is the study of dark energy through the measurement of the cosmic distance
scale with the baryon acoustic peak method as a standard ruler and through the study of the
growth of structure with redshift-space distortions. The survey will further allow measurement of
other cosmological quantities, such as neutrino mass and primordial non-Gaussianity, as well as
studies of galaxies, quasars, and stars.
The DESI survey uses a sequence of target classes to map the large-scale structure of the Universe
from redshift 0 to 3.5 (Aghamousa et al. 2016). In dark and grey time, DESI will utilize quasars,
emission-line galaxies, and luminous red galaxies. Over 4M luminous red galaxy sample will
cover 0.3 < z < 1, including coverage to z ∼ 0.8 at a density twice that of SDSS-III BOSS. The
emission-line galaxy sample is the largest set, 18M, covering 0.6 < z < 1.6 and providing the
majority of the distance scale precision. 2.4M quasars selected from their WISE excess will
extend the map. Importantly, these will yield Lyman α forest measurements along 600K
lines-of-sight, from which we will measure the acoustic oscillations at z > 2.
In bright time, DESI will conduct a flux-limited survey of 10M galaxies to r ≈ 19.5, with a
median redshift around 0.2. This will allow dense sampling of a volume over 10 times that of the
SDSS MAIN and 2dF GRS surveys, which we expect will spur development of cosmological
probes of the non-linear regime of structure formation.
In addition to extragalactic targets, DESI will observe many millions of stars. About 10M stars at
16 < G < 19 will fill unused fibers in the bright time program, and we will conduct a backup
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Location
Coordinates
Elevation
Primary mirror
Number of fibers
Field of view
Corrector design
Corrected focal ratio
Fiber density on-sky
Focal-plane scale
Fiber core diameter
Fiber pitch
Fiber patrol region
Repositioning time
Positioning accuracy
Guiding
Spectrographs
Wavelength range
Spectral resolution
Detectors
Read + dark noise
End-to-end
throughput
Survey area
Survey duration
Survey strategy
Survey samples
(baseline)
Nicholas U. Mayall Telescope
Kitt Peak National Observatory, Tohono O’odham Nation, Arizona
31°57ʹ48ʺN, 111°36ʹ00ʺW
2,120 m
3.8m diameter, f/2.8, 1.8m central DESI obscuration
Dark Energy Spectroscopic Instrument (DESI)
5,000
8.0 deg2 (corrected), 7.5 deg2 (populated)
4 lenses + 2 ADC elements
3.68 (on-axis), 3.86 (average over FOV)
667 / deg2
67.5 µm/arcsec (on axis); 70.8 µm/arcsec (average over FOV)
107 µm
10.4 mm, hexagonal close pack in 10 focal-plane wedges (“petals”)
Circular, 12 mm diameter
< 45 s
2 µm RMS
10 guide-focus-alignment cameras, 1 per petal, 29 arcmin2 each
10 x 3-arm spectrographs, 500 fibers each
360–980 nm
360–593 nm (Blue Channel): 2,000–3,200
566–772 nm (Red Channel): 3,200–4,100
747–980 nm (NIR Channel): 4,100–5,100
30 CCDs, 4,096 x 4,096 pixels, 15 µm pitch
< 4 electrons
43.0% @ 450nm, 45.4% @ 650nm, 50.5% @ 850nm
(estimated, telescope x fiber system x spectrographs x detectors)
DESI Surveys
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14,000 deg fully covered by 2,000 pointings (“tiles”)
5 years: CY2020–CY2025
Key Project (dark time): 10,000 tiles in 5 layers of 2,000 tiles each
Bright Time Survey: 6,000 in 3 layers of 2,000 tiles each
4.0 million Luminous Red Galaxies, z = 0.4 – 1.0 (dark time)
17.1 million Emission Line Galaxies, z = 0.6 – 1.6 (dark time)
1.7 million tracer QSOs, z < 2.1 (dark time)
0.7 million Ly-α QSOs, z > 2.1 (dark time)
9.8 million bright galaxies, z = 0.05 – 0.4 (bright time)
10 million Milky Way stars (bright time)
Table 1: DESI at a glance.
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program of brighter stars when observing conditions (clouds, moon, and/or seeing) prevent useful
data from being collected on extragalactic targets. Because DESI is a bench-mounted
spectrograph with sub-degree temperature stability, we anticipate velocity precision to ∼ 1 km/s.
The DESI Collaboration plans for release of annual data sets, including survey selection functions
and mock catalogs suitable for clustering analyses, following completion of its cosmology key
projects.
