CTE in the Dark
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Transcript of CTE in the Dark
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CTE in the DarkAn Empirical Pixel-Based
Correction for CTE
TIPS/JIMJanuary 21,2009Jay AndersonLuigi 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: – Charge trails– Loss of flux
Cause:– Traps within pixels that delay readout– Trap density increases linearly over time
Traditional mediation:– Photometric correction ; astrometric?– Experimental pixel-based corrections
• STIS: Bristow 2002+• ACS: Massey et al 2010
– Theoretical plus empirical
• New approach here: purely empirical
CTE/CTI
readout
observed
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One Raw Dark, post SM4
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Stack of 168 Post-SM4 Darks
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“Peak” Map
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A Purely Empirical PlanPASP Paper in preparation with L. “R”. Bedin
Inspired by– HVS project (PI-Oleg Gnedin)– Massey et al. (2010): WPs in COSMOS science data
Plan:• Examine WPs in darks• Study two dimensions:
– Profile scale: dependence on WP intensity– Profile drop-off: dependence on n
• Focus: “Just numbers”– Lots of trails– First step: mechanics of measuring the trails
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CR Tail Measurement
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Empirical TrailsFaint
Bright
No “notch”channel apparent!
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Total Power in
Tails
WP INTENSITY
TO
TA
L I
N T
AIL
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A Simple, Empirical Model• Different traps affect different electrons
– More traps affect lower-hanging electrons (q): traps per pixel at each chg level
• Different traps may have different release times– Follow release out to 100 pixels– Model: keep track of each trap’s state– All the charge?
• What about shadowing?– Well known background effect– Leading pixels?– How to resolve?
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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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A Simple, Empirical Model[1] Different traps affect different electrons: (q) = trap density (total number)[2] Different traps may have different release times
(n;q) = release profile[3] Perfect shadowing!
Instantaneous filling of traps
Readout Model’s Four Stages:
(1) Shuffle out current cloud(2) Release trapped charge(3) Shuffle in new cloud(4) Trap new electrons
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Correction Scheme
Start with a readout model– Two parameters:
1) Trap density: (q)2) Release profile: (n;q)
– Input PIX(j) output PIX(j)Iterate
– Find source function PORIG(j) that produces POBS(j)
Optimize model:– Minimize trails in darks by varying (q) and (n;q)
Independent tests:(1) Trails (2) Photometry (3) Astrometry (4) Shape
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Corrected WP Trail
Residuals
Faint
Bright
Adjust by handthe model parameters1) density: (q)2) profile: (n;q)
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Corrected WP Deep
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Two Components of the Model
Trap Density - vs - q Trap Profile(q) (n;q)
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Detailed Model Example
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The tests…
1) Aesthetic test: trails gone?
2) Photometry: 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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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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30s, 47 Tuc Outer field
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30s, 47 Tuc Outer field
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47 Tuc Calibration Catalog 53,000 stars x,y,m
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Photometric Residuals in Deep 339s 47 Tuc Images
BRIGHT
FAINT
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Photometric Residuals in Short 30s 47 Tuc Images
BRIGHT: Near Saturation
FAINT: about 50 e- max
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Astrometric Residuals in Short 30s 47 Tuc Images
BRIGHT: Near Saturation
FAINT: about 50 e- max
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What about
shape?
Bright
Faint
Corrected
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Summary
2-component model (q) and (n;q)– Pameters based solely on WPs in darks– Readout model, invert to get original pixels
Tested against stars:– Images with backgrounds of 1.5 DN2 and 15 DN2 – Trails removed– Photometry/astrometry generally restored– Shape surprisingly good
Remaining issues?
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Remaining Issues• Reminder: just a proof of concept• When best to do? _flt or _raw?
– Either is ok• Pipeline modifications?
– Use of darks, biases• Improvements:
– Speed: 5 iterations = 10 minutes/exposure– Faint and bright extremes poorly constrained
• Verify linear time behavior (pre SM4…)• Read-noise amplification
– Should apply algorithm only to real structure• X-CTE
– Yes, but…
Ask me!
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THE END
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Backup Slides
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READOUT SCHEMATIC
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30s, 47 Tuc Outer field
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30s, 47 Tuc Outer field
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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 linear trends
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Serial CTE Parameters