The Photometric Calibration of the Blanco Dark Energy Survey (DES)

Purpose of Survey:• Perform a 5000 sq deg griz imaging survey of the

Southern Galactic Cap in order to – Constrain the Dark Energy parameter w to ~5%

(stat. errors) in each of 4 complementary techniques• w=P/p (equation of state parameter)

– Begin to constrain dw/dz• Serve as a stepping stone to large-scale, next-

generation projects (e.g., LSST, SKA, JDEM)

New Equipment:• Replace the prime focus cage on the CTIO Blanco

4m telescope with a new 2.2 deg FOV optical CCD camera

• Construct instrument 2005-2009

Survey Period: • 30% of the telescope time (525 nights) from 2009-

2014 (September - February)

DES Science

Four Probes of Dark Energy– Galaxy Cluster counting

• 20,000 clusters to z=1.3 with M > 2x1014 Msun

– Weak lensing• 300 million galaxies with shape

measurements over 5000 sq deg

– Spatial clustering of galaxies• 300 million galaxies to z = 1 and

beyond

– Standard Candles• 1900 SNe Ia, z = 0.25-0.75

Photometric redshifts out to z~1.3

Good all-sky photometry (2% or better) needed for photometric redshifts (clustercounting, weak lensing, galaxy clustering) and for good light curves (SNe Ia).

H. Lin

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)

Limiting Magnitudes– Galaxies: 10σ griz = 24.6, 24.1,

24.3, 23.9– Point sources: 5σ griz = 26.1,

25.6, 25.8, 25.4

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

– gr in dark time– iz in bright time

– Multiple tilings/overlaps to optimize photometric calibrations

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

– Requirement: 2%– Goal: 1%

J. Annis

Total Area: 5000 sq deg

The DES Instrument (DECam)

62 2k x 4k image CCDs• 520 Mpix• 0.27 arcsec/pixel• 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 (%)

z band

Photometric Monitoring

• 10 micron all sky camera • Apache Point Observatory (SDSS, ARC3.5m)• Whipple Observatory (Pairitel telescope)• detects even light cirrus under a full range of moon phases (no moon to full moon)

• Optical All-Sky camera• CONCam, TASCA

• RoboDIMM seeing and flux monitor

APO 10 micron all-sky camera

• Provide real-time estimates of sky conditions for survey strategy

• E.g, “Should next image be a photometric calibration field, a science target, or something else?”

• Provide measure of the photometric quality of an image

• E.g., “This image was obtained under such-and-such conditions; is it good enough to be used for photometric calibrations?”

Nightly Absolute Calibration

(Evolving) Standard Star Observation Strategy:• Observe 3 standard star fields, each at a different airmass (X=1-2), between

nautical (12°) and astronomical (18°) twilight (evening and morning).• Observe up to 3 more standard fields (at various airmasses) throughout the night• Also can observe standard star fields when sky is photometric but seeing is too

poor for science imaging (seeing > 1.1 arcsec)• Use fields with multiple standard stars• Keep an eye on the photometricity monitors

Absolute Calibration Strategy:• Calibrate to the DES griz “natural” system

– No system response color terms in the photometric equations– Avoids coupling science images obtained in different filters

• Use u’g’r’i’z’ and ugriz standards transformed to the DES griz “natural” system– SDSS g’r’i’(z’) and gri(z) are similar to DES gri(z), so transformations should be well behaved– Transformations could be done “on the fly”

• Similar to the SDSS calibration strategy– SDSS photometry is published in the SDSS 2.5m telescope’s ugriz natural system– SDSS photometry is calibrated based upon observations of u’g’r’i’z’ standard stars by the SDSS

0.5m Photometric Telescope (“PT”)

Southern u’g’r’i’z’ Standards

• Smith, Allam, Tucker, Stute, Rodgers, Stoughton

• 13.5' x 13.5' fields, typically tens of stds. per field

• r = 9 - 18, ~60 fields, ~16,000 standards

• See talk by J . Allyn Smith

• stars as bright as r≈13 can likely be observed by DECam with 5+ second exposures under conditions of poor seeing or with de-focusing (FWHM=1.5”).