Target Selection: In preparation of target selection for DESI, the Collaboration has played a
leading role in the execution of the Legacy Survey imaging program, using nearly 1000 nights on
the Blanco, Mayall, and Bok telescopes to image 15,000 deg2 to g = 24, r = 23.4, and z = 22.5
depth, co-reduced with 5 years of WISE satellite imaging (Dey et al. 2019). This is the deepest
coverage of the full high-latitude sky in the Northern hemisphere (Figure 4). The imaging data
and catalogs have had 8 data releases, available at http://legacysurvey.org, the last of
which reaches over 19,000 deg2 by inclusion of the 5-year Dark Energy Survey. Hence, DESI has
already provided an extensive data product for the general astronomical community.
As regards this first five-year survey with DESI, we stress the opportunity of this U.S.-led project
Figure 1: The first of 10 focal plate petals installed at the Mayall prime focus (June 26, 2019).
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to conduct cutting-edge dark energy research, both in its own right and in coordination with
optical, millimeter, and X-ray imaging data sets. As well illustrated by the SDSS, the
combination of spectroscopy and imaging unlocks a wide range of applications.
Organization: DESI is being built by the DESI Collaboration with primary funding from the
U.S. Department of Energy, and additional funding from the National Science Foundation, the
Science and Technologies Facilities Council of the United Kingdom, the Gordon and Betty
Moore Foundation, the Heising-Simons Foundation, the National Council of Science and
Technology of Mexico, the French Alternative Energies and Atomic Energy Commission (CEA),
and by the DESI Member Institutions. The DESI Collaboration currently has more than 600 total
members from 79 institutions from 13 countries around the world. Of those, ∼200 are senior
members, and ∼400 are early career scientists.
With DESI, the Mayall telescope is dedicated to this single instrument configuration (unlike
DECam on the Blanco, which could also mount a secondary mirror). The DOE Office of Science
High-Energy Physics division will be the primary funder of the DESI survey, including the
operation of the Mayall telescope through the 5-year survey. The survey will utilize at least the
darkest 21 nights per lunation, plus engineering time, and potentially may use all of the telescope
Figure 2: Six of the ten 3-armed DESI spectrographs, installed in their thermal enclosure.
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time.
Cost: The DESI construction project has cost $75M, with $56M from the DOE and the balance
by other partners and institutional buy-ins. Survey operations are budgeted at ∼ $12M/year, split
approximately one-third site operations, and the rest supporting the instrument, the survey
planning, and data processing and catalog creation.
Figure 3: Measured resolution of the six installed spectrographs. Dotted lines are the system
requirements. The as-built results match exquisitely to the modeled performance.
Figure 4: The planned DESI survey footprint is shown by the shaded area; it is built on several
existing imaging surveys and extends as far south as δ = −20◦ in the SGC and −10◦ in the NGC.
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2 Beyond 2025 with DESI
Beyond the end of the first 5-year survey, DESI will remain a state-of-the-art facility for
wide-field surveys. New commitments for funding will be required. But given the time scale to
construct a facility more powerful than DESI and with no such project yet approved1 we expect
that the ground-based facility landscape in the second half of the 2020’s will look much like the
first. See Table 2 for a summary. We note that not only is DESI at the forefront of this generation,
without it, the U.S. community will not have a facility to compete with ESO and Subaru. Further,
we note that while Euclid and WFIRST will offer space-based platforms for slitless IR
spectroscopy, optical spectroscopy remains a highly efficient way to get redshifts both at z < 1.5
and z > 2.
Key Science Goals: We anticipate that a second phase of DESI will continue to offer exciting
survey opportunities. Certainly we will not have exhausted the supply of plausible targets on the
sky. Imaging surveys from HSC, LSST, Euclid, SphereX, WFIRST, eROSITA, and others will
yield improved isolation of valuable targets over areas of thousands to tens of thousands of deg2 .
Spectroscopy can provide the key leverage to realize the science potential of these candidates,
whether for redshifts or for more detailed characterization. DESI’s combination of field of view,
multiplex, throughput, and resolution makes it a great complement to the coming generation of
imaging surveys.
There are at least 5 fertile areas of potential targets for such a survey:
1) High-redshift emission-line targets: improved selection of 1 < z < 1.6 emission-line galaxies
1
We note that DESI (under the previous name BigBOSS) was identified in New Worlds New Horizons as an
exemplar of the MidScale Innovation Program. Despite timely agency support (CD-0 approved in 2012 and CD-2 in
2015), non-federal funding to conduct long-lead procurements, and no major programmatic or technical interruptions,
we will be in operations in 2020. We believe this is indicative of what projects of this complexity require, even in good
outcomes!
