Transcript Photometric and Astrometric Calibration of LSST Data David L. Burke

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
Calibration Elements
Accumulated LSST
Multi-Epoch Survey
All Sky Reconstruction
(~ Monthly; i.e. ~10 Epochs)
Science Targets
Reference Stars
( 100 per chip per image)
Auxiliary Data
Photometric Standards
(~ 1 per image)
Atmospheric Extinction
Z(az,el,n,t)
Instrumental Calibration
I(x,y,n,t)
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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.
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Six-Band Photometry
Optical Efficiency of Atmosphere, Filters, Detectors
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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?
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Sloan SDSS “Über-cal”
Southern Survey (Stripe 82)
300 deg2 along 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.
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Sloan SDSS “Über-Cal”
Channel-by-channel averages of ~ 106 stars.
Flat-fielding error.
 Channel 
Uniformity of internal zero points
in photometric conditions:
gri  5 milli-mags
uz  10 milli-mags.
gri
 Meets LSST goals.
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“Forward” Calibration
Analysis of SDSS data indicates that spatial, temporal, and/or spectral
variations in atmospheric conditions that are unobserved and unmodeled 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.
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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.)
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Back-Lit Diffuse Dome Screen
Concept Sketch and Prototype
Side-Emitting Optical Fiber
Somta Corp of Riga, Latvia
Mirror
Diffuser
Collimator
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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.
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Atmospheric Composition and Optical
Extinction
U.S. Standard Atmosphere (1976)
MODTRAN4
(USAF)
O3
O2
H2 O
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.
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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.
Auxiliary
Telescope
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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
SLAC Program Review 2007
Rayleigh Scattering and Ozone Chiappus
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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.
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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.
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Calibration Simulation
LSST Calibration Simulation
Main Program WP1
Generate Model
Simulate System Response
And Data Processing
WP2
LSST Operations
Simulator
Generate
References & Standards
WP2
Generate WP3
Test Targets
LSSTFOV(i)
WP2
Simulate
Precursor Campaign
and Priors
Standard SEDs
Standards Catalog
Target SEDs
Generate
Instrument Response
Ir(x,y,n,i)
Generate WP3
Instrument Calibration
Flats and Bias
Generate WP4
Aux Telescope Ops
AUXFOV(j)
Generate WP4
Atmosphere
Zr(az,el,n,i/j)
Compute WP1
Model Image Catalog
Simulate WP3
Calibration Pipeline
Im(x,y,n,i)
Simulate WP4
Aux Observing
Aux Object Catalog
Model Image
Catalog
Simulation Analysis
and Reporting WP1
WP5
Simulate
Image Processing Pipeline
Object Catalog
and Zm(az,el,n,i/j)
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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.
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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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