The%20Photometric%20Calibration%20of%20the%20Blanco%20Dark%20Energy%20Survey%20(DES)

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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
• wP/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)

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DES Science

• Four Probes of Dark Energy
• Galaxy Cluster counting
• 20,000 clusters to z1.3 with M gt 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

H. Lin
Good all-sky photometry (2 or better) needed
for photometric redshifts (cluster counting, weak
lensing, galaxy clustering) and for good light
curves (SNe Ia).
Photometric redshifts out to z1.3
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Basic Survey Parameters
Overlap with South Pole Telescope Survey (4000
sq deg)
Survey Area

• Limiting Magnitudes
• Galaxies 10s griz 24.6, 24.1, 24.3, 23.9
• Point sources 5s 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

Connector region (800 sq deg)
J. Annis
Overlap with SDSS equatorial Stripe 82 for
calibration (200 sq deg)
Total Area 5000 sq deg
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The DES Instrument (DECam)
DES Focal Plane The Hex
z band
B. Flaugher

• 62 2k x 4k image CCDs
• 520 Mpix
• 0.27 arcsec/pixel
• LBL design
• fully depleted, 250-micron thick CCDs
• 17 second readout time
• QEgt 50 at 1000 nm

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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

• 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?

APO 10 micron all-sky camera
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Nightly Absolute Calibration

• (Evolving) Standard Star Observation Strategy
• Observe 3 standard star fields, each at a
  different airmass (X1-2), 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 gt 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 ugriz and ugriz standards transformed to
  the DES griz natural system
• SDSS gri(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
  telescopes ugriz natural system
• SDSS photometry is calibrated based upon
  observations of ugriz standard stars by the
  SDSS 0.5m Photometric Telescope (PT)

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Southern ugriz 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 r13 can likely be observed by
  DECam with 5 second exposures under conditions
  of poor seeing or with de-focusing (FWHM1.5).

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SDSS Stripe 82 ugriz Standards

• 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 gt 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

(Others VST OmegaCam stds (Verdoes Kleign)?
SkyMapper stds (Bessell)?)
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Global Relative CalibrationsHex-to-Hex
Zeropoint Offsets

• We cover the sky twice per year per filter. This
  is called tiling.
• It takes 1700 hexes to tile the whole survey
  area

The Hex
Jim Annis DES Collaboration Meeting, May 5-7,
2005
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Global Relative CalibrationsStar Flats

• Due to vignetting and stray light, a detectors
  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)

U
B
V
R
R
V
I
Manfroid, Selman, Jones 2001, ESO WFI using
dithered exposures (3rd degree polynomial fits)
Koch et al. 2004, ESO WFI star flats based on
SDSS Stripe 82 observations (2nd order polynomial
fits)
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Global Relative Calibrations Simulation

• 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

scaling bar is 0.20 mags to 0.20 mags
Jim Annis DES Collaboration Meeting, May 5-7,
2005
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Global Absolute Calibration

• 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

• 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 BlancoDECam
• Calculate the zeropoint zp via the relation,
• 10-0.4(mag zp) Fexp

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Global Absolute Calibration

• Filter transmission, CCD QE, and optical
  throughput for the BlancoDECam can be measured
  via a monochromator (but see also Chris Stubbs
  talk on a tunable dye laser system and David
  Burkes 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 gt 13.0
• Wont saturate DECam at an exposure time of 5
  seconds (FWHM 1.5arcsec)

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Extra Slides
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DES Simulations Feed DM Challenges

• 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

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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/m2)
• answers the question, What is the zeropoint flux
  F0 for mag m0?

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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

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Is Absolute Calibration Necessaryand/or Useful?

• Type Ia SNe (0.3 lt z lt 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

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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/m2)

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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 BlancoDECam
• Calculate the zeropoint zp via the relation,
  
  10(-0.4mag zp) Fexp

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Possible Spectrophotometric Stds

• Vega
• V0.03, RA183656.3, DEC384701
• NIST-calibrated. Too bright! Too far north!
• BD17 4708
• V9.47, RA211131.4, DEC180534
• SDSS fundamental standard. F subdwarf. Too
  bright.
• G191 B2B
• V11.77, RA050530.1, DEC524947
• HST White Dwarf standard. Too bright? Too far
  north!
• GD 71
• V13.03, RA055227.5, DEC155317
• HST White Dwarf standard.
• P330E
• V13.01, RA163134.3, DEC300852
• HST solar analog. A bit northerly.

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Possible Southern Spectro. Stds.

• Several potentials in Stone Baldwin, 1983,
  MNRAS, 204, 347
• G158 -100
• r'14.691, RA003354.6, DEC-120758.9
• HST White Dwarf standard.
• Filippenko Greenstein, 1984 PASP, 96, 530
• GD 50
• V14.06, RA034850, DEC-005831
• HST White Dwarf standard.
• Stone, 1996 ApJS, 107, 423
• G162-66
• V13.0, r'13.227, RA103343, DEC-114139
• HST White Dwarf standard.
• Stone, 1996 ApJS, 107, 423

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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

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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

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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

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Structure (III)

• Explicit examples for DES filters
• g ginst ? ag ? bg ( (g-r) ? (g-r)0 ) ?
  kgX (3a)
• r rinst ? ar ? br ( (g-r) ? (g-r)0 ) ?
  krX (3b)
• i iinst ? ai ? bi ( (i-z) ? (i-z)0 ) ?
  kiX (3c)
• z zinst ? az ? bz ( (i-z) ? (i-z)0 ) ?
  kzX (3d)
• These assume that only two filters will be
  observed each night (either g and r, or i and z).

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Global Relative Photometry
? CMB style mapping strategy ? y A x N ? y
observations ? Ratios 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 map ? Scale factor for a given hex
image ? N noise ? A survey mapping ? 0 if no
overlap ? 1/3 if 2nd, 3rd, tiling overlap ? ½ if
4th, and higher tile overlaps

• 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

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