Mitigating CTE losses: Charge Injection and Pre/Post Flash November 21, 2002 Curing CTE degradation

Curing CTE degradation Traps and CTE losses CTE loss is caused by traps formed in lattice by cosmic radiation. During charge transfer operations, charge from transiting packets is captured and retained by “traps”, thus lost to the packet. Traps retain charge for some time and then releases it. Charge released by traps can add to nearby following packets. Process is stochastic in nature: packets loose, but also gain, charge in a random way: Space-dependent photometric bias Increased scatter Decreased S/N Net charge is lost Fluctuations increase November 21, 2002 Curing CTE degradation

Curing CTE degradation Measuring CTE Losses Effects of CTE loss can be reproduced in the LAB on CCDs subject to radiation damage of controlled magnitude. Sources of known flux provided by radioactive isotopes, such as 55Fe. For example, an X-ray line from 55Fe promotes 1620 e- in the CCD detector Equivalent to a flat f-l source with V=25.84 in a 1,800 sec exposure. Photometry of CR hits made with Sextractor. Used circular aperture (5 pix d.) and isophotal aperture Used CCD43-152 irradiated to 0 year 2.5 year 5 year worth of damage November 21, 2002 Curing CTE degradation

CTE degradation: 2.5 and 5 year damage Comparison of photometry of CR hits: new detector 2.5 year irradiated detector 5-year irradiated CCD Effects of CTE losses: degradation of photometric uniformity loss of sensitivity charge is lost additional noise introduced November 21, 2002 Curing CTE degradation

Curing CTE degradation Mitigating CTE Losses Degradation of CTE mitigated by filling traps with charge. Filled traps become passive and do not subtract additional charge from transiting packets. Two methods to dispense charge: Charge injection, Discrete continuous Post or pre-flash with light Shown here is charge injection of ~104 e- every 200 lines in 2.5yr CCD Unfortunately, traps release charge after some time, becoming active again. November 21, 2002 Curing CTE degradation

Curing CTE degradation Release of Charge Release of charge by traps diminishes effectiveness of of added charge to mitigate CTE losses. Spacing between injected lines is key parameter for Discrete Charge Injection . Spacing must be such that traps are not allowed to “dry up” without charge and become active. November 21, 2002 Curing CTE degradation

Discrete Charge Injection: every 25 lines By injecting charge more frequently, one can mitigate the charge release problem. Shown here is charge injection of 104 e- every 25 lines in 5yr CCD. Note that CTE losses, released charge from injected lines is much less than in the previous case. November 21, 2002 Curing CTE degradation

Pre Flash and Continuous Charge Injection Filling traps with charge can be done by either: Post/Pre Flash (injection by light) Charge Injection (electronically) Discrete Charge Injection Continuous Charge Injection Shows here is the pattern of C.C.I. with ~10,000 e-/pix November 21, 2002 Curing CTE degradation

Curing CTE degradation C.C.I. Residual Map Noise: s = 15 e- rms C.C.I. repeatable and “calibratable”. November 21, 2002 Curing CTE degradation

Curing CTE degradation Pre Flash at 5 year - 1 Shown here are the curves relative to pre-flash with 100 and 200 electrons. Also shown are the curves for the undamaged CCD and for the 5 year CCD Improvement in the photometric uniformity is modest and overall similar to D.C.I. at 25 lines. Photometric scatter is better than D.C.I. (probably due to filling all traps) However, note the lower S/N ratios. November 21, 2002 Curing CTE degradation

Curing CTE degradation Pre Flash at 5 year - 2 Shown here are the curves relative to pre-flash with 500, 1000 and 2000 electrons. Also shown are the curves for the undamaged CCD and for the 5 year CCD Improvement in the photometric uniformity is good and overall similar to D.C.I. at 25 lines for the 2.5 year CCD. Photometric scatter is significantly better than D.C.I (probably due to filling all traps). However, note the lower S/N ratios. November 21, 2002 Curing CTE degradation

Continuous Charge Injection vs. Pre Flash at 5 yr Continuous Charge Injection (CCI) very promising: Same remedial effects as P.F. In principle, much less noise Curves relative to Pre Flash with 2000 electrons, and C.C.I. With 10,000 electrons. Also shown are the curves for the undamaged CCD and for the 5 year CCD Improvement in the photometric uniformity is very similar C.C.I. has very high photometric scatter and lower S/N ratios. November 21, 2002 Curing CTE degradation

Effects of isophotes’ size Isophotal apertures Vs. Fixed Apertures (5 pix) November 21, 2002 Curing CTE degradation

Curing CTE degradation Faint Source Limit Case of faint sources not empirically tested for WFC3 CCD. Studies with WFPC2 CCD (e.g. Whitmore et al. 2002) showed that fainter sources proportionally more affected by CTE losses than brighter ones: Flux no P.F 25 e- 250 e- 1700 e- 20-50 DN 37.7 +/- 4.7 11.8 +/- 2.4 3.9 +/- 5.3 not enough stars 50-200 DN 23.3 +/- 2.1 8.3 +/- 1.4 3.3 +/- 2.0 5.8 +/- 3.4 % 200-500 DN 16.6 +/- 4.0 8.7 +/- 1.8 5.5 +/- 2.2 -1.8 +/- 2.8 % 500-2000 DN 8.8 +/- 5.6 10.2 +/- 4.3 -1.2 +/- 4.2 2.3 +/- 1.6 % Dm ~ 0.0 0.4 0.9 1.5 Low-level pre-flash effective in mitigating faint/bright difference. However, CTE mitigation with low-level pre-flash not terribly effective for ~1600 e- source (e.g. still 10% losses with 100 e-) in WFC3 CCD. WFPC2 case suggests similar losses at faint levels, at best. High-level P.F. has devastating effects of Poisson noise. This suggests that C.C.I. is still optimal solution: increased noise from 5 to 15 e- corresponds to Dm~0.3 in V-band for a V~28 (220 e-) point source. November 21, 2002 Curing CTE degradation

Curing CTE degradation Conclusions CTE losses significantly degrade CCD performance: Space-dependent photometric bias, photometric scatter Decreased sensitivity (S/N), e.g.: Dm~0.8 loss in limiting flux at 5 years Decreased photometric accuracy (increased scatter) D.C.I. (25 lines) and P.F. (2000 e-) provide comparable mitigation to CTE loss C.C.I. superior to both P.F. and D.C.I. at 5 year with relatively good noise performance (15 e-) SOC recommended to implement C.C.I. capability Work ongoing to further reduce C.C.I. noise. November 21, 2002 Curing CTE degradation