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

2 Outline LSST Mission and Photometric Goals Celestial Approach and Existing Surveys Toward Absolute LSST Photometry Camera Production Calibration Plan

3 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).

4 Goals for Stellar Photometry 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).

5 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 = 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.

6 Rapid-Paced Multi-Epoch Surveys Sloan 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 Workshop Blankenberge, Southern Survey 300 deg 2 along celestial equator. Photometry for 870,000 stars observed in multiple epochs. Projections of main sequence locus in gri and riz.

7 Sloan SDSS Precursor Systematic 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 average within fixed detector boundaries of ensemble of stars. gri Dec (proxy for detector channel number)

8 Outline LSST Mission and Photometric Goals Celestial Approach and Existing Surveys Toward Absolute LSST Photometry Camera Production Calibration Plan

9 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:

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

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

12 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.

13 X-Ray In-Situ Calibration of FPA Decouple optical and electronic contributions by measuring the X-ray response of sensors, alone or “biased” by dome screen illumination. Absolute gain in e - /ADU. Offset and linearity. Read noise. Charge transfer efficiency. 1μCi 55 Fe with 15 sec sweep.  0.5% of pixels hit with 2500 hits per electronic segment. Windscreen Wiper Design Study

14 Outline LSST Mission and Photometric Goals Celestial Approach and Existing Surveys Toward Absolute LSST Photometry Camera Production Calibration Plan

15 Individual Sensor Tests at BNL sensor PC 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 controller x-y-z stage with pinhole projector lamp temp. controller dewar 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.

16 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, ).  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).

17 Assembled Rafts at BNL Raft-level metrology. FEE ASIC pulse tests. Optical flat-field QE (photon statistics) CTE and gain Read-out noise X-Rays CTE and gain Cosmetics Linearity (with optical) Raft Deliverables Raft-level metrology to specifications. Monochromatic flat-field to 1%. “Operational” parameters (those expected to be stable). Possible Optical Calibration Set-Ups Monochromatic screen (Harvard). Integrating sphere, or point source and lens. Laser raster. With tunable or red/green laser(s), precision photodiode monitors, baffling, etc.

18 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”.

19 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.

20 Laser “Star” Schematic Reference Photodiode Photodiode Array (or Telescope) Not To Scale L1 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.)