Photometric and Astrometric Calibration of LSST Data David L. Burke Kavli Institute for Particle...

Photometric and AstrometricCalibration of LSST Data

David L. BurkeKavli Institute for Particle Astrophysics and Cosmology

Stanford Linear Accelerator Center

DOE HEP SLAC Program ReviewJune 13, 2007

SLAC Program Review 2007

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Calibration Elements

All Sky Reconstruction(~ Monthly; i.e. ~10 Epochs)

Instrumental CalibrationI(x,y,,t)

Atmospheric ExtinctionZ(az,el,,t)

Reference Stars( 100 per chip per image)

Photometric Standards(~ 1 per image)

Science Targets

Accumulated LSST Multi-Epoch Survey

Auxiliary Data


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Photometric Design Specifications

• 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.

Except as noted, specifications are given for isolated bright stars (17 < r < 20).


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Six-Band Photometry

Optical Efficiency of Atmosphere, Filters, Detectors


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?


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Sloan SDSS “Über-cal”

Main sequence stellar color locus is quite narrow.

Averages of stars with r < 20 define photometric zero-points.

Southern Survey (Stripe 82)

300 deg2 along celestial equator. Multiple (30-40) epochs.

Projections of main sequence locus in gri and riz.


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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 ~ 106 stars.

Channel

gri


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“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.


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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 ~ 10-3.

Harvard/LSST – PanSTARRSCollaboration

(C. Stubbs, J. Tonry, et al.)


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Somta Corp of Riga, Latvia

Side-Emitting Optical Fiber

Mirror

Diffuser

Collimator

Back-Lit Diffuse Dome ScreenConcept Sketch and Prototype


11

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.


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H2OO2

O3

Atmospheric Composition and Optical Extinction

Rayleigh and aerosol scattering extinction coefficients

kscat() = a · -4.05 + 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)


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Auxiliary Telescope

• Spectroscopy (R = /d 100) or photometry with 10-12 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.

AuxiliaryTelescope


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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

Rayleigh Scattering and Ozone Chiappus


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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.


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Stromlo Southern Sky Survey

S/N = 5 in 110 sec exposures.

1.3 meter telescope8 square degree FOV

Complete southern sky: 20,000 square degrees – first light 2007.

Developing joint analysis with SkyMapper team.


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WP2

WP3

LSST OperationsSimulator

LSSTFOV(i)

Standard SEDs

Target SEDs

Standards Catalog

Zr(az,el,,i/j) Aux Object Catalog

Object Catalogand Zm(az,el,,i/j)

Model Image Catalog

LSST Calibration SimulationMain Program WP1

GenerateReferences & Standards

Generate Atmosphere

WP4

Im(x,y,,i)

Ir(x,y,,i) Generate Instrument Response

SimulateImage Processing Pipeline

WP5ComputeModel Image Catalog

WP1

Generate Test Targets

WP2

AUXFOV(j)Generate Aux Telescope Ops

WP4

Simulate Aux Observing

WP4

SimulateCalibration Pipeline

WP3

SimulatePrecursor Campaign

and Priors

WP2

Flats and BiasGenerateInstrument Calibration

WP3

Simulation Analysis and Reporting WP1

Generate Model Simulate System ResponseAnd Data Processing

Calibration Simulation


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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.

Photometric and Astrometric Calibration of LSST Data David L. Burke Kavli Institute for Particle...

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