1 TIPS 2011 May Persistence in the WFC3 IR detector Knox S. Long.

1TIPS 2011 May

Persistence in the WFC3 IR detector

Knox S. Long

2TIPS 2011 May

Overview

Persistence is a residual image observed in most types of IR arrays

There is physical model for this persistence involving traps that are accessed by free electrons or holes when a pixel is near saturation

The WFC3 IR detectors exhibit persistence from sources that approach or exceed full well

Typically persistence results in signals of 0.3 electrons s-1 , 1000 s after a saturated exposure.

The persistence is primarily a function of the amount of saturation of a pixel and the time since saturation

Persistence decays as a power law of time (g~1)

In most cases, post-processing can remove about 90% of the persistence signal with algorithms that track the history of the stimulus in the preceding 6 hours

3TIPS 2011 May

Basic reason for persistence is understood

-- - --

+ ++++next dark exp.

(small bias reduction) The released charge reduces the bias voltage. persistence

- - -

+ + +

-- - --

+ ++++

high flux signal

(low bias)As signal accumulates the depletion width is reduced. Traps newly exposed to charge can capture some mobile carriers.

Trapped holes

Trapped electrons

- - -

+ + ++ +++

+

-- - --

reset

(large reverse bias)

At “reset” the wider depletion region is restored, but trapped charge stays behind.

Depleted

Mobile electrons

Mobile holes

dark idle

(large reverse bias)All traps have released their charge in depletion region

+ ++++

-- - --

-+P

N

R.Smith, SPIE 7021-22, Marseille 2008-06-24

4TIPS 2011 May

Persistence Examples

Easy case Look for obvious patterns Look for objects that

appear mushy

Hard case Use multidrizzle to find

residuals Subtract the last single

science image from the first

If you suspect persistence, use MAST search to see if there were IR observations that preceded yours

Ex. 1

Ex. 2

http://archive.stsci.edu/hst/history_search.html

5TIPS 2011 May

Cycle 18 Calibration - Omega Cen

3 visits all consisting of an undithered Omega Cen exposure followed by a sequence of darks

Two visits successful, and the third to be repeated in a few weeks

Dark current measured from 234 s to 5800 s after the stimulus

6TIPS 2011 May

Persistence follows a Fermi-like distribution

Note – Not individual pixels but the mean persistence at different stimulus levels. WFC3 nominal saturation value is 70,000 e

Per

sist

ence

(e

s-1)

230 s

5800 s

7TIPS 2011 May

Persistence is ~ a power law function of time

8TIPS 2011 May

Persistence Model

Where q and t are the stimulus (in electrons) and time (in seconds) N is the normalization factor (at 1000 s)

qo is the “Fermi energy” at which the stimulus is “half”

dq is the “Fermi kT” a is the power law slope correction to the Fermi function. g defines the power slope for the decay with time

9TIPS 2011 May

Results (Visit 1)

Global fits good with systematic departure near knee

“Mean value” of persistence 1.43 0.05 e/s

RMS error 0.02 0.0052 e/s

Maximum error 0.04 0.02 e/s

Visit 3 similar

10TIPS 2011 May

Persistence subtraction S/W

HST archive does not currently provide an automated way to subtract persistence from images

However, WFC3 group is providing persistence subtracted images to users on request

Implements the model described above with a set of Python scripts

Provides Persistence image

• External – generated by early visits

• Total – internal + external persistence

Corrected flt file

Original

External Persistence

Total. Persistence

Corrected

11TIPS 2011 May

Open item - Persistence is not completely uniform

Tungsten lamp Dark after Tungsten lamp

Tungsten lamp is brightest in lower right quadrantPersistence is brightest in upper left quadrant

12TIPS 2011 May

Persistence is not completely uniform

Omega Cen Dark after model subtraction

13TIPS 2011 May

Open item – Persistence may be a function of time saturated

• Cycle 18 test – turn the tungsten lamp off at mid-exposure• At 80,000 e, there seems to be a correlation of persistence with hold time• At 140,000 e, the small amount of data in hand is not easily understood

Smith et al. model predicts persistence is a function of time a pixel held at saturation

14TIPS 2011 May

Summary – Persistence is not a virtue, but …

Today IR observations prohibited

after some “bad actors” Substantial Cycle 18

calibration program underway A working model exists for

persistence• Based on prior history of

illumination

A tool exists to mitigate persistence

• Results have been provided to individual users on a case by case basis

• Tool has been tested on a large fraction of the data

Future Ready to run existing

persistence subtraction tool on al the WFC3/IR data and then to advertise the existence of the persistence images

There are additional effects that may be able to improve the model

• Persistence as a function of position

• Add effects including dependency of the decay on illumination and time at saturation