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

14 January 2003 Special LASP Seminar at GSFC 2

Outline

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

14 January 2003 Special LASP Seminar at GSFC 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.

14 January 2003 Special LASP Seminar at GSFC 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

14 January 2003 Special LASP Seminar at GSFC 5

JWST Near Infrared (NIR) Detector Requirements

14 January 2003 Special LASP Seminar at GSFC 6

Detector Testing at STScI/JHU:Independent Detector Testing Laboratory

14 January 2003 Special LASP Seminar at GSFC 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

14 January 2003 Special LASP Seminar at GSFC 9

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

14 January 2003 Special LASP Seminar at GSFC 10

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

IDTL Measurements: Read Noise

14 January 2003 Special LASP Seminar at GSFC 11

IDTL Measurements: Conversion Gain

Per correlateddouble sample

14 January 2003 Special LASP Seminar at GSFC 12

Hawaii 1R with 5 um Cutoff

14 January 2003 Special LASP Seminar at GSFC 13

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.

14 January 2003 Special LASP Seminar at GSFC 14

Relative QE Maps• Relative QE maps show significant structure

14 January 2003 Special LASP Seminar at GSFC 15

IDTL Test System

Hawaii Detector

Hawaii Shirt

14 January 2003 Special LASP Seminar at GSFC 16

Then & Now

November 2000

November 2002

14 January 2003 Special LASP Seminar at GSFC 17

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)

14 January 2003 Special LASP Seminar at GSFC 18

IDTL Test SystemLeach II Controller Electronics

Vacuum Hose

He Lines

EntranceWindow

Dewar

14 January 2003 Special LASP Seminar at GSFC 19

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

14 January 2003 Special LASP Seminar at GSFC 20

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)

14 January 2003 Special LASP Seminar at GSFC 21

Close-up ofDetector Customization Circuits

(DCCs)

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

14 January 2003 Special LASP Seminar at GSFC 22

Optimal Use

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

14 January 2003 Special LASP Seminar at GSFC 23

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

14 January 2003 Special LASP Seminar at GSFC 24

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

14 January 2003 Special LASP Seminar at GSFC 25

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

14 January 2003 Special LASP Seminar at GSFC 26

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

14 January 2003 Special LASP Seminar at GSFC 27

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.

14 January 2003 Special LASP Seminar at GSFC 28

NICMOS Detector Effects: 2

• QE– Can depend on

wavelength and temperature.

• Dark current “bump”– This is a curious

effect seen in NICMOS.

14 January 2003 Special LASP Seminar at GSFC 29

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

14 January 2003 Special LASP Seminar at GSFC 30

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

14 January 2003 Special LASP Seminar at GSFC 31

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.

14 January 2003 Special LASP Seminar at GSFC 32

Averaging small numbersof reference pixels adds noise

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

Before After

14 January 2003 Special LASP Seminar at GSFC 33

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

14 January 2003 Special LASP Seminar at GSFC 34

Spatial Averaging: Before & After

Before After

14 January 2003 Special LASP Seminar at GSFC 35

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

14 January 2003 Special LASP Seminar at GSFC 36

Temporal Averaging: Before & After

Before After

14 January 2003 Special LASP Seminar at GSFC 37

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.

14 January 2003 Special LASP Seminar at GSFC 38

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