JWST's Near-Infrared Detectors:
Ultra-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
16 January 2003 STScI TIPS 1

• 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 2

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 3

JWST Needs Very Good Near Infrared Detectors!
• Completing the JWST Design
Reference Mission “on time” requires background limited nearinfrared (NIR) broadband imaging
• Zodiacal light is the dominant background component in the NIR
• The total NIR detector noise requirement is therefore =10 erms 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
1.E+02
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
JWST requirement
JWST goal
R=5
Zodiacal Light
Sunshield
R=1000
1.E-04
0.1
1
Wavelength [ m m]
10
16 January 2003 STScI TIPS 4

JWST Near Infrared (NIR)
Detector Requirements
Parameter
Detector Noise per image
Read Noise (n
Fowler

16)
Dark Current
Requirement
9 e

RMS
NR
NR
Goal
2.5 e

RMS
NR
NR
InSb
Status
10.8 e

RMS
10.6 e

RMS
0.004 e

/s
QE
0.6
1.0 m m <

< 1.0 m m m m <

< 5.0 m m
Pixel-to-pixel uniformity
Operability
Latent Image
Fill Factor
Radiation Immunity

70%

80%
 
TBD

98%


0.1%
>95%
<4% inop EOM



90%
95%
TBD
99.5%
0
100% no effect unknown
>80%
~10%
NA
<0.5%
>98 unknown
Frame Time
Temperature
MTF
12 s
30 K < T < 37 K
0.53
<12 s allow

2 K
>0.53
<12s in range unknown a Assumes 1000 second exposure and quadrature sum of read noise and shot noise from dark current.
HgCdTe
6.7 e

RMS
4.6 e

RMS
0.02 e

/s
~10%
NA
<0.5 %
>98% unknown
<12 s in range unknown
16 January 2003 STScI TIPS 5

Detector Testing at STScI/JHU:
Independent Detector Testing Laboratory
16 January 2003 STScI TIPS 6

Past and present personnel
Eddie Bergeron
Data Analyst
Tom Reeves
Lab Technician
Mike Telewicz
Intern
Gretchen Greene
Mechanical Engineer
Monica Rivera
Intern
Russ Pelton
Technician
Bernie Rauscher
Project Scientist
Steve McCandliss
JHU Lead
Scott Fels
Intern
Sito Balleza
Systems Engineer
16 January 2003 STScI TIPS
Robert Barkhouser
Optical Engineer
Utkarsh Sharma
Graduate Student
Ernie Morse
Data Analyst
Don Figer
Director
Mike Regan
System Scientist
7

• Lowest measured dark current is ~0.006 e  /s/pixel.
16 January 2003 STScI TIPS 9

• Read noise is ~10 e  for Fowler-8. (system read noise is ~2.5 e  )
16 January 2003 STScI TIPS 10

IDTL Measurements: Conversion Gain
Per correlated double sample
16 January 2003 STScI TIPS 11

Hawaii Shirt
16 January 2003 STScI TIPS
Hawaii Detector
12

November 2000
November 2002
16 January 2003 STScI TIPS 13

Raytheon ALADDIN
Rockwell HAWAII-1R Rockwell HAWAII-1RG
Jan. ‘01 (MUX) Feb. ‘02 (MUX) Apr. ‘02 (SCA)
Raytheon SB-304
Jun. ‘02 (MUX) Jul. ‘02 (SCA)
Rockwell HAWAII-2RG
16 January 2003
Nov. ‘02 (MUX)
STScI TIPS
Jan. ‘03 (MUX)
14

Leach II Controller Electronics
Vacuum Hose
He Lines
16 January 2003 STScI TIPS
Dewar
Entrance
Window
15

T=30-50 K
Unix Instrument
Control Computer
COTS Leach II IR
Array Controller
Warm Harness
T~293 K
JWST SCA
16 January 2003
Detector Customization
Circuit
STScI TIPS
Cryogenic Harness
16

• 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 17

Rockwell HAWAII-2RG
16 January 2003
Raytheon SB-290/SB-304
STScI TIPS 18

Optimal Use
• JWST Detector Readout Strategies
• Use of Reference Pixels
16 January 2003 STScI TIPS 19

MULTIACCUM 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.
16 January 2003
MULTIACCUM parameters: t expose t
1
= frame time, and t
2
= exposure time,
= group time. The small overhead associated with finishing the last group of samples is not included in the exposure time.
STScI TIPS 20

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 -> reference pixels work!
• Reference pixel subtraction is a standard part of IDTL data reduction pipeline
Raytheon 2Kx2K
NIR Module
Rockwell 2Kx2K
NIR Module
Raytheon 1024x1024
MIR MUX
16 January 2003 STScI TIPS 21

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 22

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 23

Averaging small numbers of reference pixels adds noise
• Averaged the last 4 columns in each row and performed rowby-row subtraction
16 January 2003
Before
STScI TIPS
After
24

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 standard part of the IDTL data calibration pipeline
16 January 2003 STScI TIPS 25

16 January 2003
Before
STScI TIPS
After
26

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).
16 January 2003
Fit to reference columns using Savitzky-Golay filtering to smooth averaged reference pixel data in each row..
STScI TIPS 27

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 28

• 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 29

Temporal Averaging: Before & After
Before
16 January 2003 STScI TIPS
After
30

• 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
16 January 2003 STScI TIPS 31