An Empirical Pixel-Based Correction for CTE
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An Empirical Pixel-BasedCorrection for CTE
HST Calibration Workshop 2010
Jay Anderson & Luigi Bedin
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30s, 47 Tuc Outer field
Shuffle
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Steadily increasing problem for:– STIS, ACS’s WFC, … WFC3?– Was also bad for WFPC2, HRC
Symptoms: – Loss of flux– Charge trails
Cause:– Traps within pixels that delay readout– Trap density increases linearly over time
CTE/CTI
readout
observed
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A New Empirical ApproachWill be published in September PASP
Inspired by:– Hyper-velocity star project (PI-Oleg Gnedin)– Massey et al. (2010): WPs in COSMOS science data
Plan:• Examine WPs in darks• Study two dimensions:
– Trail intensity: dependence on WP height– Profile drop-off: dependence on n (distance along trail)
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One Raw Dark, post SM4
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Stack of 168 Post-SM4 Darks
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WP Map
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CR Tail Measurement
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Empirical Trails
Faint
Bright
No “notch”channel apparent!
WP 4321
1%
100%
Fractionalheight greaterfor fainter WPs
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Start with a readout model– Two parameters:
1) Trap density: (q)2) Release profile: (n;q)
– Shadowing? – Monitor filling/emptying of traps at all charge q levels– PORIG(j) POBS(j)
Invert – Find source function that produces observations– Optimize model by varying (q) and (n;q)– Efficiencies…
Correction Scheme
TRAILINTENSITY
TRAILPROFILE
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Corrected WP Trail
Residuals
Faint
Bright
Adjust by handthe model parameters1) density: (q)2) profile: (n;q)
Original
Corrected
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Corrected WP Deep
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The tests…
1) Aesthetic test: trails gone?
2) Photometry: is all the flux back?
3) Astrometry: flux in right place?
4) Shape: flux really in the right place?
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339s, 47 Tuc Outer field
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30s, 47 Tuc Outer field
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30s, 47 Tuc Outer field
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10s -vs- 1200s
ASTROMETRYPHOTOMETRY
BKGD ~ 2 e
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Comparisonwith
Chiabergecorrection
INSTRUMENTAL MAGNITUDE
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Shape… looks good!
AFTER BEFORE
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Summary2-component model (q) and (n;q) based on WPs in darksTested against stars:
– Images with backgrounds of 3 e and 25 e
– Trails removed!– Photometry/astrometry generally restored– Shape surprisingly good
Remaining issues…
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Remaining Issues…• Read-noise mitigation• Time/temperature dependence• X-CTE: present• More exploration of low bkgd levels
– short darks coming!
• Implications for dark subtraction: darks w/CTE are non-linear• ACS team’s plans
– (any day…) PASP paper on site
– (continuing) Evaluation of model
– (Oct 2010) Come up with stand-alone routine for _flt’s
– (Mid2011) Consideration of how/whether to modify
the pipeline
• Other instruments: – Current: STIS, UVIS
– Legacy: HRC, WFPC2
BackupSlides
Shadowing
Detailed Model
Shape
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BACKUP SLIDES
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SHAPE
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What about
shape?
Bright
Faint
Corrected
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READNOISE MITIGATION
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Original
Decomposition
“Smoothed” RN Component
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Actual Change
Original Repaired Original Repaired Modified
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Just the change
Change for Original Change for RN-Smoothed
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SERIAL CTE
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Serial CTE
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Serial CTE linear trends
2NDPIXEL
FIRSTPIXEL
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Serial CTE Parameters
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DETAILED MODEL EXAMPLE
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Detailed Model Example
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SHADOWING
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WP
C R
C R
WP~2500
WP~5000What about Shadowing?
?
?
Yes! Shadowing is essentially “perfect” !
X
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Total Power in
Tails
WP INTENSITY
TO
TA
L I
N T
AIL