Name
Telescope
# Fibers
DESI
Mayall 4-m
5000
PFS
Subaru 8-m
2400
4MOST
VISTA 4-m
2436
WEAVE
WHT 4-m
960
SDSS-V Sloan & DuPont 2.5-m
1000
FOV (deg2 )
8
1.5
5
3
7
Bandpass (nm)
360–980
380–1260
370–950
370–960
360–1700
Resolution
mid
mid
mid & high
mid & high
mid (opt) & high (IR)
Table 2: A brief comparison of multi-fiber facilities under construction. Mid-resolution is typically
a few thousand; high-resolution is typically around 20K, but for a more limited bandpass. DESI
will offer the highest multiplex and largest field of view of these next-generation facilities; only
PFS has more instantaneous light-gathering power, but it is not a dedicated platform. Of the current
generation of facilities, LAMOST is operating 4000 fibers in a 20 deg2 field of view, but with
performance limited to bright galaxies and stars.
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from deeper imaging; selection of Lyα emission candidates from deep imaging in the blue; or
follow-up of low-quality emission-line candidates from Euclid and SPHEREx. Such a survey
would increase the sampling of the large volume available at higher redshift.
2) Increased depth and sampling in the Lyman-α forest, reobserving known targets and adding
fainter candidates from deeper imaging.
3) A high-density galaxy survey at z < 1, building on the DESI bright galaxy survey. These
candidates are readily identified, but a high-density sample with precise spectroscopic redshifts
would allow identification of groups and redshift-space distortions in the non-linear regime within
the cosmic acceleration epoch.
4) A high-multiplex survey of the Milky Way, with O(100) million stars, to yield radial velocities
and stellar abundances to pair with the exquisite Gaia astrometry. The rapid reconfiguration time
of DESI (< 2 minutes) makes short exposures an effective strategy.
5) Time-domain spectroscopy and transient host spectroscopy, building on SDSS-V and
time-domain imaging surveys such as ZTF, LSST, and TESS.
Technical Drivers: While the DESI instrument could continue to be usefully operated in the
same configuration as the pre-2025 phase, there may be opportunities for augmentations. Notably,
the spectrographs are modular and could be altered or replaced, subject to cost and space
constraints, if the adopted science goals called for it.
Organization, Status, and Schedule: A science collaboration for post-2025 operations has not
yet been formed, but we expect that many of the current participants would be interested in
continuing. The Mayall telescope remains property of the National Science Foundation, while the
DESI equipment is DOE property.
We expect that planning for such surveys will pick up speed in 2021 with the arrival of early DESI
data, which will solidify the on-sky performance and give a tactile sense of the target selections.
We note that there has been mention of the idea of moving DESI south to the Blanco in 2025. On
the plus side, such a move would increase sky overlap with LSST. On the down side, it is an
expensive proposition: it has taken over 1.5 years to install DESI at the Mayall, and much of that
work would need to re-occur. We therefore expect that a move would result in substantial
downtime for both telescopes, along with financial cost. Such a decision will require a detailed
cost-benefit analysis. Given the large amount of near-equatorial sky visible jointly from Arizona
and Chile, we suspect that many post-2025 survey options could be well performed without a
move, potentially in collaboration with an instrument of similar or even lesser capability in the
south.
Cost Estimate: The budget for a second phase of DESI operations would depend on the survey
choices made as well as on the assessment of costs of ongoing instrument support, presumably
informed by experience in the coming years of operations. However, the cost is likely
O($10 − 15)M/year (inclusive), comparable to those of other mid-scale facilities that deliver
highly processed data products. Hence, we expect ongoing operations to fall in the Medium class
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of ground-based activities.
In conclusion, we expect that the Mayall telescope with DESI will remain a world-class facility
for high-multiplex optical mid-resolution spectroscopy in the latter half of this decade, offering
the U.S. the opportunity to continue its leadership in spectroscopic wide-field surveys.
References
Dey, A., et al., ”Overview of the DESI Legacy Imaging Surveys,” ApJ, 157, 168 (2019)
Martini, P., et al., “Overview of the Dark Energy Spectroscopic Instrument,” SPIE, 107021F,
(2018).
DESI Collaboration, Aghamousa, A., et al., “The DESI Experiment Part I: Science,Targeting, and
Survey Design,” arXiv:1611.00036 (2016)
DESI Collaboration, Aghamousa, A., et al., “The DESI Experiment Part II: Instrument Design,”
arXiv:1611.00037 (2016)
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