Overview – LSST Camera, sensors, science raft subsystem

P. O’Connor BNLJan. 25, 2012

LSST PROJECT

LSST project• Imaging survey of the entire sky with a large ground-based telescope• NSF/DOE joint project:

• Key innovations:– Wide field of view– Wide spectral coverage– Fast readout– Camera located in optical beam

NSF (lead) DOE

Provides:

Telescope Camera

Site

Data management

Science goals:

solar system inventory Dark energyMilky way map Dark matterOptical transients

Preliminary Design Review • Tucson, Arizona • August 29th – September 2nd, 2011 5

The LSST Optical System - Modified Mersenne-Schmidt/Paul-Baker Design

Primary and Tertiary Mirrors

Secondary Mirror

Camera Lenses

Rest of camera

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STAR LSST

size compared to STARdetector

8.4m

LSST CAMERA AND FOCAL PLANE

Camera – cross section

1.6m

Focal plane

Science rafts(3 x 3 CCDs)

Corner rafts

Guide sensors

Wavefront sensors

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TOWER• CCDs + front end electronics• 180K operation• An autonomous, fully-testable and serviceable 144 Mpixel camera

thermal straps

FEE boards

housing (cold mass)

cooling planes

RAFT• 9 CCDs• coplanarity 13.5mm 12.5 cm

4K x 4K CCD•10mm pixels = 0’’.2• extended red response• 16 outputs• 5mm flatness

4 cm

CRYOSTATFOCAL PLANE WITH 21 SCIENCE RAFTS

21 “science rafts” make up the 3.1Gpix focal plane

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LSST’s camera will surpass existing survey instruments in four areas:

The largest focal planeLSST: 3.1Gpix (189 CCDs)PanSTARRS GPC1: 1.4Gpix (60 CCDs)HyperSuprimeCam: 940Mpix (112 CCDs)DECam: 500Mpix (60 CCDs)CFHT MegaCam: 340Mpix (36 CCDs)

The fastest focal ratioLSST: f/1.23SuprimeCam: f/1.87DECam: f/2.7PanSTARRS: f/4CFHT MegaCam: f/4.2

The fastest readout timeLSST: 2sPanSTARRS GPC1: 6sDECam: 17sCFHT MegaCam: 40sSuprime-Cam: 18s

The highest data rateLSST: 1.0TB/hrPanSTARRS GPC1: 0.22HSC 0.03DECam: 0.004CFHT MegaCam: 0.003

DECam

HSC

MegaCam

GPC1

LSST

REQUIREMENTS

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LSST system performance goals

• High etendue: – primary mirror effective aperture 6.7m, field of view diameter 3.5 degrees

• High throughput:– > 80% (temporal), > 90% (spatial), > 80% spectral efficiency (visible)

• Short exposures:– two back-to-back 15s exposures per “visit”

• Image quality: – the system contribution to delivered image size should not exceed 15%

• Sensitivity: – achieve 5s image depth of r=24.7 in 30s (about 10-4 photons/cm2/s)

• Photometric repeatability:– 0.5%, accuracy 1%

• Astrometric accuracy:– .05” (240nrad)

• Survey lifetime: – 10 years (3x106 visits over 20,000 square degrees of sky, ~40PB image data)

LSST’s angular resolution is about 0.5 arcsec

…15 more lines

60,000 pixels

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Unique science goals drive sensor design

Large field of view implies physically large focal plane (64cm F)

Modular mosaic focal plane construction

21 rafts, 9 (4K)2 CCDs/raft189 CCDs total3.1Gpix

Fast f/1.2 beam, shallow depth of focus

Tight alignment and flatness tolerance

Flatness: 5mmAlignment (z axis): 10mm

Plate scale 20”/mm Small pixels, close butting

Pixel: 10mmChip-chip gap: 250mm

Fast readout (2s) with low noise (5 e-)

Highly parallel readout electronics

16 amplifiers/(4K)2 CCD

Broadband, high spectral sensitivity

Thick silicon sensor, back illuminated, AR coat

100mm thickness for IR sensitivityThin conductive window

Seeing-limited image quality

Internal electric field to minimize diffusion

High resistivity, biased silicon (> 3 kW-cm, -50V)

LSST CD-1 Review • SLAC, Menlo Park, CA • November 1 - 3, 2011 16

LSST science demands a camera sensor that pushes the state of the art in imaging technology

• Broadband wavelength coverage with a single detector type– Sensitivity from 320 – 1050 nm

• Flat imaging surface in f/1.23 beam– Shallow depth of focus (13.5mm p-v within a raft)

