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

2. 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

3. 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 CCD irradiated to
0 year
2.5 year
5 year
worth of damage
November 21, 2002
Curing CTE degradation

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 sensitivity
charge is lost
additional noise introduced
November 21, 2002
Curing CTE degradation

5. 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

6. 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

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

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

9. 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

10. 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

11. 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

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

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

14. 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 e e e-
DN / / / not enough stars
DN / / / /- 3.4 %
DN / / / /- 2.8 %
DN / / / / %
Dm ~
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

15. 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