LSST Photometric Calibration

D. BurkeSLAC/KIPAC

DOE SLAC Program ReviewJune 6-7, 2006

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The LSST Mission

Photometric survey of half the sky ( 20,000 square degrees).

Multi-epoch data set with return to each point on the sky every 4-5 nights for up to 10 years.

Rapid cadence (new pointing every 40 seconds) with prompt transient alerts.

Deliverables

Archive 3 billion galaxies with photometric redshifts to z = 3.

Detect 250,000 Type 1a supernovae per year (with photo-z < 0.8).

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Goals for Stellar Photometry

• 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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LSST Celestial CalibrationFull Advantage of Cadence and Replication

LSST Calibration Standards

Start with existing catalogs – e.g. SDSS to r < 20.LSST single-visit depth (5) r = 24.5.LSST single-image saturation r 17.

Use of photometric nights to build LSST standards catalogs.

Hydrogen white dwarfs becoming the standard of choice.SDSS ~ 2000 confirmed equatorial WDs (18 < r < 20).Cross check with WDs (few dozen) observed with HST.

LSST Calibration Sentinels

Expect 100 main-sequence stars r < 20 every chip every image.Overlapped tiling of the sky in each epoch.Each point on the sky in repeated epochs.

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Rapid-Paced Multi-Epoch SurveysSloan SDSS Precursor

Main sequence stellar color locus is quite narrow. Use this to evaluate and monitor instrumental and observational parameters.

Z. Ivezic, et al. (SDSS)Standards WorkshopBlankenberge, 2006.

Southern Survey

300 deg2 along celestial equator. Photometry for 870,000 stars observed in multiple epochs.

Projections of main sequence locus in gri and riz.

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Sloan SDSS PrecursorSystematic Errors and Uniformity of Photometry

Uniformity of zero points:

gri 5 milli-mags uz 10 milli-mags.

Meets LSST goals.

Errors in photometric flat-fielding determined from averaging large numbers of stars across the sky within fixed detector boundaries.

gri

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Toward Absolute LSST Photometry

1. Instrumental “flat field” and calibration. Stable and uniform reconstruction of photons in the telescope pupil.

Absolute calibration of detector.

2. Measure atmospheric extinction and emission. Photons at the top of the atmosphere.

3. Image processing, standardization, and verification. Algorithms and celestial standards.

An R&D program.

Separate the problem into three parts:

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Instrumental Flat FieldingLSST and PanSTARRS Collaboration

Calibrated Photodiodes

Dome Screen

Tunable Laser

nmQ

E Wavelength dependence calibrated at NIST with relative accuracy of a part in a thousand or better.

Product is a “flat-cube” of combined optical efficiency and electronic response at coordinates (i, j, ).

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Somta Corp of Riga, Latvia 800 m fused silica.

450-1100 nm bandpass.

Side-Emitting Optical Fiber

Mirror

Diffuser

Collimator

Back-Lit Diffuse Dome ScreenConcept Sketch

Y. Brown (Harvard)

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FPA Optical Calibration

Calibrated photodiodes in FPA – absolute sensor response.

• Monitor flux at focal plane during instrumental flat-fielding.

• Scan standard stars across photodiodes and sensors.

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Camera ProductionIndividual Sensor Tests at BNL

sensorPC

CfA controller

temp. controller

dewar

Full Prototype Testing

QE Fringe patterns Dark current CTE and cosmetics Crosstalk Full well Gain and RON Persistent image

stagecontroller

x-y-z stage withpinhole projector

lamp

temp. controller

dewarlamp

integratingsphere

filtershutter

calibratedphotodiode

light-tight box

Production SensorsVendor and BNL optical and electronic acceptance tests.Precise metrology done to specifications.

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Camera ProductionOptical Calibration of Camera Subsystems

Will do optical calibrations of assembled rafts and final camera.

Optical flat-field of raft at BNL.Goal is ~ 1% relative calibration at (i, j, ).

Optical flat-field of assembled camera at SLAC.Goal is ~ 0.5% relative calibration at (i, j, ).

But subsystem calibrations at BNL and SLAC are not well defined in terms of SRD specifications since we can not create the LSST optical beam.

Other issues:Uniformity of illumination - vignette and scattered light (any set-up).Thermal calibration and control.Cleanliness and repeatability of test set up (especially raft-level).

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Integrated Camera Test and Calibration

When …Camera is completed and sitting in SLAC assembly room.Electronics and DAQ working.Peripherals (shutter, filters, etc) in place and working.

GoalVerify we are ready to ship the Camera to the mountain.

MethodRun the camera as if it were taking data on the telescope!

Images to Record and AnalyzeBias frames.Darks (long and short).Flats.Laser “stars”.

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Assembled Camera Optical Calibration

Challenge to obtain uniform illumination through all and/or part of refractive optics.

Fiber driven screen?Integrating sphere?

Calibrated Photodiodes

Tunable Laser

Camera

Laboratory Screen (TBD) – Illumination? – Shape?

Goal is “flat-field” at 0.5%, with transfer of calibration to camera on the telescope uncertain by perhaps a “smooth” function.

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Laser “Star” Schematic

Reference Photodiode

Photodiode Array

(or Telescope)

Not To ScaleL1

L2

Filter

L3

FPA

30 m (Approximate FWHM of LSST PSF at 0.6 arc-sec seeing.)

Laser Source(2.6 cm aperture)

14 – 23.6 degrees

Reflectivity R ~ 0.3%. (Need ray-trace of the optics.)

300 m(4cm away)

(Not all reflections shown.)