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The Dark Energy Survey (DES)

Douglas L. Tucker (Fermilab)

DES Calibrations Scientist

Southern Connecticut State University, 12 March 2010

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Part 0: Some Background

The Universe is Expanding

The distance between galaxies increases with time

The wavelength of light grows with time at the same rate

(In this 2D representation, imagine yourself at the North Pole of this expanding globe…)

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Part 0: Some Background

Cosmological Redshift (z)… is the redshift caused by the expansion of the Universe:

R0 / R = (1+z)

where R0 is the “size” of the Universe now, and R is the “size” of the Universe at redshift z.

It can be measured by the shift in wavelength of lines in the spectra of distant galaxies:

λ / λ0 = (1+z)

where λ0 is the rest-frame wavelength and λ is the observed wavelength for redshift z.

Z=0.004

Z=0.05

Z=0.07

Z=0.13

Z=0.20

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Part I: What is Dark Energy?

To begin to answer this question, we must first consider what makes up the total mass-energy density of the Universe...

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What is the Universe Composed of?

• “Normal Matter” (protons, neutrons, electrons, etc...)• Luminous

NGC 2403

~20 kp

c

SDSS (Optical)

~250 kp

c

Fornax Cluster

Chandra (X-ray)

[ 1 parsec = 3.26 light-year, and 1000 parsecs = 1 kiloparsec (1 kpc) ]

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Horsehead Nebula

Credit: NOAO/AURA/NSF

What is the Universe Composed of?

• “Normal Matter” (protons, neutrons, electrons, etc...)• Non-luminous

(Baryonic Dark Matter)

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• Non-Baryonic Dark Matter• Cold Dark Matter (non-relativistic velocities)

(“WIMPs” = Weakly Interacting Massive Particles)

What is the Universe Composed of?

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• Incidentals• Neutrinos with non-zero restmass• Neutrino Oscillation measurements: me > 0.1 eV

• Hot Dark Matter (relativistic velocities)

• Cosmic Microwave Background photons• T=2.73K

• E=mc2

All-sky map of CMB photons from ~380,000 years after the Big Bang

WMAP-5

What is the Universe Composed of?

Credit: WMAP Science Team

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Until 1998, these seemed to be all there was...

In that year, two independent groups discovered evidence for another, major component of the mass-energy density of the Universe.

-- Supernova Cosmology Project -- Perlmutter et al.

-- High-Z Supernova Project -- Riess et al.

What is the Universe Composed of?

1998 and 2003 Science breakthroughs of the year

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What is the Universe Composed of?

• These two teams were using supernovae in other galaxies as “Standard Candles” to measure the deceleration of the expansion of the Universe due to the mutual gravitational attraction of the mass within the Universe.

Average Distance Between Galaxies

Standard Candles• Astronomical objects

which, as a class, have very uniform intrinsic luminosities (intrinsic brightnesses)

• Can use inverse square law to determine their distances

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SN2001V (Type 1a)Credit: Rafael Ferrando

• Type Ia Supernovae (SNe Ia) Very luminous

• at peak luminosity, ~ 5 billion times the energy output of the Sun

• can be seen over cosmological distances

Very uniform• after correction, the scatter in

the peak luminosity for a sample of SNe Ia is only about 10-15%

Moderately numerous• ~ 1 SNIa / galaxy / 100 years

The SN Ia Mechanism

• an accreting White Dwarf with a mass near the Chandrasekhar limit (1.4 M

sun) undergoes a

massive thermonuclear explosion, completing disrupting the star

• light curve powered by radioactive decay of 56Ni and 56Co generated in the explosion into 56Fe.

Type Ia Supernovas as Standard Candles

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Perlmutter, Physics Today, April 2003

Measurements of Dark Energy via SNe Ia

Before the discovery of Dark Energy

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Perlmutter, Physics Today, April 2003

Measurements of Dark Energy via SNe Ia

After the discovery of Dark Energy

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Perlmutter, Physics Today, April 2003

Measurements of Dark Energy via SNe Ia

Acceleration --> Dark Energy is a repulsive force counteracting gravity!