• Already part of the DES survey strategy.

• Readily observable at a range of airmasses throughout most nights during the DES program.

• 2.5° wide (compares favorably with DECam's FOV (≈2.2°).

• Good star/galaxy classification to r ≈ 21.

• Value-added catalogue of tertiary standards is being made

– Area of Stripe 82 has been observed by SDSS > 10x under photometric conditions

– ~ 1 million tertiary SDSS ugriz standards (r = 14.5 - 21)!

– ~ 4000 per sq deg (on average)– Ivezic et al. 2006, in prep. – See talk by Zeljko Ivezic

SDSS Stripe 82 ugriz Standards

(Others: VST OmegaCam stds (Verdoes Kleign)? SkyMapper stds (Bessell)?)

1 tiling 3 tilings2 tilings

1 tiling 2 tilings 3 tilings

• We cover the sky twice per year per filter. This is called tiling.

• It takes ~ 1700 hexes to tile the whole survey area

Recipe:Tile the planeThen, tile the plane with

hex offset half hex over and up

This gives 30% overlap with three hexagons

Repeat, with different offsets

Large overlaps provide very robust hex-to-hex relative calibrations

Similar to PanStarrs strategy (see E. Magnier’s talk)

The Hex

Global Relative Calibrations:Hex-to-Hex Zeropoint Offsets

Jim AnnisDES Collaboration Meeting, May 5-7, 2005

Global Relative Calibrations:Star Flats

Koch et al. 2004, ESO WFI star flats based on SDSS Stripe 82 observations (2nd order polynomial fits)

Manfroid, Selman, & Jones 2001, ESO WFI using dithered exposures (3rd degree polynomial fits)

U B

RV

I

R V

• Due to vignetting and stray light, a detector’s response function differs for point sources and extended sources

• Standard flat fields (domes, twilights, skies) may flatten an image sky background well, but not the stellar photometry

• The solution: star flats (Manfroid 1995)• offset a field (like an open cluster) multiple times and fit a spatial function to the magnitude differences for matched stars from the different exposures• can also just observe a well-calibrated field once (Manfroid 1996)

INSTRUMENT MODEL:

A multiplicative flat field gradient of amplitude 3% from east to west

An additive scattered light pattern with a amplitude from the optical axis, 3% at the edge of the camera

An additive 3% rms scattered light per CCD

Solution:• Simultaneous least squares solution to

the underlying relative photometry given the observations

Relative Calibration

Tiling (rms of hex ZPs)

1 0.035

2 0.018

5 0.010

scaling bar is –0.20 mags to +0.20 mags

Global Relative Calibrations: Simulation

Jim AnnisDES Collaboration Meeting, May 5-7, 2005

Global Absolute Calibration

• Calculate the expected photon flux Fexp for each std star in each filter passband (synthetic photometry)

• Measure the magnitude for each standard star in each filter passband with the Blanco+DECam

• Calculate the zeropoint zp via the relation,

10**[-0.4*(mag – zp)] = Fexp

• Need:– one or more spectrophotometric standard stars which have been calibrated (directly or

indirectly) to a NIST standard source

– an accurately measured total system response for each filter passband for at least one CCD

• filter transmissions, CCD QE, optical throughput, atmospheric transmission

• Filter transmission, CCD QE, and optical throughput for the Blanco+DECam can be measured via a monochromator (but see also Chris Stubbs’ talk on a tunable dye laser system and David Burke’s talk on LSST calibration)

• The atmospheric transmission spectrum for CTIO has been measured (Stone & Baldwin 1983, Baldwin & Stone 1984, Hamuy et al. 1992, 1994)

• Several potentially useful spectrophotometric standards are available– E.g., GD 71, G158-100, GD 50, and G162-66

• All are HST WD spectrophotometric standards• All are visible from CTIO• All are V >= 13.0

– Won’t saturate DECam at an exposure time of 5 seconds (FWHM ~ 1.5arcsec)

Global Absolute Calibration

Extra Slides

• 2004 Level 0 Image Simulations → DM Challenge 0: Done!– Reformatted SDSS data used to simulate DES images

