Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

Design of the Dark Energy Survey

James Annis

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

Science Goals to Science Objective

• To achieve our science goals:– Cluster counting to z > 1

– Spatial angular power spectra of galaxies to z = 1

– Weak lensing, shear-galaxy and shear-shear

– 2000 z<0.8 supernova light curves

• We have chosen our science objective:– 5000 sq-degree imaging survey

• Complete cluster catalog to z = 1, photometric redshifts to z=1.3• 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

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

Science Requirements

1. 5000 sq-degrees• Significantly overlapping the

SPT SZ survey area• To be completed in 5 years

with a 30% duty cycle

2. 4 bandpasses covering 390 to 1100 nm• SDSS g,r,i,z• z modified with Y cutoff

3. Limiting magnitudes• g,r,i,z = 24,24,24,23.6• 10σ for small galaxies

4. Photometric calibration to 2%• 1% enhanced goal

5. Astrometric calibration to 0.1”

6. Point spread function• Seeing < 1.1” FWHM

• Median seeing <= 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.

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

Limiting Magnitude

Limiting magnitude (10σ 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

0 redshift 1.5

0 redshift 1.5

Mag of ½ L* galaxy

photo-z – spectro-z

i = 23-24

Red Galaxy

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

Photometric Redshifts

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*

1.0x1014 M0

Clusters

Red galaxies

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

The Footprint

Requirements• Overlap with SPT SZ survey• Redshift survey overlap

Footprint• -60 <= Dec <= -30• SDSS Stripe 82 + VLT surveys

Overlap target Right Ascension (deg)

Declination (deg)

Area (sq. deg.)

SPT -60 to 105 -75 to -60

-30 to -65-45 to –65

4000

SDSS Stripe 82 -50 to 50 -1.0 to 1.0 200

Connection region

20 to 50 -30 to –1.0 800

DIRBE dust map, galactic coordinates

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

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.

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

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: abs=2.5% rel=1.2%– Clusters to z=0.8– 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: abs=<2% rel=<1%– Clusters to z=1.3– 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.

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

DES Time Allocation Model

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

Time to the Community and to the Dark Energy Survey

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

Time Allocation

• 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

Probability of obtaining 8 tilings per year over survey area. Dark is 100%, light yellow ~50%

=> DES time allocation model just sufficient to achieve science objective.

CTIO mean weather year

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

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

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

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 Weschler’s Hubble Volume n-body– Albert Stebbins’s multi-gaussian

approximation– Mike Gladder’s empirical halo model

• Image level simulations– Erin Sheldon for weak lensing– Doug Tucker and Chris Stoughton

• Terapix skyMaker• Massey’s Shapelets code

Jim Annis for the DES Collaboration BIRP Meeting August 12, 2004 Tucson

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