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Location of this peak is very sensitiveto total mass-energydensity of the Universe

Further Evidence via the CMB:Baryonic Acoustic Oscillations

Characteristic angular scale set by sound horizon at recombination:Standard ruler (geometric probe)

Need Dark Energy to explain total mass-energy density of the Universe.

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(0.03%)

(0.47%)

(4%)

(25%)

(0.5%)

(70%)CMB PhotonsΩ =0.0001 (0.01%)

What is the Universe Composed of?

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• A Cosmological Constant? Einstein's “Blunder” (originally added to make Universe static) Vacuum energy “Fine Tuning Problem”: observed value 10120 too small

• The decay of a new, ultralight particle?

• Evidence for extra dimensions?

• Breakdown of General Relativity?

• We don’t know.

What is Dark Energy?

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Consider the “size” R of the Universe at some time t compared with its “size” R0 at the present-day...

• Density of matter m = m,0 x (R0/R)3

• Density of radiation rad= rad,0 x (R0/R)4

• Density of dark energy = ,0 x (R0/R)3(1+w)

– Equation of State parameter w ≡ P/c2

– for a cosmological constant, w = -1, or = ,0

– for a “Big Rip”, w < -1

– w might vary with time (w’)

• Recall that R0/R = (1+z)

– E.g., at z=2, the Universe was 1/3rd the size it is today (at z=0)

How does Dark Energy Behave?

Recall that z is the “Cosmological Redshift”

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• Currently, observations are consistent with Dark Energy being a cosmological constant (w=-1)

= ,0 (density constant with time)

w ≡ P/c2 = -1 ⇒ P = wc2 ⇒ P = -c2 ⇒ negative pressure!

• Consider a piston chamber containing normal gas expansion leads to reduction in energy density / cooling of gas (positive) gas pressure is doing work on piston

• For a cosmological constant, however, the energy density remains unchanged (total energy increases)!

dW=PdV

How does Dark Energy Behave?

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Probing Dark Energy: I. Geometry

1. Standard Candles• Expansion rate of the Universe as

a function of time

• Basically makes use of the inverse square law to estimate distances (time since Big Bang)

1. Standard Rulers• Curved (Riemannian) vs. Flat

(Euclidean) Space time

• Angular sizes of objects of known physical size vs. distance

Average Distance Between Galaxies

Time since Big Bang

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• Galaxy clustering as a function of redshift depends on the characteristics of Dark Energy (among other things)

• Measure mass, number, and spatial distribution of galaxies and galaxy clusters as a function of z (time)

43 Mpc (present day)see http://cosmicweb.uchicago.edu/filaments.htmlCredit: Andrey Kravtsov

Probing Dark Energy: II. Growth of Structure

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Conclusion to “Part I: What is Dark Energy?”

• We don’t know what Dark Energy is, other than some mysterious force that appears to counteract the effects of gravity on cosmological scales.

• We do have ways of measuring the effects of Dark Energy (e.g, via geometric measures and measures of the growth of structures in the Universe).

• We also have ways to parameterize Dark Energy (e.g., the Dark Energy equation of state parameter, w, and its derivative with respect to time w’ (or a variation of w’ called wa).

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Part II: What is the Dark Energy Survey (DES)?

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The Dark Energy Survey (DES)

• Plan:– Perform a 5000 sq. deg. survey of the

southern galactic cap in 5 optical and near-infrared filters (g, r, i, z, y)

– Measure dark energy with 4 complementary techniques

• Cluster counts, weak lensing, baryonic acoustic oscillations, supernovae

• New Instrument (DECam):– Replace the prime-focus cage on the

CTIO Blanco 4m telescope with a new 2.2° field-of-view, 520 Mega-pixel CCD camera + optics

• Survey:– 525 nights during 2011-2016

(September - February) – 30% of the telescope time

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DES Participating Institutions

