16 January 2003 STScI TIPS 1
JWST's Near-Infrared Detectors:Ultra-Low Background Operation and
Testing
Bernie Rauscher, Don Figer, Mike Regan, Sito Balleza, Robert Barkhouser, Eddie Bergeron, Gretchen Greene, Ernie Morse,
Steve McCandliss, Russ Pelton & Tom Reeves
And coming soon!
16 January 2003 STScI TIPS 2
Outline
• What is a Near-Infrared Array Detector?• JWST Science Drivers• Detector Requirements• Detector testing at STScI/JHU• Optimal Use• Summary
16 January 2003 STScI TIPS 3
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.
16 January 2003 STScI TIPS 4
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]
Sig
na
l [e
-/s
ec
/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
16 January 2003 STScI TIPS 7
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
16 January 2003 STScI TIPS 10
• Read noise is ~10 e for Fowler-8. (system read noise is ~2.5 e)
IDTL Measurements: Read Noise
16 January 2003 STScI TIPS 14
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)
16 January 2003 STScI TIPS 15
IDTL Test SystemLeach II Controller Electronics
Vacuum Hose
He Lines
EntranceWindow
Dewar
16 January 2003 STScI TIPS 16
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
16 January 2003 STScI TIPS 17
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)
16 January 2003 STScI TIPS 18
Close-up ofDetector Customization Circuits
(DCCs)
Rockwell HAWAII-2RG Raytheon SB-290/SB-304
16 January 2003 STScI TIPS 19
Optimal Use
• JWST Detector Readout Strategies• Use of Reference Pixels
16 January 2003 STScI TIPS 20
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
16 January 2003 STScI TIPS 21
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
16 January 2003 STScI TIPS 22
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
16 January 2003 STScI TIPS 23
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.
16 January 2003 STScI TIPS 24
Averaging small numbersof reference pixels adds noise
• Averaged the last 4 columns in each row and performed row-by-row subtraction
Before After
16 January 2003 STScI TIPS 25
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
16 January 2003 STScI TIPS 27
Spatial Averaging:Example using Rockwell HAWAII-1RG Detector
Rockwell HAWAII-1RG Double Correlated Sampling image. Read noise is ~15 e- rms (=5.3 e- using Fowler-8 sampling).
Fit to reference columns using Savitzky-Golay filtering to smooth averaged reference pixel data in each row..
16 January 2003 STScI TIPS 28
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.
16 January 2003 STScI TIPS 29
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
16 January 2003 STScI TIPS 31
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