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Calibration Elements

Photometric and Astrometric Calibration of LSST Data David L. Burke Kavli Institute for Particle Astrophysics and Cosmology Stanford Linear Accelerator Center DOE HEP SLAC Program Review June 13, 2007. Science Targets. Reference Stars (  100 per chip per image). All Sky Reconstruction

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Calibration Elements

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  1. Photometric and AstrometricCalibration of LSST Data David L. BurkeKavli Institute for Particle Astrophysics and CosmologyStanford Linear Accelerator Center DOE HEP SLAC Program ReviewJune 13, 2007

  2. Science Targets Reference Stars ( 100 per chip per image) All Sky Reconstruction (~ Monthly; i.e. ~10 Epochs) Photometric Standards (~ 1 per image) Atmospheric Extinction Z(az,el,n,t) Instrumental Calibration I(x,y,n,t) Calibration Elements Accumulated LSST Multi-Epoch Survey Auxiliary Data

  3. Photometric Design Specifications Except as noted, specifications are given for isolated bright stars (17 < r < 20). • Repeatability of measured flux over epochs of 0.005 mag (rms). • Internal zero-point uniformity for all stars across the sky 0.010 mag (rms) in g,r,i ; 0.020 in other bands. • Transformations between internal photometric bands known to 0.005 mag (rms) in g,r,i; 0.010 to other bands. • (This is a specification on the absolute accuracy of measured colors.) • Transformation to a physical scale with accuracy of 0.020 mag.

  4. Optical Efficiency of Atmosphere, Filters, Detectors Six-Band Photometry

  5. Photometric Calibration Philosophy • Reduce accumulated all-sky multi-epoch survey to a single arbitrary scale for each filter band. • Reference stars:  108 main-sequence stars (105 per image). • Determine six filter-band zero-points. • Photometric standards:  2000 hydrogen white dwarf stars. • Physical scale • Conventional (Landolt, Stetson) standard stars. • HST 1% photometry – DA WDs. • NIST laboratory calibrated detectors?

  6. Sloan SDSS “Über-cal” Southern Survey (Stripe 82) 300 deg2along celestial equator. Multiple (30-40) epochs. Main sequence stellar color locus is quite narrow. Averages of stars with r < 20 define photometric zero-points. Projections of main sequence locus in gri and riz.

  7. Flat-fielding error. gri Sloan SDSS “Über-Cal” Channel-by-channel averages of ~ 106 stars. Channel  Uniformity of internal zero points in photometric conditions: gri 5 milli-mags uz  10 milli-mags.  Meets LSST goals.

  8. “Forward” Calibration Analysis of SDSS data indicates that spatial, temporal, and/or spectral variations in atmospheric conditions that are unobserved and un-modeled dominate residual calibration errors. We believe better control of these residual errors will be required to meet LSST goals for performance and observing efficiency. • 1. Calibrate telescope and camera instrumentation. • I(x,y,n,t) • Reconstruction of photons in the telescope pupil. 2. Measure atmospheric extinction. Z(az,el,n,t) Photons at the top of the atmosphere to the telescope pupil.

  9. Instrumental Optical Calibration Dome Screen Every point on the screen must provide uniform (Lambertian) illumination of the angular FOV – fill LSST étendue. Tunable Laser Calibrated Photodiode Calibrate at NIST across wavelength (griz) to part in ~ 10-3. Harvard/LSST – PanSTARRS Collaboration (C. Stubbs, J. Tonry, et al.)

  10. Back-Lit Diffuse Dome Screen Concept Sketch and Prototype Side-Emitting Optical Fiber Somta Corp of Riga, Latvia Mirror Diffuser Collimator

  11. Performance and Issues • Illumination uniformity. • Stray and scattered light. • Mechanical construction. Test on CTIO Blanco. Comparison of Blanco r-filter facility reference bandpass and dome illumination measurements.

  12. O3 H2O O2 Atmospheric Composition and Optical Extinction U.S. Standard Atmosphere (1976) MODTRAN4 (USAF) Rayleigh and aerosol scattering extinction coefficients kscat(l) = a · l-4.05 + b · l-a Aerosol spectral index a depends on particulate size and shape and varies between 0.5 – 1.5. Telluric absorption varies nonlinearly with airmass - saturated.

  13. Auxiliary Telescope Auxiliary Telescope Measure (changes in) atmospheric transmission with sufficient resolution in wavelength to accurately compute spectral extinction across all wavelengths, e.g. with MODTRAN4. • Spectroscopy (R = l/dl 100) or photometry with  10-12 appropriately chosen bands; B, A, and F stars < 15 magnitude. Spectrographic and photometric standards. →Extract Z(az,el,n,t) relative to standard atmosphere.

  14. Observing Tests at CTIO (Tololo) Approved Runs 3 nights in April – completed. 3 more in June – this weekend. Real-time analysis of April data looks good. Oxygen A-line Equivalent Width Rayleigh Scattering and Ozone Chiappus

  15. AstrometrySpecification and Approach • Relative astrometry – stacking images raft-by-raft to  10 milliarcsec, and across the 3 FOV to  15 mas. • Consistent multi-color observations; refraction. • Zero proper motion for galaxies and QSOs; parallax of stars. • Absolute astrometry – transformation to external system to  50 mas. • Reference catalogs and QSO solutions. • Radio sources? • Image-by-image, chip-by-chip: (x,y) (RA,DEC). • Best fit (6 parameter/chip) to accumulated multi-epoch survey. • Stability of relative and absolute solutions.

  16. Stromlo Southern Sky Survey Complete southern sky: 20,000 square degrees – first light 2007. 1.3 meter telescope 8 square degree FOV S/N = 5 in 110 sec exposures. Developing joint analysis with SkyMapper team.

  17. Simulate System Response And Data Processing LSST Calibration Simulation Main Program Generate Model WP1 LSST Operations Simulator Simulate Precursor Campaign and Priors WP2 Generate References & Standards Target SEDs Zr(az,el,n,i/j) Ir(x,y,n,i) Im(x,y,n,i) Standards Catalog AUXFOV(j) Standard SEDs LSSTFOV(i) Generate Test Targets WP2 Generate Instrument Response Simulate Calibration Pipeline WP3 Generate Instrument Calibration WP3 Flats and Bias Generate Aux Telescope Ops WP4 Generate Atmosphere WP4 Simulate Aux Observing WP4 Aux Object Catalog Simulate Image Processing Pipeline WP5 Model Image Catalog Compute Model Image Catalog WP1 Object Catalog and Zm(az,el,n,i/j) Simulation Analysis and Reporting WP1 Calibration Simulation WP2 WP3

  18. Calibration Simulation Responsibilities WP1.Simulation Main Program.Bogdan Popescu, Margaret Hanson, Brian Meadows, Mike Sokoloff (U. of Cincinnati), and David Cinabro (Wayne State University). WP2.Standards and Targets.Lynne Jones and Zeljko Ivezic (U. of Washington, Seattle). WP3.Instrument and Hardware Calibration.Raul Armendariz, Jim Frank, and John Haggerty(Brookhaven and Harvard University). WP4. Auxiliary Instrumentation and Atmosphere.Jim Bartlett (APC Paris 7) and David Burke (SLAC). WP5.Pipelines. Tim Axelrod (LSSTC), Lynne Jones (U. of Washington), and representatives from WP1-WP4.

  19. Broad Front of Activity • Analysis of data from present-day observatories and surveys. • Design of LSST algorithms and calibration equipment. • In the field observing and testing (in collaboration with PanSTARRS and SkyMapper). • Simulation of LSST calibration data and pipelines.

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