• Fermilab• University of Illinois at Urbana-Champaign• University of Chicago• Lawrence Berkeley National Laboratory• University of Michigan• NOAO/CTIO• Spain-DES Collaboration:

Institut d'Estudis Espacials de Catalunya (IEEC/CSIC), Institut de Fisica d'Altes Energies (IFAE), CIEMAT-Madrid

• United Kingdom-DES Collaboration:University College London, University of Cambridge, University of Edinburgh, University of Portsmouth, University of Sussex

• The University of Pennsylvania• Brazil-DES Consortium:

Observatorio Nacional, Centro Brasileiro de Pesquisas Fisicas, Universidade Federal do Rio de Janeiro, Universidade Federal do Rio Grande do Sul

• The Ohio State University• Argonne National Laboratory• Santa Cruz – SLAC – Stanford Consortium

13 participating institutions and >100 participants

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Basic Survey Parameters

Survey AreaOverlap with South PoleTelescope Survey (4000 sq deg)

Overlap with SDSS equatorial Stripe 82 for calibration (200 sq deg)

Connector region(800 sq deg)

J. Annis

Total Area: 5000 sq deg

Limiting Magnitudes– Galaxies: 10σ grizy = 24.6,

24.1, 24.3, 23.8, 21.8– Point sources: 5σ grizy = 26.1,

25.6, 25.8, 25.3, 23.3

Observation Strategy– 100 sec exposures– 2 filters per pointing (typically)

– gr in dark time (moon down)– izy in bright time (moon up)

– Multiple tilings/overlaps to optimize photometric calibrations

– 2 survey tilings/filter/year– All-sky photometric accuracy

– Requirement: 2%– Goal: 1%

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I. Galaxy Cluster counts vs. redshift– 20,000 clusters to z=1 with M >

2x1014 Msun

II. Weak lensing– 300 million galaxies with shape

measurements over 5000 sq deg

III. Baryonic Acoustic Oscillations / Galaxy Clustering

– 300 million galaxies to z = 1 and beyond

IV. Supernovae as Standard CandlesI. 1900 SNe Ia, z = 0.25-0.75

DES’s Four Probes of Dark Energy

Growth of Structure (+Geometry/Volume)

Growth of StructureAlso for calibration of cluster

masses for cluster counting

Standard Ruler (the “bump” in the power spectrum seen in the Cosmic Microwave Background, but at z = 0 – 1)

Standard Candle

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Forecast DES Constraints on Dark Energy Equation of State

Note: a = R / R0 = 1 / (1 + z)

DES Goal: Reduce area of red oval by a factor of ~4 from current dark energy experiments

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What are the Characteristics of the Dark Energy Camera (DECam)?

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DES Instrument: DECam

Hexapod

Optical Lenses

CCDReadout Filters

Shutter

Mechanical Interface of DECam Project to the Blanco

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DECam CCDs and Focal Plane

62 2k x 4k image CCDs• 520 Mpix• 0.27 arcsec/pixel (15-micron pixels)• FOV: 2.2º (3 sq deg)• LBL design

– fully depleted, 250-micron thick CCDs– 17 second readout time– QE> 50% at 1000 nm

DES Focal Plane: The Hex

B. Flaugher

DECam / Mosaic II QE comparison

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

QE, LBNL (%)QE, SITe (%)

DES CCD QE

Current Mosaic II QE

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DECam OpticsUK, Michigan, FNAL

C1

C2 - C3

C4

C5, vacuum window

Filters &Shutter

Focal plane

BipodsAttachment ring

C1

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Filter Changer - UMichigan

Capacity for 8 filters.Filter diameter: 620 mm.Filter thickness: ~13mm

DES g,r,i,z,y filtersCommunity filters: u-band very likely.Others?

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Some Tests of the CCDs and the Front-End Electronics

Imager Prototype/Multi-CCD Test VesselFNAL and UChicago

CTIO-1m + DECam 2kx2k CCDApril 2008

CTIO-1m + DECam 2kx2k CCDOct 2008 (credit: J. Estrada)

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Community Use of DECam

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Epilogue

Mentioned only Dark Energy...

