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An Empirical Pixel-Based Correction for CTE. HST Calibration Workshop 2010 Jay Anderson & Luigi Bedin. 30s, 47 Tuc Outer field. Shuffle. CTE/CTI. Steadily increasing problem for: STIS, ACS’s WFC, … WFC3? Was also bad for WFPC2, HRC Symptoms: Loss of flux Charge trails Cause:
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An Empirical Pixel-BasedCorrection for CTE HST Calibration Workshop 2010 Jay Anderson & Luigi Bedin
30s, 47 Tuc Outer field Shuffle
CTE/CTI 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 readout observed
A New Empirical Approach Will 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)
100% 1% WP 1 2 3 4 Empirical Trails Faint No “notch” channel apparent! Fractional height greater for fainter WPs Bright
Correction Scheme 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… TRAIL INTENSITY TRAIL PROFILE
Faint Corrected WP Trail Residuals Original Corrected Adjust by hand the model parameters 1) density: (q) 2) profile: (n;q) Bright
The tests… • Aesthetic test: trails gone? • Photometry: is all the flux back? • Astrometry: flux in right place? • Shape: flux really in the right place?
PHOTOMETRY ASTROMETRY 10s -vs- 1200s BKGD ~ 2 e
ComparisonwithChiabergecorrection INSTRUMENTAL MAGNITUDE
Shape… looks good! AFTER BEFORE
Summary 2-component model (q) and (n;q) based on WPs in darks Tested against stars: • Images with backgrounds of 3 e and 25 e • Trails removed! • Photometry/astrometry generally restored • Shape surprisingly good Remaining issues…
Backup Slides 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 Shape Shadowing Detailed Model
Bright What about shape? Faint Corrected
Original “Smoothed” RN Component Decomposition
Original Repaired Original Repaired Modified Actual Change
Change for Original Change for RN-Smoothed Just the change
Serial CTE linear trends FIRST PIXEL 2ND PIXEL
WP~5000 What about Shadowing? Yes! Shadowing is essentially “perfect”! WP~2500 WP ? C R X ? C R
Total Power in Tails TOTAL IN TAIL WP INTENSITY