1. LSST Photometric Calibration
D. Burke
SLAC/KIPAC
DOE SLAC Program Review
June 6-7, 2006

2. 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).

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

4. LSST Celestial Calibration Full 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.

5. Rapid-Paced Multi-Epoch Surveys Sloan SDSS Precursor
Southern Survey
300 deg2 along celestial equator.
Photometry for 870,000 stars observed in multiple epochs.
Main sequence stellar color locus is quite narrow. Use this to evaluate and monitor instrumental and observational parameters.
Projections of main sequence locus in gri and riz.
Z. Ivezic, et al. (SDSS)
Standards Workshop
Blankenberge, 2006.

6. Sloan SDSS Precursor Systematic Errors and Uniformity of Photometry
Errors in photometric flat-fielding determined from averaging large numbers of stars across the sky within fixed detector boundaries.
gri
Uniformity of zero points:
gri  5 milli-mags
uz  10 milli-mags.
 Meets LSST goals.

7. Toward Absolute LSST Photometry
An R&D program.
Separate the problem into three parts:
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.

8. Instrumental Flat Fielding LSST and PanSTARRS Collaboration
Calibrated Photodiodes
Dome Screen
Tunable Laser
 nm QE
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, ).

9. Back-Lit Diffuse Dome Screen
Concept Sketch
Side-Emitting Optical Fiber
Mirror
Diffuser
Collimator
Somta Corp of Riga, Latvia
800 m fused silica.
nm bandpass.
Y. Brown (Harvard)

10. 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.

11. Camera Production Individual Sensor Tests at BNLPC
CfA controller
temp.
controller
dewar
Full Prototype Testing QE
Fringe patterns
Dark current
CTE and cosmetics
Crosstalk
Full well
Gain and RON
Persistent image
stage
x-y-z stage with
pinhole projector
lamp
integrating
sphere
filter
shutter
calibrated
photodiode
light-tight box
Production Sensors
Vendor and BNL optical and electronic acceptance tests.
Precise metrology done to specifications.

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

13. 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.
Goal
Verify we are ready to ship the Camera to the mountain.
Method
Run the camera as if it were taking data on the telescope!
Images to Record and Analyze
Bias frames.
Darks (long and short).
Flats.
Laser “stars”.

14. Assembled Camera Optical Calibration
Challenge to obtain uniform illumination through all and/or part of refractive optics.
Fiber driven screen?
Integrating sphere?
Laboratory Screen (TBD)
– Illumination?
– Shape?
Camera
Goal is “flat-field” at 0.5%, with transfer of calibration to camera on the telescope uncertain by perhaps a “smooth” function.
Calibrated Photodiodes
Tunable Laser

15. Laser “Star” Schematic
Photodiode Array
(or Telescope)
Reference Photodiode
Laser Source
(2.6 cm aperture)
14 – 23.6 degrees L1 L2
Filter L3
FPA
Not To Scale
(Need ray-trace of the optics.)
Reflectivity R ~ 0.3%.
(Not all reflections shown.)
300 m
(4cm away)
30 m (Approximate FWHM of LSST PSF at 0.6 arc-sec seeing.)