... but the DES is very similar to the SDSS imaging survey (grizy vs. ugriz)...

... but deeper! (r to 24 instead of r to 22)

Like with SDSS, can do:

–Quasars

–Galaxy environments

–Superclusters

–Stellar streams associated with Milky Way

– ...

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visualization: David W. Hogg (NYU)with help from Blanton, Finkbeiner,

Padmanabhan, Schlegel, Wherry

data: Sloan Digital Sky Surveyand the Bright Star Catalog

The Power of Large Imaging Surveys

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Extra Slides

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Hubble Diagram

5252 CMB Maps

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I. Galaxy Cluster counting– 20,000 clusters to z=1 with M

> 2x1014 Msun

II. Weak lensing– 300 million galaxies with

shape measurements over 5000 sq deg

III. Baryonic Acoustic Oscillations / Galaxy Clustering

– 300 million galaxies to z = 1 and beyond

IV. SNe as Standard CandlesI. 1900 SNe Ia, z = 0.25-0.75

Survey AreaOverlap with South PoleTelescope Survey (4000 sq deg)

Overlap with SDSS equatorial Stripe 82 for calibration (200 sq deg)

Connector region(800 sq deg)

J. Annis

Total Area: 5000 sq deg

DES’s Four Probes of Dark Energy

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Photometric Redshifts

• Measure relative flux in five filters grizy: track the 4000 A break

• Estimate individual galaxy redshifts with accuracy (z) < 0.1 (~0.02 for clusters)

• Precision is sufficient for Dark Energy probes, provided error distributions well measured.

• Note: good detector response in z band filter needed to reach z>1

Elliptical galaxy spectrum

( + JHK data from VISTA Hemispheric Survey )

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I. DES Galaxy Clusters

• Galaxy cluster abundance, mass function, and correlations sensitive to cosmology via effects on volume and on growth rate of perturbations

• Complementary cluster samples• DES optical data provide accurate

cluster photometric redshifts• South Pole Telescope (SPT)

Sunyaev-Zel’dovich effect (SZE) data provides robust cluster masses

• Tens of thousands of clusters in 4000 deg2 area of DES-SPT overlap out to z=1

• Multiple cluster mass estimators (optical richness, SZE, weak lensing) and cross-checks of sample selection effects

MohrVolume Growth

Number of clusters above observable mass threshold

Dark Energy equation of state

M > 2x1014 Msun

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Cluster Cosmology with DES

• 3 Techniques for Cluster Selection and Mass Estimation:

• Optical galaxy concentration• Weak Lensing • Sunyaev-Zel’dovich effect (SZE)

• Cross-compare these techniques to reduce systematic errors

• Additional cross-checks:

shape of mass function;

cluster correlations

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DES photometric redshifts accurate to dz~0.02 for clusters out to z~1.3

z=0.041

z=0.138

z=0.277

z=0.377

Examples from SDSS

Clusters in the Optical

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Clusters from the Sunyaez-Zeldovich Effect:Synergy with the South Pole Telescope

Credit: L. Van Speybroek Sunyaev & Zeldovich (1980)

The South Pole Telescope

• John Carlstrom (PI)

• 10m submm telescope

• 150 & ~250 GHz

• 1000 element bolometer array

• 4000 sq. deg. survey Southern Galactic cap

• measures cluster masses using SZ effect

• First Light: 16 February 2007

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Finding Clusters with the SZE

Credit: Carlstrom et al. (2002)

Pros:– can identify galaxy clusters nearly

independent of redshift

z=0.17

z=0.55

z=0.83z=0.89

z=0.55

z=0.25

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Finding Clusters with the SZE

Credit: Carlstrom et al. (2002)

Pros:– can identify galaxy clusters nearly

independent of redshift

z=0.17

z=0.55

z=0.83z=0.89

z=0.55

z=0.25

Cons:– can identify galaxy clusters nearly

independent of redshift

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Observer

Dark matter halos

Background sources

Statistical measure of shear pattern, ~1% distortion Radial distances depend on geometry of Universe Foreground mass distribution depends on growth of structure

