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CTE in the Dark

CTE in the Dark. An Empirical Pixel-Based Correction for CTE. TIPS/JIM January 21,2009 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: Charge trails Loss of flux

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CTE in the Dark

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  1. CTE in the Dark An Empirical Pixel-Based Correction for CTE TIPS/JIM January 21,2009 Jay Anderson Luigi Bedin

  2. 30s, 47 Tuc Outer field Shuffle

  3. CTE/CTI 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 readout observed

  4. One Raw Dark, post SM4

  5. Stack of 168 Post-SM4 Darks

  6. “Peak” Map

  7. A Purely Empirical Plan PASP 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

  8. CR Tail Measurement

  9. Empirical Trails Faint No “notch” channel apparent! Bright

  10. Total Power in Tails TOTAL IN TAIL WP INTENSITY

  11. 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?

  12. WP~5000 What about Shadowing? Yes! Shadowing is essentially “perfect”! WP~2500 WP  ? C R X ? C R

  13. 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

  14. 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 

  15. Faint Corrected WP Trail Residuals Adjust by hand the model parameters 1) density: (q) 2) profile: (n;q) Bright

  16. Corrected WP Deep

  17. Trap Density - vs - q Trap Profile (n;q) (q) Two Components of the Model

  18. Detailed Model Example

  19. The tests… • Aesthetic test: trails gone? • Photometry: flux back? • Astrometry: flux in right place? • Shape: flux really in the right place?

  20. 339s, 47 Tuc Outer field

  21. 339s, 47 Tuc Outer field

  22. 30s, 47 Tuc Outer field

  23. 30s, 47 Tuc Outer field

  24. 30s, 47 Tuc Outer field

  25. 30s, 47 Tuc Outer field

  26. 47 Tuc Calibration Catalog 53,000 stars x,y,m

  27. BRIGHT Photometric Residuals in Deep 339s 47 Tuc Images FAINT

  28. BRIGHT: Near Saturation Photometric Residuals in Short 30s 47 Tuc Images FAINT: about 50 e- max

  29. BRIGHT: Near Saturation Astrometric Residuals in Short 30s 47 Tuc Images FAINT: about 50 e- max

  30. Bright What about shape? Faint Corrected

  31. 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?

  32. Ask me! 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…

  33. THE END

  34. Backup Slides

  35. READOUT SCHEMATIC

  36. 30s, 47 Tuc Outer field

  37. 30s, 47 Tuc Outer field

  38. Original “Smoothed” RN Component Decomposition

  39. Original Repaired Original Repaired Modified Actual Change

  40. Change for Original Change for RN-Smoothed Just the change

  41. Serial CTE

  42. Serial CTE linear trends

  43. Serial CTE Parameters

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