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

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JWST's Near-Infrared DetectorsUltra-Low
Background Operation and Testing
And coming soon!

• Bernie Rauscher, Don Figer, Mike Regan, Sito
  Balleza, Robert Barkhouser, Eddie Bergeron,
  Gretchen Greene, Ernie Morse, Steve McCandliss,
  Russ Pelton Tom Reeves

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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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JWSTs 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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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 t1000 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/JHUIndependent
Detector Testing Laboratory
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• Past and present personnel

Eddie Bergeron Data Analyst
Tom Reeves Lab Technician
Robert Barkhouser Optical Engineer
Bernie Rauscher Project Scientist
Utkarsh Sharma Graduate Student
Mike Telewicz Intern
Gretchen Greene Mechanical Engineer
Steve McCandliss JHU Lead
Ernie Morse Data Analyst
Monica Rivera Intern
Scott Fels Intern
Don Figer Director
Sito Balleza Systems Engineer
Mike Regan System Scientist
Russ Pelton Technician
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IDTL Experiments

• Read noise
• Conversion Gain
• Dark current
• Linearity
• Electronic Gain
• Latent charge (persistence)
• Relative and Absolute Quantum efficiency (QE)
• Intra-pixel sensitivity

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Dark Current

• Lowest measured dark current is 0.006
  e-/s/pixel.

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IDTL Measurements Read Noise

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

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IDTL Measurements Conversion Gain
Per correlateddouble sample
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Hawaii Shirt
IDTL Test System
Hawaii Detector
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Then Now
November 2000
November 2002
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IDTL First Light Images
Rockwell HAWAII-1RG
Jun. 02 (MUX)
Jul. 02 (SCA)
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IDTL Test System
Leach II Controller Electronics
Dewar
Entrance Window
Vacuum Hose
He Lines
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Detector Readout System
T30-50 K
Unix Instrument Control Computer
Warm Harness
COTS Leach II IR Array Controller
T293 K
Cryogenic Harness
Detector Customization Circuit
JWST SCA
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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.

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Close-up ofDetector Customization Circuits (DCCs)
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Optimal Use

• JWST Detector Readout Strategies
• 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.

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Reference Pixels

• 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 -gt
  reference pixels work!
• Reference pixel subtraction is a standard part of
  IDTL data reduction pipeline

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Use of Reference Pixels

• 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

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Spatial Averaging
This is a standardpart of the IDTL
datacalibration pipeline

• 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

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Spatial Averaging Before After
Before
After
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Spatial AveragingExample using Rockwell
HAWAII-1RG Detector
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Spatial Averaging Works!

• IDTL dark ramp. Astrisks include reference pixel
  correction using the Spatial Averaging method.
  Pluses do not. Fitted slope is 0.006 .001
  e-/s/pixel.

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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

• The Independent Detector Testing Laboratory
  (IDTL) at STScI/JHU is up and running
• Test results including dark current, read noise,
  conversion gain, and persistence are in good
  agreement with other JWST test labs
• Reference pixels work and are an invaluable part
  of the data calibration pipeline
• Spatial averaging works well and is robust