II. DES Weak Lensing & Cosmic Shear

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Weak Lensing of Faint Galaxies: distortion of shapes

BackgroundSourceshape

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Weak Lensing of Faint Galaxies: distortion of shapes

ForegroundCluster

BackgroundSourceshape

Note: the effect has been greatly exaggerated here

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Lensing of real (elliptically shaped) galaxies

ForegroundCluster

Coadd signal around a number of clusters

BackgroundSourceshape

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Mapping the Dark Matter in a Cluster of Galaxies

via WeakGravitational Lensing

Data fromBlanco 4-meterat CTIO

Joffre et al.

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•Cosmic Shear Angular Power Spectrum in 4 Photo-z Slices

•Shapes of ~300 million

galaxies, median redshift z = 0.7

•Primary Systematics: photo-z’s, PSF anisotropy, shear calibration

Weak Lensing Tomography

DES WL forecasts conservatively assume 0.9” PSF = median delivered to existing Blanco camera: DECam should do better & be more stable

Huterer

Statistical errorsshown

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Weak Lensing Tomography

Huterer

Statistical errorsshown

• Cosmic Shear Angular Power Spectrum as a function of redshift

• Shapes of ~300 million galaxies

• Median redshift <z> = 0.7

4 redshift bins

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CMBAngularPowerSpectrum

SDSS galaxycorrelation function

Acoustic series in P(k) becomes a single peak in (r)

Eisenstein et al

CMB Angular Power Spectrum

III. DES Baryonic Acoustic Oscillations (BAO)

Cosmic Microwave Background(z~1000)

Luminous Red Galaxies from SDSS(z~0.35)

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Baryon Acoustic Oscillations: CMB & Galaxies

CMBAngularPowerSpectrum

SDSS galaxycorrelation function

Acoustic series in P(k) becomes a single peak in (r)

Eisenstein et al

CMB Angular Power Spectrum

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Measuring BAO vs. Redshift in DES

Wiggles due to BAO

Blake & BridleFosalba & Gaztanaga

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Credit: Will Percival

III. Baryonic Acoustic Oscillations (BAO):Galaxy Angular Power Spectrum in the DES

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IV. DES Supernovae

• Geometric Probe of Dark Energy

(brightness vs. redshift)

• Repeat observations of 5 fields– CDF-S, SDSS Stripe 82, SNLS D1/Virmos,

XMM-LSS, ELIAS S1

– all are within DES wide-area survey footprint)

– 2 deep fields (10 min in g, 20 min r, 30 min i, 50 min z), 3 shallower

• Mean cadence of ~5 days

• Mixture of photometric and non-photometric time (10% of survey time)

• ~ 1900 well-measured SN Ia lightcurves,

0.25 < z < 0.75

SDSS

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DES Filters

Wavelength [Å]

Tra

nsm

issi

on,

Rel

. P

hoto

n F

lux

G191-B2B

g r i z Y

• DES grizy filters are being manufactured by Asahi, Japan

• Filters are large, 620 mm in diameter

• Care has been taken to specify spatial uniformity requirements on filter transmission vs. wavelength, in order to meet our 2% photometric calibration requirement

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Forecast Constraints

•DES+Stage II combined = Factor 4.6 improvement over Stage II combined•Consistent with DETF range for Stage III DES-like project•Large uncertainties in systematics remain, but FoM is robust to uncertainties in any one probe, and we haven’t made use of all the information

DETF FoM

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Survey Power (approximate)

TelescopeMirror Diameter

(meters)

Field of View

(deg2)A

SDSS 2.5 3.9 3

CFHT 3.6 1 4

PanSTARRS 1

PanSTARRS 4

1.8

4x1.8

7

4x7

15

60

DES 4 3 30

LSST 6.5 7 200

Partial Source: Pan-STARRS Website