1. LSST Sensor Requirements and Characterization of prototype LSST CCDs V. Radeka, J. Frank, J.C. Geary, D.K. Gilmore, I. Kotov, P. O’Connor, P. Takacs, J.A. Tyson

2. Large Synoptic Survey Telescope WIDE A large aperture, wide field survey telescope and 3200 Megapixel camera to image faint astronomical objects across the sky. FAST rapidly scan the sky, charting objects that change or move: from exploding supernovae to potentially hazardous near-Earth asteroids. DEEP trace billions of remote galaxies, providing multiple probes of the mysterious dark matter and dark energy.

3. Large Synoptic Survey Telescope WIDE close to the whole sky coverage. The survey area will include 30,000 deg2 and ~ 20,000 deg2 region will be observed ~1000 times FAST a single visit exposure time is less than 40 seconds DEEP single visit depth should reach to r~24 (5 sigma for point sources). Science drivers: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way

4. Large Synoptic Survey Telescope f-ratio: f/1.23 apperture: 8.4 m field-of-view 9.6 sq.deg etendue: 318 m2 deg2 plate diameter: 65 cm plate scale: 0.2”/pixel pixels: 3.2 Gigapixel band: 320-1050 nm single visit depth: r~24 exposure time: 2*15 sec readout time: 2 sec The image quality is limited by the atmosphere and telescope hardware should not degrade this level of the image quality. The LSST field-of-view area is maximized to its practical limit 9.6 sq.deg. The primary mirror size is driven by required survey depth (for adopted sky coverage and survey lifetime).

5. Large Synoptic Survey Telescope

7. camera & sensors LSST camera should be –large ~65 cm (FOV&mirror size) –flat +/- 5 micron (fast optics) –fast 2 s readout time (= dead time) –high quantum efficiency (QE) over the broad wavelength band (depth) –low noise level (depth) CCD technology was chosen for focal plane sensors

8. CCD technology challenge Science driverChallengecriteria broadband QEfully depleted thick SiQE(1000) > 30% thin back contactQE(400) > 40% image qualitypixel size10 micron low charge diff.<3.2 micron rms read noise< 5 e- dark current< 2e-/pix/sec persistence< 10 -4 full well> 90,000 flatness< 5 micron p-v high throughputmultiport output16 outputs 4K*4K fill factor> 93%

9. sensor tradeoffs QE and Point Spread Function (thickness) QE and Dark Current (temperature) readout speed and noise & segmentation

10. ~300μm at 173K ~4nm at 350nm QE and PSF problem QE and Window problem QE

11. Light spot, cone, absorption→ionization, charge diffusion → PSF Point Spread Function (PSF) in Si Simulation by P. Takacs, BNL: LSST (f~1.2!)

12. CCD test facility

13. Purpose-built test equipment Point projector Fringe projector In-cryostat X-ray source swing arm LN2 autofill

14. 55 Fe calibration: gain, CTE

15. CTE: 200808 data set, device 106-07, T=-140C initial distributionafter correction X direction, serial transfer CTE=0.999911 Y direction, parallel transfer CTE=0.999996 55 Fe calibration: CTE

16. Temp., C kT, eV CTE CTI serial parallel serial parallel 55 Fe calibration: CTE vs temperature

17. QE

18. PSF: 4 methods under study VIRTUAL KNIFE EDGE MODULATION TRANSFER FUNCTION X-RAY CLUSTER SIZE COSMIC RAY TRACK WIDTH 5.0 4.5 4.0 meas.  m

19. Readout Speed, Noise, Dark Current science driver – depth of single visit exposure  5e- @ 2s readout time 5e- achievable @ 500 kpix/s (source follower analysis)  1 Mpix segments  16 segments for 4K*4K sensor

20. Readout Speed, Noise, Dark Current thermally generated dark current is another equally important contribution to the system noise  limit is 2e-/pix/s

21. Full Well & Linearity The full well is about 300,000 electrons. The response is linear to within 0.5% from zero to 90% of full well. flat field illumination at increasing exposure times

22. Persistent Image There is no persistence image up to the level of measurements accuracy 3*10 -4 laser exposure followed by 20 laser off exposures, was repeated sixteen times without moving the laser spot

23. Reference Design 100mm-thick, p-type silicon, resistivity 5 – 10 kW-cm 4K x 4K CCD; 16 segments, 512x2048 pixel 16 low power, low noise source follower amplifier substrate bias voltage, applied from the front side of the device, biases the thin conductive window on the backside fully-depleted sensor average electric field of 5 – 10 kV/cm.

24. Flatness Estimated effect of the defocus for 550nm light,

25. Metrology station optical con-focal displacement sensor Keyence LT-9030

26. Float glass tiles, differential adjusters

27. Surface flatness of two material samples 0.65mm Si wafer, laser cut, CA glue to AlN ceramic 2.4mm float glass, epoxy to Al frame 90um 3 um

28. Flatness adjustment using lapped spacers

29. Best achieved flatness Lapped spacers Differential screw adjustors 1.8  m p-v best fit plane to each tile (remove warpage)

30. Summary and Outlook The LSST sensor study program has been an important step towards development of the advanced sensors needed to achieve the survey’s ambitious science goals. Vendors participating in the study program have demonstrated viable approaches to producing devices having extended red response with 100mm-thick, fully-depleted silicon; transparent back window contact; low dark current; negligible image persistence; high full well; flat silicon surface; and multiport output for fast readout. A characterization laboratory at BNL with fully automated image acquisition and analysis has verified the performance characteristics of the devices. The collaboration now plans to work with vendors to develop a full prototype meeting all critical LSST specifications that will allow procurement to begin.