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

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

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

SLAC Program Review Six-Band Photometry Optical Efficiency of Atmosphere, Filters, Detectors

SLAC Program Review Photometric Calibration Philosophy Reduce accumulated all-sky multi-epoch survey to a single arbitrary scale for each filter band. –Reference stars:  10 8 main-sequence stars (10 5 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?

SLAC Program Review Sloan SDSS “ Ü ber-cal” Main sequence stellar color locus is quite narrow. Averages of stars with r < 20 define photometric zero-points. Southern Survey (Stripe 82) 300 deg 2 along celestial equator. Multiple (30-40) epochs. Projections of main sequence locus in gri and riz.

SLAC Program Review Flat-fielding error. Sloan SDSS “Über-Cal” Uniformity of internal zero points in photometric conditions: gri  5 milli-mags uz  10 milli-mags.  Meets LSST goals. Channel-by-channel averages of ~ 10 6 stars.  Channel  gri

SLAC Program Review “Forward” Calibration 1. Calibrate telescope and camera instrumentation.  I(x,y,,t)  Reconstruction of photons in the telescope pupil. 2. Measure atmospheric extinction.  Z(az,el,,t)  Photons at the top of the atmosphere to the telescope pupil. 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.

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

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

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

SLAC Program Review H2OH2O O2O2 O3O3 Atmospheric Composition and Optical Extinction Rayleigh and aerosol scattering extinction coefficients k scat ( ) = a · b · -  Aerosol spectral index  depends on particulate size and shape and varies between 0.5 – 1.5. Telluric absorption varies nonlinearly with airmass - saturated. MODTRAN4 (USAF) U.S. Standard Atmosphere (1976)

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

SLAC Program Review 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

SLAC Program Review Astrometry Specification 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.

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

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

SLAC Program Review 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.

SLAC Program Review 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.