JWST's Near-Infrared Detectors: Ultra-Low Background Operation and Testing

14 January 2003 Special LASP Seminar at GSFC 1

JWST's Near-Infrared Detectors:Ultra-Low Background Operation and

Testing

Bernard J. RauscherSpace Telescope Science Institute

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Outline• What is a Near-Infrared Array Detector?• JWST Science Drivers• Detector Requirements• Detector testing at STScI/JHU• Optimal Use• Summary

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JWST’s IR Arrays are “Hybrid” Sensors• PN junctions are

“bump bonded” to a silicon readout multiplexer (MUX).

• Silicon technology is more advanced than other semiconductor electronics technology.

• The “bump bonds” are made of indium.

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1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

0.1 1 10

Wavelength [m]

Sign

al [e

-/sec

/pix

]

Zodiacal Light

Sunshield

JWST requirement

JWST goal

R=5

R=1000

JWST Needs Very Good Near Infrared Detectors!• Completing the JWST Design

Reference Mission “on time” requires background limited near-infrared (NIR) broadband imaging

• Zodiacal light is the dominant background component in the NIR

• The total NIR detector noise requirement is therefore =10 e- rms in a t=1000 seconds exposure.

• NIRSpec will probably be detector noise limited. The total noise goal is =3 e- rms per 1000 seconds exposure

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JWST Near Infrared (NIR) Detector Requirements

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Detector Testing at STScI/JHU:Independent Detector Testing Laboratory

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Past and present personnelEddie BergeronData Analyst

Mike TelewiczIntern

Gretchen GreeneMechanical Engineer

Monica RiveraIntern

Russ PeltonTechnician

Tom ReevesLab Technician

Bernie RauscherProject Scientist

Steve McCandlissJHU Lead

Scott FelsIntern

Sito BallezaSystems Engineer

Robert BarkhouserOptical Engineer

Utkarsh SharmaGraduate Student

Ernie MorseData Analyst

Don FigerDirector

Mike ReganSystem Scientist

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Dark Current• Lowest measured dark current is ~0.006 e/s/pixel.

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• Read noise is ~10 e for Fowler-8. (system read noise is ~2.5 e)

IDTL Measurements: Read Noise

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IDTL Measurements: Conversion GainPer correlateddouble sample

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Hawaii 1R with 5 um Cutoff

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IDTL Measurements:Relative and Absolute Quantum Efficiency

Relative Quantum Efficiency for H1RG, 10/18/2002

We are currently working on better calibration to enable measurements of absolute QE vs. wavelength.

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Relative QE Maps• Relative QE maps show significant structure

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IDTL Test System

Hawaii Detector

Hawaii Shirt

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Then & Now

November 2000

November 2002

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IDTL First Light Images

Jan. ‘01 (MUX)

Raytheon ALADDIN

Feb. ‘02 (MUX)

Apr. ‘02 (SCA)

Rockwell HAWAII-1R Rockwell HAWAII-1RG

Jun. ‘02 (MUX) Jul. ‘02 (SCA)

Raytheon SB-304

Nov. ‘02 (MUX)

Rockwell HAWAII-2RG

Jan. ‘03 (MUX)

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IDTL Test SystemLeach II Controller Electronics

Vacuum Hose

He Lines

EntranceWindow

Dewar

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Detector Readout System

Unix Instrument Control Computer

COTS Leach II IR Array Controller

Warm Harness

Cryogenic Harness

Detector Customization Circuit

JWST SCA

T~293 K

T=30-50 K

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An Adaptable Readout System• The only hardware change

required to run a different detector is swap-in a DCC.

• We have DCCs for the following detectors.

– Raytheon• SB-290• SB-304

– Rockwell• HAWAII-1R• HAWAII-1RG• HAWAII-2RG

• Each DCC is a multi-layer PCB. Extensive use of surface mount technology. Includes flexible “neck” to simplify interfacing.

Rockwell HAWAII-2RG Detector Customization Circuit (DCC)

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Close-up ofDetector Customization Circuits

(DCCs)

Rockwell HAWAII-2RG Raytheon SB-290/SB-304

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Optimal Use

• JWST Detector Readout Strategies• Anomalies seen in other instruments• Other effects…• Use of Reference Pixels

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Detector Readout• JWST science requires

MULTIACCUM and SUBARRAY readout.

• Other readout “modes” can be implemented using parameters.– For example, Fowler-8 can be

implemented as MULTIACCUM-2x8.

• Cosmic rays may be rejected either on the ground or on-orbit. MULTIACCUM parameters: texpose = exposure time,

t1 = frame time, and t2 = group time. The small overhead associated with finishing the last group of samples is not included in the exposure time.

