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Mitigating CTE losses: Charge Injection and Pre/Post Flash. 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. - PowerPoint PPT Presentation
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November 21, 2002 Curing CTE degradation Mauro Giavalisco 1
Mitigating CTE losses: Charge Injection and Pre/Post Flash
November 21, 2002 Curing CTE degradation Mauro Giavalisco 2
Traps and CTE losses
CTE loss is caused by traps formedin 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 timeand then releases it.
Charge released by traps can add tonearby 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 Mauro Giavalisco 3
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 yearworth of damage
November 21, 2002 Curing CTE degradation Mauro Giavalisco 4
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 sensitivity1. charge is lost2. additional noise
introduced
November 21, 2002 Curing CTE degradation Mauro Giavalisco 5
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:1. Charge injection,
• Discrete• continuous
2. 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 Mauro Giavalisco 6
Release of Charge
Release of charge by trapsdiminishes effectiveness of of added charge to mitigate CTE losses.
Spacing between injectedlines is key parameter for Discrete Charge Injection .
Spacing must be such that traps are not allowed to “dry up” without charge and becomeactive.
November 21, 2002 Curing CTE degradation Mauro Giavalisco 7
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 Mauro Giavalisco 8
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 Mauro Giavalisco 9
C.C.I. Residual Map
Noise: s = 15 e- rms
C.C.I. repeatable and “calibratable”.
November 21, 2002 Curing CTE degradation Mauro Giavalisco 10
Pre Flash at 5 year - 1
Shown here are the curvesrelative 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 Mauro Giavalisco 11
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 Mauro Giavalisco 12
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 Mauro Giavalisco 13
Effects of isophotes’ size
Isophotal aperturesVs.Fixed Apertures (5 pix)
November 21, 2002 Curing CTE degradation Mauro Giavalisco 14
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 Mauro Giavalisco 15
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.