• Fewest possible wasted photons– 90% packing fraction mosaic focal plane– 80% of time spent exposing

• PSF size and shape control– <10% degradation due to sensors

• Read noise < darkest sky background shot noise– 7 e- rms

• Manufacturability– Largest number of CCDs in any astronomical camera– Reproducible characteristics, no per-device tuning

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Requirements

• Quantum Efficiency• Image Quality

– Image size and ellipticity• Charge diffusion• Charge transfer inefficiency• Beam divergence in silicon• Non-flatness and z-height variation

– Full well capacity– Residual image– Dark current and cosmetic defects

• Electrical Requirements– Read noise and readout time– Linearity– Crosstalk– Power dissipation

• Fill factor and pixel size

LSST CD-1 Review • SLAC, Menlo Park, CA • November 1 - 3, 2011 18

Summary of the SRFT requirements to which sensors contribute

Parameter Min Max unitQE u 41 %

QE g 78 %

QE r 88 %

QE i 81 %

QE z 75 %

QE y4 14 %

Pixel size 10 10 mm

Diffusion 5.5 mm rms

Read time 2 sec

Read noise/pixel/exposure 7 e- rms

Video crosstalk .05 %

Gain stability .05 %

Nonplanarity 13.5 mm p-v in raft

Inactive area 8 %

Complete set of SRFT requirements captured in controlled document LCA-57

sensors

sensors + electronics

sensors + RSA

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Noise

• LSST signal-to-noise ratio should be limited by sky background:

BAND u g r i z y4sky (e-/pixel/15s) 43 234 543 900 1388 1807sky noise (rms e-/pixel) 6.5 15.3 23.3 30.0 37.3 42.5

• The dominant noise contribution at SNR=5 is the sky in all bands except the u-band.

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(a)Estimated read noise for a 4K x 4K sensor constrained to 2s readout time. Noise estimates based on CCD output transistor properties with three values of sense node capacitance. (b) Proposed layout of a 16-fold segmented, 4K x 4K CCD for fast, low-noise readout.

Segmentation vs. Read Noise

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Additional electrical requirements

• Linearity3% for signals from 100e- to full well limit

Driven by requirements on photometric accuracy

• Crosstalk10-4 from all electrical effects (sensor + readout electronics)

• Power dissipation– Sensors dissipate power in their source-follower amplifiers and while clocks are being driven.– Target power dissipation of same order as IR head load from L3 lens + warm cryostat walls.– Approx. 0.4W per 4Kx4K CCD

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Technology selection

• Sensor technologies considered:– Monolithic and hybrid CMOS– CCD

• CMOS technologies promising due to:– low power– integrated electronics, flexible addressing modes– No CTI– “electronic shutter”

• Monolithic CMOS difficult to reach high sensitivity and NIR response• Hybrid p-i-n/CMOS limited use in astronomy to date:

– Noise higher than CCD at same frame rate– Significant number of isolated dark pixels (bump bond failures)– Dark current density relatively high– Interpixel capacitance induces undesired correlations– Problems with oversaturated illumination:

• Residual image at the level of 8e-/pix/s at 120K, persists for hours after 100X full well illumination

• Permanent threshold shift of ROIC input transistor• Although hybrid and monolithic CMOS would simplify some engineering

aspects of LSST’s large-area focal plane, concerns about performance, technological maturity, and high cost led us to choose CCDs as the baseline technology for the science, wavefront sensing, and guiding arrays.

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Mechanical design

• Non-imaging periphery of a FDCCD has to include space for amplifiers, electrical busses, and field-terminating structures (guard rings).– Guard ring area > 2.5X chip thickness to keep lateral fields in substrate negligibly small.

• 4K x 4K, 10mm pixel CCD– Require 4-side buttable package on 42.5mm pitch.

• Imaging surface flatness, package tip,tilt, and piston controlled to maintain focus:– Require imaging surface of all chips 13.000 .005mm above baseplate.

• 0.8W heat removed through package frame and mounting points to the raft:– Require thermal impedance ≤ 5°C/W.

SENSORS: IMPLEMENTATION

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These design choices lead to the LSST sensor reference design

• 100mm-thick, high resistivity silicon CCDs, fully-depleted with transparent conductive window– for broadband QE and small

PSF• 4K x 4K format with 16-fold parallel

output – for fast, low noise readout

• 10mm pixels – for optimum sampling at LSST

plate scale• Buttable, >92% fill factor packaging

– for minimum inactive area• Flatness and alignment tolerance to

bring image surface to 13.000 ±0.0065mm from baseplate– for use in fast f/1.2 beam

• Mounting and alignment feature geometry specified– ensure mechanical compatibility

between vendors

Vendors allowed to develop implementation details (window formation, amplifier design, package, electrical interface, etc.)