MULTIACCUM Detector Readout

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NICMOS Anomalies (& how JWST will avoid them)• Dark current

– JWST detectors already designed to minimize glow

– Careful detector characterization & selection– Do not exceed max temp. requirement!

• Bias drifts– Good electronic design

• Avoid power supply coupling• Avoid ground coupling• Reference pixels will help• Synchronous readout can help

• QE variations– Careful detector characterization & selection

• Amplifier glow– JWST detectors should be much better than

NICMOS

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NICMOS Anomalies: 2• Persistence

– There will be persistence on JWST

– Strongly dependent on detector fabrication process

– Careful detector characterization & selection needed to choose best detectors

– In IDTL, we are exploring mitigation measures

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NICMOS Anomalies: 3• DC bias level drift

– Good electronic design is first line of defense

– Reference pixels should eliminated “Pedestal” drifts.

– Depending on reference pixel layout, reference pixels may help reject “bands”.

• Ghosts– In NICMOS, may result from

ground plane coupling within the MUX.

– Also seen in SIRTF InSb radiation testing.

– Good cable harness and electronic design help

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NICMOS Detector Effects• Linearity

– In NICMOS, ~10% intrinsic non-linearity can be calibrated out to within ~0.2%.

• Well depth– Well-depth is a function of reverse

bias in photo-voltaic detectors.– Well-depth can also depend on

temperature.– In the IDTL, we will study well

depth as a function of reverse bias and temperature.

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NICMOS Detector Effects: 2

• QE– Can depend on

wavelength and temperature.

• Dark current “bump”– This is a curious

effect seen in NICMOS.

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Reference Pixels

Raytheon 2Kx2K NIR Module

Rockwell 2Kx2K NIR Module

• All candidate JWST detectors have reference pixels

• Reference pixels are insensitive to light

• In all other ways, designed to mimic a regular light-sensitive pixel

• NIR detector testing at University of Rochester, University of Hawaii, and in the IDTL at STScI -> reference pixels work!

• Reference pixel subtraction is a standard part of IDTL data reduction pipeline

Raytheon 1024x1024 MIR MUX

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Use of Reference Pixels• JWST’s NIR reference pixels will be grouped in columns and possibly

rows• Most fundamentally

– reference pixels should be read out in exactly the same manner as any “normal” pixel

– Data from many reference pixels should be averaged to avoid adding noise to data

• We have begun to explore how reference pixels should be used. Approaches considered include the following.– Maximal averaging (average all reference pixels together and subtract the

mean)– Spatial averaging– Temporal averaging

• Spatial averaging is now a standard part of IDTL calibration pipeline

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A Picture of IDTL System Noise

• Shorting resistor mounted at SCA location• 1/f “tail” causes horizontal banding.• Total noise is =7 e- rms per correlated double sample.

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Averaging small numbersof reference pixels adds noise

• Averaged the last 4 columns in each row and performed row-by-row subtraction

Before After

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Spatial Averaging• In spatial averaging, data from

many (~64 rows) of reference pixels are used to calibrate each row in the image

• A Savitzky-Golay smoothing filter is used to fit a smooth and continuous reference column

• This reference column is subtracted from each column in the image

• Using this technique, we can remove some 1/f noise power within individual frames

• In practice, this technique works very well

This is a standardpart of the IDTL datacalibration pipeline

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Spatial Averaging: Before & After

Before After

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Temporal Averaging• Dwell on the reference

pixel and sample many times before clocking next pixel

• Potentially removes most 1/f

• Not tried this in IDTL yet. U. Hawaii has reported some problems with reference pixels heating up

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Temporal Averaging: Before & After

Before After

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Summary of Reference Pixel Calibration Methods• Spatial averaging works

well using a Rockwell HAWAII-1RG detector

• Based on conversations with U. Rochester, we foresee no problems with SB-304

• Temporal Averaging is promising. More work needed using real detectors.

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Summary• The Independent Detector Testing Laboratory (IDTL) at STScI/JHU is up

and running• Test results including dark current, read noise, conversion gain,

relative quantum efficiency, and persistence are in good agreement with other JWST test labs

• Reference pixels work and are an invaluable part of the data calibration pipeline

• We have explored three techniques for using reference pixels– Maximal averaging,– Spatial averaging, &– Temporal averaging

• Spatial averaging works well and is robust• Early reports from U. Hawaii using temporal averaging are not

encouraging due to reference pixel self-heating. More work is planned in the IDTL