Persistence in the WFC3 IR detector
Knox S. Long
TIPS 2011 May
1
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
TIPS 2011 May
2
Basic reason for persistence is understood
Trapped Trapped
electrons holes
Mobile
electrons
Depleted
Mobile
holes
- - - --+
- - - -- - -
- - - -- - -
++ +
+ +
+ ++
++ +
+ +
+ ++
++ +
+ +
high flux
signal
reset
dark idle
(large reverse bias)
All traps have released
their charge in depletion
region
R.Smith, SPIE 7021-22, Marseille 2008-06-24
(low bias)
As signal
accumulates the
depletion width is
reduced. Traps
newly exposed to
charge can
capture some
mobile carriers.
TIPS 2011 May
(large reverse
bias)
At “reset” the
wider depletion
region is restored,
but trapped charge
stays behind.
- - - -++ +
+ +
N
P
next dark exp.
(small bias
reduction)
The released charge
reduces the bias
voltage. persistence
3
Persistence Examples
 Easy case

Look for obvious patterns

Look for objects that
appear mushy
Ex. 1
 Hard case

Use multidrizzle to find
residuals

Subtract the last single
science image from the
first
Ex. 2
 If you suspect persistence,
use MAST search to see if
there were IR observations
that preceded yours
http://archive.stsci.edu/hst/history_search.html
TIPS 2011 May
4
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
TIPS 2011 May
5
Persistence follows a Fermi-like distribution
Persistence (e s-1)
230 s
5800 s
Note – Not individual pixels but the mean
persistence at different stimulus levels.
WFC3 nominal saturation value is 70,000 e
TIPS 2011 May
6
Persistence is ~ a power law function of time
TIPS 2011 May
7
Persistence Model
a
-g
æ
ö
æ
ö q æ t ö
1
P(q, t) = N ç (q-qo )/dq ÷ç ÷ ç
÷
èe
+1 øè qo ø è 1000s ø
 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
TIPS 2011 May
8
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
TIPS 2011 May
9
Persistence subtraction S/W
 HST archive does not currently
provide an automated way to
subtract persistence from images
Original
Total. Persistence
 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
Corrected
TIPS 2011 May
External Persistence
10
Open item - Persistence is not completely uniform
Tungsten lamp
Dark after Tungsten lamp
Tungsten lamp is brightest in lower right quadrant
Persistence is brightest in upper left quadrant
TIPS 2011 May
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Persistence is not completely uniform
Omega Cen Dark after model subtraction
TIPS 2011 May
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Open item – Persistence may be a function of time saturated
Smith et al. model predicts persistence is a function of time
a pixel held at saturation
• 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
TIPS 2011 May
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Summary – Persistence is not a virtue, but …
 Today
 Future

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

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
• Results have been provided to
individual users on a case by
case basis
• Tool has been tested on a large
fraction of the data
TIPS 2011 May
• Add effects including
dependency of the decay on
illumination and time at
saturation
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