1. 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)

3. DES Science Four Probes of Dark Energy –Galaxy Cluster counting 20,000 clusters to z=1.3 with M > 2x10 14 M sun –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 (cluster counting, weak lensing, galaxy clustering) and for good light curves (SNe Ia). H. Lin

4. Basic Survey Parameters Survey Area Overlap with South Pole Telescope 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

5. 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 z band

7. 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?”

8. 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”)

9. 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”).

10. 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)?)

11. 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 plane Then, 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 Annis DES Collaboration Meeting, May 5-7, 2005

12. 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) UB R V I RV 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)

13. 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) 10.035 20.018 50.010 scaling bar is –0.20 mags to +0.20 mags Global Relative Calibrations: Simulation Jim Annis DES Collaboration Meeting, May 5-7, 2005

14. Global Absolute Calibration Calculate the expected photon flux F exp 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)] = F exp 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

15. 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

17. Extra Slides

19. 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

20. 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?”

21. Is Absolute Calibration Necessary and/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

22. Is Absolute Calibration Necessary and/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

23. 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)

24. 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

25. 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.

26. 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

27. 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

28. 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 = m inst ‒ a ‒ kX(1) m is the standard (“true”) mag of the standard star m inst is the instrumental mag, m inst = -2.5log(counts/sec) a is the photometric zeropoint k is the first-order extinction X is the airmass

29. 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 = m inst ‒ a ‒ b(color ‒ color 0 ) ‒ kX(2) color is a color index, e.g., (g-r) color 0 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 = m inst ‒ a ‒ kX. follows SDSS photometric calibration strategy

30. Structure (III) Explicit examples for DES filters: g = g inst ‒ a g ‒ b g ( (g-r) ‒ (g-r) 0 ) ‒ k g X(3a) r = r inst ‒ a r ‒ b r ( (g-r) ‒ (g-r) 0 ) ‒ k r X(3b) i = i inst ‒ a i ‒ b i ( (i-z) ‒ (i-z) 0 ) ‒ k i X(3c) z = z inst ‒ a z ‒ b z ( (i-z) ‒ (i-z) 0 ) ‒ k z X(3d) These assume that only two filters will be observed each night (either g and r, or i and z).

31. Global Relative Photometry Solutions – x = W y – Simple average coadd W coadd = [A t A] -1 A t – Weighted averaging W = [A t N -1 A] -1 A t N -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 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 2 nd, 3 rd, tiling overlap ½ if 4 th, and higher tile overlaps Jim Annis, DES Collaboration Meeting, May 5-7, 2005