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 5

JWST Near Infrared (NIR) Detector Requirements

16 January 2003 STScI TIPS 6

Detector Testing at STScI/JHU:Independent Detector Testing Laboratory

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 9

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

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 11

IDTL Measurements: Conversion Gain

Per correlateddouble sample

16 January 2003 STScI TIPS 12

IDTL Test System

Hawaii Detector

Hawaii Shirt

16 January 2003 STScI TIPS 13

Then & Now

November 2000

November 2002

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 26

Spatial Averaging: Before & After

Before After

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 30

Temporal Averaging: Before & After

Before After

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