• 2005-06 Level 1 Catalog &Image Sim. → DM Chal. 1: Done!– 500 sq. deg. catalog; 500 GB of images; FNAL and UChicago computing used

• 2006-07 Level 2 Catalog and Image Sim. In progress– 5000 sq. deg. catalog; 5 TB of images– FermiGrid & MareNostrum SuperComputer (Barcelona)– Higher resolution N-body simulation, more realistic galaxy properties, and more

sophisticated atmosphere and instrument models (noise, ghosts)– Recover input cosmology from catalogs using 4 DES key project methods

• 2007-8 Level 3 Catalog and Image Simulations– Suite of full-DES catalogs (i.e., different input cosmologies)– Synergy with DOE SciDAC proposal (with many DES collaborators) to produce

large cosmological simulations for dark energy studies– 1 year of DES imaging data– Recovery of input cosmologies from catalogs and images– Stress test of full data processing system

DES Simulations Feed DM Challenges

Some Definitions

• Relative Photometric Calibration– m – m0 = -2.5log(F/F0)– the ratio F/F0 is important, not absolute value of F0

• F0 need not be measured directly• (F/F0,m0) can be defined arbitrarily (e.g., relative to Vega).

• Absolute Photometric Calibration– connects mags, which are relative by nature, to real physical

fluxes (e.g., photons/s/m**2)– answers the question, “What is the zeropoint flux F0 for mag m0?”

Is Absolute Calibration Necessaryand/or Useful?

Absolute calibration is necessary in order to make use of results from models or from other experiments, either for input into the current experiment or for sanity checks

• Red Sequence Cluster Finding

– could in principle be done purely based upon relative calibration internal to the DES (empirical calibration of red sequence)

– could lose benefits of modelling with synthetic photometry• Photo-z's

– could also, in principle, be done purely based upon relative calibration

– could also lose benefits of modeling with synthetic photometry

Is Absolute Calibration Necessaryand/or Useful?

• Type Ia SNe (0.3 < z < 0.8)– absolute calibration needed to accurately compare rest-frame

photometry of high-redshift SNe against the rest-frame photometry of low-redshift SNe within the DES SNe sample

• strictly speaking, only an absolute color calibration is needed– the zeropoints for the 4 photometric bands need to be the same– the zeropoints for the 4 photometric bands can all be wrong by the same

amount, though– see SNAP calibration requirements

– absolute calibration needed in order to combine DES SNIa results with other SNIa experiments, which are done in other filter systems

• Galaxy evolution – absolute calibration needed in order to compare DES results with

those of galaxy evolution models

The Absolute Calibration Experiment

• Assume a perfectly flat relative calibration across the full 5000 sq deg of the DES

– chip-to-chip and tile-to-tile offsets and color terms are known perfectly and applied perfectly to all the data

– all that are needed for the absolute calibration of the entire survey are the 4 zeropoints – one for each filter passband – to convert the measured mags in each passband into a calibrated flux with real flux units (e.g., photons/s/m**2)

The Absolute Calibration Experiment

• Need:– one or more spectrophotometric standard stars which have been

calibrated (directly or indirectly) to a NIST std source– an accurately measured total system response for each filter

passband for at least one CCD• filter transmissions, CCD QE, optical throughput, atmospheric

transmission

• Calculate the expected photon flux Fexp for each std star in each filter passband (synthetic photometry)

• Measure the magnitude for each standard star in each filter passband with the Blanco+DECam

• Calculate the zeropoint zp via the relation, 10**(-0.4*mag – zp) = Fexp

Possible Spectrophotometric Stds

• Vega– V=0.03, RA=18:36:56.3, DEC=+38:47:01

– NIST-calibrated. Too bright! Too far north!

• BD+17 4708– V=9.47, RA=21:11:31.4, DEC=+18:05:34

– SDSS fundamental standard. F subdwarf. Too bright.

• G191 B2B– V=11.77, RA=05:05:30.1, DEC=+52:49:47

– HST White Dwarf standard. Too bright? Too far north!

