Design of the Dark Energy Survey

1
Design of the Dark Energy Survey

• James Annis

2
Science Goals to Science Objective

• To achieve our science goals
• Cluster counting to z gt 1
• Spatial angular power spectra of galaxies to z
  1
• Weak lensing, shear-galaxy and shear-shear
• 2000 zlt0.8 supernova light curves
• We have chosen our science objective
• 5000 sq-degree imaging survey
• Complete cluster catalog to z 1, photometric
  redshifts to z1.3
• Overlapping the South Pole Telescope SZ survey
• 30 telescope time over 5 years
• 40 sq-degree time domain survey
• 5 year, 6 months/year, 1 hour/night, 3 day
  cadence

3
Science Requirements

• 5000 sq-degrees
• Significantly overlapping the SPT SZ survey area
• To be completed in 5 years with a 30 duty cycle
• 4 bandpasses covering 390 to 1100 nm
• SDSS g,r,i,z
• z modified with Y cutoff
• Limiting magnitudes
• g,r,i,z 24,24,24,23.6
• 10s for small galaxies
• Photometric calibration to 2
• 1 enhanced goal

• Astrometric calibration to 0.1
• Point spread function
• Seeing lt 1.1 FWHM
• Median seeing lt 0.9
• g-band PSF can be 10 worse
• Stable to 0.1 over 9 sq-arcminute scales
• From chapter 3 of NOAO proposal version 3 of
  requirements.
• Version 4, under review, will be a formal science
  requirements document.

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Limiting Magnitude
Red Galaxy

• Limiting magnitude (10s for small galaxies) was
  set by flow down of science goals
• ½ L cluster galaxies at redshift 4000A break
  leaving blue filter
• g,r,i,z 22.8,23.4,24.0,23.3
• Complete cluster catalog
• Galaxy catalog completeness
• g,r,i,z 22.8,23.4,24.0,23.6
• Simple selection function
• Blue galaxy photo-z at faint mags
• g,r,i,z 24.0,24.0,24.0,23.6
• Photo-z for angular power spectra and weak
  lensing

Mag of ½ L galaxy
0 redshift 1.5
i 23-24
photo-z spectro-z
0 redshift 1.5
5
Photometric Redshifts
Red galaxies

• Resulting limiting magnitudes give very good
  photometric redshifts
• Monte Carlo simulations of photometric redshift
  precision
• Evolving old stellar pop. SED
• Redshifted and convolved with filter curves.
  Noise added.
• Polynomial fit to photo-z
• For clusters, averaging all galaxies in the
  cluster above limiting magnitude.
• Template fit for photo-z
• These are sufficient to achieve our science goals.

½ L
2 L
Clusters
1.0x1014 M0
6
The Footprint

• Requirements
• Overlap with SPT SZ survey
• Redshift survey overlap
• Footprint
• -60 lt Dec lt -30
• SDSS Stripe 82 VLT surveys

DIRBE dust map, galactic coordinates
7
Survey Strategy I

• Design decision 1 area is more important than
  depth
• Image the entire survey area multiple times
• Design decision 2 tilings are important for
  calibration
• An imaging of the entire area is a tiling
• Multiple tilings are a core means of meeting the
  photometric calibration requirement offset
  tilings, not dithers
• Design decision 3 substantial science with year
  2 data
• We will aim for substantial science publications
  jointly with the public release of the year 2
  data.

8
Survey Strategy II

• Year 2
• g,r,i,z 100 sec exposures
• g,r,i,z 24.6, 24.1, 23.6, 23.0
• Calibration abs2.5 rel1.2
• Clusters to z0.8
• Weak lensing at 12 gals/sq-arcmin
• Year 5
• z 400 sec exposures
• g,r,i,z 24.6, 24.1, 24.3, 23.9
• Calibration abslt2 rellt1
• Clusters to z1.3
• Weak lensing at 28 gals/sq-arcmin

• Two tilings/year/bandpass
• In year 1-2, 100 sec/exp
• In year 3, drop g,r and devote time to i,z 200
  sec/exp
• In year 5, drop i and devote time to z 400
  sec/exp
• If year 1 or 2 include an El Nino event, we lose
  1 tiling, leaving three tilings at the end of
  year 2. This is sufficient to produce substantial
  key project science.

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DES Time Allocation Model
Time to the Community and to
the Dark Energy Survey

• September 4 bright 4 dark nights
  22 nights
• October 4 bright 5 dark nights
  22 nights
• November 4 bright 4 dark nights
  22 nights
• December 4 bright 4 dark nights
  21 nights
• Telescope shut down Dec 25,
  31
• January 4 bright 5 dark nights
  11 nights
• and the 2nd half of all
  nights
• February 3 bright 3 dark nights
  11 nights
• and the 2nd half of all
  nights
• March August all
  none
• Total 257 nights
  108 nights

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Time Allocation
CTIO mean weather year

• Analytic calculation of time available
• 30 year CTIO weather statistics
• 5 year moving averages
• Calculate photometric time
• Can complete imaging survey and time domain
  survey with 3 sq-degree field of view camera
• Simulations of observing process
• Use mean weather year
• Survey geometry
• Observing overhead
• NOAO time allocation model
• High probability of completing core survey area
  in time allocated

gt DES time allocation model just sufficient to
achieve science objective.
Probability of obtaining 8 tilings per year over
survey area. Dark is 100, light yellow 50
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Photometric Calibration Strategy

• Calibrate system response
• Convolve calibrated spectrum with system response
  curves to predict colors to 2
• Dedicated measurement response system integrated
  into instrument
• Absolute calibration
• Absolute calibration should be good to 0.5
• Per bandpass magnitudes, not colors
• Given flat map, the problem reduces to
  judiciously spaced standard stars

• Relative calibration
• Photometry good to 2
• Per bandpass mags, not colors
• Use offset tilings to do relative photometry
• Multiple observations of same stars through
  different parts of the camera allow reduction of
  systematic errors
• Hexagon tiling
• 3 tilings at 3x30 overlap
• 3 more at 2x40 overlap
• Aim is to produce rigid flat map of single
  bandpass
• Check using colors
• Stellar locus principal colors

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Survey Simulation

• We plan a full scale simulation effort
• Led by Huan Lin
• Centered at Fermilab and Chicago
• Using analytic, catalog and full image simulation
  techniques
• Over 4 years
• Underway, starting with photometric redshift
  simulations
• Use the simulations in 3 ways
• Check reduction code
• Mock data reduction challenge
• Chris Stoughton
• Prepare analysis codes
• Mock data analysis challenge
• Josh Frieman
• Prepare for science

• Survey simulations
• Jim Annis
• Catalog level simulations
• Lin, Frieman, students for photo-z and galaxy
  distributions
• Risa Weschlers Hubble Volume n-body
• Albert Stebbinss multi-gaussian approximation
• Mike Gladders empirical halo model
• Image level simulations
• Erin Sheldon for weak lensing
• Doug Tucker and Chris Stoughton
• Terapix skyMaker
• Masseys Shapelets code

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Survey Planning Summary

• We have well defined science goals and a well
  defined science objective
• A 5000 sq-degree survey substantially overlapping
  the SPT survey
• A time domain survey using 10 of time
• The science requirements are achievable.
• A good seeing, 4 bandpass, 2 calibration, i 24
  survey
• Multiple tilings of the survey area the core of
  the survey strategy and photometric calibration.
• The survey can be completed using
• 22 nights a month between September and October
• 21 nights in December
• 22 half nights a month in January and February