4K x 4K science sensorFunctional characteristics

• 10 um pixels• 16 segments each 2k x 512• 16 readouts per CCD• 100um thick silicon, back-

illuminated• Fully depleted • Operated at -100C

Key requirements:• Broadband spectral response• Fast parallel readout

• 500 kpix/s 2s• Flat imaging surface

• 5um peak-valley• Small PSF

• 5.5um rms• Low read noise

• 5 e- rms

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wirebonds

frontside bond pads

bump bonds

multilayer ceramic

frame

alignment pins

backthinned CCD

(a)

(b)

Buttable Package Construction

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Fill factors

Sensor imaging area package Ratiox y Area x y Area

40.04 40.96 1640.038 42.00 42.00 1764.00 93.0%

Raft Sensor active area Raft Ratiox y Area

9 x 1640.04 = 14760.35 126.5 126.5 16002.25 92.24%

Focal Plane Raft active area Raft pitch RatioArea x y Area14760.35 127 127 16129 91.51%

RSA fill factors

Gap between outermost imaging pixels on adjacent chips0.75 – 1.8mm (15 – 37 arcsec)

RAFT TOWER MODULE

12K x 12K science raft tower

FEC

RCC

Raft-SensorAssembly (RSA)

Front End Cage (FEC)

Raft Control Crate (RCC)

Components of the science Raft Tower Module (RTM)

3x3 array of CCDs on flat SiC baseplate-100C

Clock and bias bufferingAnalog signal processingDifferential line drivers/receivers-100C

Video digitizingData serializerMonitoring and diagnosticsSlow controls-40C

Raft-Sensor Assembly (RSA)

Raft Control Crate (RCC)

Front End Cage (FEC)

power, control, cooling pixel data

LSST 2009 Camera Meeting – Raft Tower Modules 32

Deliverables List• 21 Science Raft Tower Modules (RTMs), each of which consists of:

– Nine 4Kx4K science CCDs– Raft baseplate – Front-End Electronics (FEE)

• Three FEE boards Type A• Three FEE boards Type B• FEE board supports• FEE enclosure

– Raft thermal management components• Thermal planes to cryoplate• Raft thermal straps• Raft temp sensors• Raft heaters

– Raft mechanical hold-down components– Raft conductance barrier– Shipping box– Performance report

• Spare raft tower modules– Quantity TBD

• Documentation– Operating manual– System safety information (thermal/electrical limits)– Installation/repair/rework procedures

CONSTRUCTION PLAN

LSST 2009 Camera Meeting – Raft Tower Modules 34

Integration flow in production

CCDCCD

CCD

raftraft

raft

Electrical/optical test

Metrology

Assemble and align(warm)

Database

raftCCD CCD CCD

Integrate electronics

test(warm)

Coldacceptance

test;ship to SLAC

Database

(225 units)

(25)

(180)

FEBFEBFEB

ASICs(1200)

purchased component

purchased component

LPNHE,Paris

U. Penn

Harvard

(30)

RSA

BEE

RTM

LSST CD-1 Review • SLAC, Menlo Park, CA • November 1 - 3, 2011 35

We have put in place a phased prototype development program with external vendors

Phase 1Technology development

Phase 2Full spec prototypes

2a – pkg2b – operable

Phase 3Extended mfg. demo

First articles

CD1 CD3a CD2

3 vendors

1 vendor

Production

CD0

39 mo

A more detailed schedule will be shown in the subsystem section of the talk

2 vendorsAdd 3rd vendor?

Teledynee2vITL/STA

e2vITL/STA

2 vendors

RESULTS

LSST CD-1 Review • SLAC, Menlo Park, CA • November 1 - 3, 2011 37

CCD prototypes from 2 vendors

e2v CCD250 (operable)

STA3800 (mechanical sample) (cold probe, Aug. 24)

LSST 2009 Camera Meeting – Raft Tower Modules 38

CCD250 measurements at BNL

37-wire interface, side A

CCD250

handling jigshorting plug

connector saver

6”CF w/triple 150-pin feedthrough

LSST 2009 Camera Meeting – Raft Tower Modules 39

Two CCD250 buttable packages on raft (e2v)

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Package mechanical samples on silicon carbide raft prototype

LSST 2009 Camera Meeting – Raft Tower Modules 41

Initial assembly of mechanical samples to raft

13.5mm window

Expect improvement with• flatter sensors• torque and spring control