• GD 71– V=13.03, RA=05:52:27.5, DEC=+15:53:17

– HST White Dwarf standard.

• P330E– V=13.01, RA=16:31:34.3, DEC=+30:08:52

– HST solar analog. A bit northerly.

Possible Southern Spectro. Stds.

• Several potentials in Stone & Baldwin, 1983, MNRAS, 204, 347

• G158 -100– r'=14.691, RA=00:33:54.6, DEC=-12:07:58.9

– HST White Dwarf standard.

– Filippenko & Greenstein, 1984 PASP, 96, 530

• GD 50– V=14.06, RA=03:48:50, DEC=-00:58:31

– HST White Dwarf standard.

– Stone, 1996 ApJS, 107, 423

• G162-66– V=13.0, r'=13.227, RA=10:33:43, DEC=-11:41:39

– HST White Dwarf standard.

– Stone, 1996 ApJS, 107, 423

The Shutter

Spec's from SOAR shutter (A. Walker):• 1 percent UNIFORMITY at 1 sec exposure time

– actual exp. time anywhere on CCD should be no more than 1% different from anywhere else at this exposure time

• Shutter exposure time REPEATABILITY better than 0.005 sec (5 milliseconds)

• Shutter exposure time ACCURACY should be such that the offset from the nominal exposure time should be less than 0.05 sec (50 milliseconds)

• Shutter should allow exposures from 1 sec upwards– capability to take shorter exposures (e.g., down to 0.2) would be

useful as a GOAL, not a specification

Structure (I)

• The Photometric Standards Module is basically a big least squares solver, fitting the observed mags of a set of standard stars to their “true” mags via a simple model (photometric equation).

• In one of its simplest forms, the photometric equation looks like this:

m = minst

‒ a ‒ kX (1)

• m is the standard (“true”) mag of the standard star

• minst

is the instrumental mag, minst

= -2.5log(counts/sec)

• a is the photometric zeropoint• k is the first-order extinction• X is the airmass

Structure (II)

• Since we will likely be using standard stars which are on a system that closely approximates but does not exactly match the DES natural system, we will probably want to add an instrumental color term to the photometric equations:

m = minst

‒ a ‒ b(color ‒ color0) ‒ kX (2)

• color is a color index, e.g., (g-r)

• color0 is a constant indicating the “crossing color” between the DES

natural system and the standard star photometric system• converts standard stars to DES system, and not the other way around!• use only for standard star observations; when applying the results to

target data, use m = minst

‒ a ‒ kX.

• follows SDSS photometric calibration strategy

Structure (III)

• Explicit examples for DES filters:

g = ginst

‒ ag ‒ b

g ( (g-r) ‒ (g-r)

0 ) ‒ k

gX (3a)

r = rinst

‒ ar ‒ b

r ( (g-r) ‒ (g-r)

0 ) ‒ k

rX (3b)

i = iinst

‒ ai ‒ b

i ( (i-z) ‒ (i-z)

0 ) ‒ k

iX (3c)

z = zinst

‒ az ‒ b

z ( (i-z) ‒ (i-z)

0 ) ‒ k

zX (3d)

• These assume that only two filters will be observed each night (either g and r, or i and z).

Global Relative Photometry

• Solutions– x = W y– Simple average coadd

• Wcoadd = [AtA] -1At

– Weighted averaging• W = [At N -1 A] -1AtN -1 • N is the noise covariance matrix• Minimum variance for Gaussian

noise

• Provides least squares flux scalings• That is, the flat map

• Inverting large matrices (??)– Year 1: 4 matrices of 6000x4000– Year 2: 4 matrices of 30,000x8000

CMB style mapping strategyy = A x + Ny = observationsRatios of instrumental star fluxes between pairs

of hexes (62 ccds = 1 hex)Includes effects of uncorrected flat field

problems and scattered light problems

x = scale factor mapScale factor for a given hex image

N = noiseA = survey mapping 0 if no overlap1/3 if 2nd, 3rd, tiling overlap½ if 4th, and higher tile overlaps

Jim Annis, DES Collaboration Meeting, May 5-7, 2005