1. Following the Photons… Empirical, Pixel-Based Corrections for CTE Jay Anderson STScI October 12, 2011 Back-Tracking the Electrons

2. 30s, 47 Tuc Outer field Shuffle

3. Plan for the Talk Introduce the CTE problem Brief history Version 1.0: My initial solution Version 2.0: Soon-to-be-pipelined solution WFC3/UVIS Version 3.0: Additional needed improvements

4. Steadily increasing problem for: –STIS, ACS’s WFC, … WFC3? –Is also bad for archival WFPC2, HRC The Problem: CTE/CTI readout CTE=Charge-Transfer Efficiency CTI = Charge-Transfer Inefficiency readout observed Symptoms: –Loss of flux in source –Increase of flux in trails Cause: –Traps within silicon pixels that delay individual electrons –Number of traps increases over time

5. A Brief History Many Approaches Laboratory work: – 55 Fe 1620 e  events ; FPR ; EPER  two trap species –Also computer modeling of distribution within pixel –Limited array of tools used, incomplete picture Post-hoc corrections –Common wisdom: CTE worst for faint sources on low background –Empirical photometric corrections (Riess, Mack, Ciaberge, Goudfrooij, … ) Problem 1D: Observed flux + sky, time, location  initial flux What about astrometry? Shape? Pixel-based corrections/reconstructions –The holy grail –STIS: Bristow, Alexeev –WFPC2: Riess –ACS: Massey et al 2010 on COSMOS data Limited focus (medium/high backgrounds) Proof of concept: generated renewed excitement at ST EPER parallel overscan image pixels (flat) readout FPR up-shifted flat charge grabbed charge let go

6. My Model 1.0 Previous: –Bristow: Sources –Riess: CRs –Massey: WPs in science frames Trail data from lab tests Assume mini-channel from manufacturing expectations Modeled specific representative trap locations Model 1.0: WPs in dark images –Explore lower backgrounds than GOODS sky (50 e  ) –Purely empirical: Just look-up tables Trap density:  (q)  traps per marginal electron Trap release:  (n;q)  short + long trails –Trap and release assumptions Trapping deterministic Release probabilistic Keep track of state of each trap during transfer –Modeling the pixel array: continuum of fractional traps in each pixel code economizing for speed: 2048 steps  1 to 5 steps iterate for to get input distribution (like Massey) = 0.01% chance OBS’N MODEL INPUT

7. Animation of Model Parameters of Model: Trap density:  (q) Trail profile:  (t;q)

8. One Raw Dark, post SM4

9. Stack of 168 Post-SM4 Darks

10. CR Tail Measurement

11. Empirical Trails Faint Bright No “notch” channel apparent! consistent with common wisdom that CTE worse for faint sources

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

13. Corrected WP Deep

14. 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?

15. 339s, 47 Tuc Outer field

17. 30s, 47 Tuc Outer field

21. 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?

22. Limitations of PB approach –Read-noise problem –S/N loss Limitations of v1.0 –Time dependence (assumed linear) –Temperature dependence of trails –Even darks not dark –Need to explore lowest packets (10 e  ) Not the end of the story… OBS’N MODEL INPUT +RN

23. Model 2.0 Will soon be released as standard pipeline product Compare long darks + short darks –Can see the 10 e  WP events –Absolute handle on losses Known WPs! Same WPs? 1000s, 1000 e- 100s, 100 e- 

24. Creeping CTE TOP OF CHIP BRIGHTER WPFAINT WPBOT OF CHIP

25. Model 2.0 cross section  (q) (DN) Will soon be released as standard pipeline product Compare long darks + short darks –Can see the 10 e  WP events –Absolute handle on losses –Truly pathological losses… 15 e   < 1 e 

26. Aging fast Why? –SBC, mini-channel, etc?! Maybe, but useless…. –Solar cycle –Different observing regime Lower background Narrow filters, UV, low dark current, few WPs True mitigation available –Charge-injection: every 10, 17, or 25 lines –Benefit, but limited… WFC3/UVIS

27. Aging fast Why? –SBC, mini-channel, etc?! Maybe, but useless…. –Solar cycle –Different observing regime Lower background Narrow filters, UV, low dark current, few WPs True mitigation available –Charge-injection: every 10, 17, or 25 lines –Benefit, but limited… True mitigation, but add noise model dependence (get better model)

28. Model 3.0 Realization that dark current important –Readout ~ 90s, but many WPs… –Even bias frames have 15 e  at top! –Do the correction on raw frames Study everything –Column by column dependence –WPs in all exposures over time –EPER parallel overscan over time –Pin-down UVIS model, using charge-injection –Explore UVIS CI mitigation Goddard exploring possible injection mitigation…

29. THE END