JWST Radiation Environment 1March 13, 2003

Reference pixels and readout modes: What we have learned thus far

Don Figer, Bernie Rauscher, Mike ReganMarch 13, 2003

JWST Radiation Environment 2March 13, 2003

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

0

1

2

3

4

2 4 6 8

Read noise per exposure [electrons]

Du

rati

on

of

DR

M N

IR O

bs

erv

ati

on

s [

yrs

]

Images

Spectra

Dark current =

0.126 e /sec

0.020 e /sec

0.003 e /sec

Detectors Are Important for JWST

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NIR Detector Characteristics

Dark current

Read noise

Linearity

Latent charge (persistence)

Quantum efficiency (QE)

Intra-pixel sensitivity

Thermal stability

Radiation immunity

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IDTL Test System

Controller Electronics

Vacuum Hose

He Lines

EntranceWindow

Dewar

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JWST MIR Detector Requirements

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Dark Current

Lowest measured dark current is ~0.005 e/s/pixel.

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

IDTL Measurements: Read Noise

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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

JWST Radiation Environment 9March 13, 2003

Use of Reference Pixels

JWST’s NIR reference pixels are grouped in columns and 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.

– row-by-row subtraction

– 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

JWST Radiation Environment 10March 13, 2003

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

Before After

JWST Radiation Environment 12March 13, 2003

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

JWST Radiation Environment 13March 13, 2003

Spatial Averaging: Before & After

Before After

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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

JWST Radiation Environment 15March 13, 2003

Temporal Averaging: Before & After

Before After

JWST Radiation Environment 16March 13, 2003

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.

JWST Radiation Environment 17March 13, 2003

Summary

Reference pixels work and are an invaluable part of the data calibration pipeline

We have explored three techniques for using reference pixels

– row-by-row subtractions,

– maximal averaging,

– spatial averaging, &

– temporal averaging

Averaging at the end of row will not work

Spatial averaging works well and is robust

We have found:

– dark current is low (~0.01 e-/s/pixel)

– glow is very small

– noise goes down as roughly 1/root(N) up to 8 reads (at least)

– persistence is observed

– JWST requirements seem realizable

– saving all the data are necessary to mitigate unforeseen detector effects, such as the non-linear bias drift after reset ("shading" in NICMOS). Note that ref pixels do not get rid of all of the effect.

Cosmic ray rejection requires careful handling of reference pixels, output voltage drifts, and knowledge about previous history (persistence)

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Appendix

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NIR Detector Effects - NICMOS

Dark current

Bias drifts

QE variations

Amplifier glow

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NIR Detector Effects - NICMOS

Persistence

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NIR Detector Effects - NICMOS

DC bias level drift

Ghosts

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NIR Detector Effects - NICMOS

Linearity

Well depth

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NIR Detector Effects - NICMOS

QE

Dark current “bump”