Goddard Space Flight Center
HST Project
ACS-R
SMOV Optimization Campaign
Iteration 2 Summary
June 5, 2009
Ed Cheng
Goddard Space Flight Center
Summary
HST Project
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Performance continues to be outstanding.
 Preliminary Iteration 2 results confirm Iteration 1 findings.
 Half-speed noise successfully recovered after realtime commanding to change the
offsets before Iteration 2.
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Optimization Campaign formally truncated after Iteration 4.
Realtime uplink commanding is requested to investigate one strange
result.
 Will not affect normal operation.
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Investigating charge injection as a way to mitigate CTE degradation.
- Similar scheme implemented for WFC3.
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Goddard Space Flight Center
Preliminary Half-Speed Noise
HST Project
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No Action for SDCR Adjustment
HST Project
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Additional analysis indicates that the SDCR test is at level of
measurement uncertainty.
Noise variations during the test are at the level of 0.2 e-.
 Probably not worth chasing after even if the measurement could be improved.
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Goddard Space Flight Center
Optimization Campaign Timeline
HST Project
Deleted
Now
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Iteration 1 Main Discoveries
HST Project
Discovery
Action
Minor timing irregularities noted in
oscilloscope mode data.
Bug in Flight Assembly Code found and
resolved in ground testing. No impact to✔
normal operations. Will be repaired in
final FAC drop.
Half-speed mode noise is unrealistically
low.
Saturation of ADCs because offset is not
optimized for half-speed mode. Offset ✔
change requested in realtime uplink.
Science Data Control Register
Examining result more carefully. Taking
optimization shows lower noise for a non- a little more time before deciding on
✔
default value.
uplinking any new values.
Reset Gate voltage is well-behaved at a
lower value than pre-failure.
Keeping the new default (8V) instead of
the pre-failure value (12V). Slight noise
✔
benefit and stability benefit. Margin
demonstrated by proper operation at 7V.
“Hypersensitivity” of Quadrant to C a
small change in Vod.
Investigation in progress. Default value
appears to give good performance.
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Goddard Space Flight Center
Observed VOD Anomaly
HST Project
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VDD: +27V, VLG: -3V (defaults)
VOD: +14V (default is +15V)
Bias
Illumination 1
Illumination 2
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System Block Diagram
HST Project
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Clocks and Biases Defined
Goddard Space Flight Center
HST Project
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VOD
VDD
VOS
VLG
RG
SW
Sx
Px
TG
VOFD
- Bias applied to CCD Reset Drain, (often called VRD)
- Bias applied to CCD Output Drain
- CCD video signal output
- Bias applied to CCD Last Gate
- Reset Gate for CCD amplifier
- Summing Well gate
- Serial Register Clock gates
- Parallel Register Clock gates
- Transfer Gate controlling CCD overflow drain
- Bias applied to the CCD overflow drain
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Goddard Space Flight Center
VOD Behavior Is Not Expected
HST Project
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Small change of the VOD from nominal setting of 15V to 14V results in
unexpected features in the image which appear to be charge injection.
Ground test at DCL of ACS flight spare demonstrates that VOD change
from 15V to 10V does not produce the effect observed in space.
Ground test confirms that observed on-orbit effect can be produced by
changing potentials of the VOFD and TG.
 Leads to conclusion that commanded changes to VOD result in unintended
alteration of VOFD or TG or both.
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Such effects are expected to affect the entire CCD so that Quadrants C
and D are expected to be similar.
 Quadrant D prescan is unusual, but image area appears normal.
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Goddard Space Flight Center
VOD Anomaly Resolution Path
HST Project
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Carefully reviewed all schematics and software/operations products.
 No “Black Peter” identified yet.
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Anomalous data are subarray data.
 May affect expected similarities between Quadrants C and D.
- Data were not simultaneous.
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Uplink request sets the value of VOD to what was seen in the anomalous
images.
 Allows checking using a more complete data set acquired during the performance
summary tests (Visit A).
 In particular, Visit A includes many full-frame images in various readout modes that
should show whether the subarray readout is a factor.
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Continue DCL tests to understand the effects better, and to check EM3
CEB-R behavior.
Even if we fail to understand the mechanism for this anomaly, the default
settings are performing normally.
 Only happens when we change a particular bias to an off-nominal value.
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Goddard Space Flight Center
Potential CTE Mitigation
HST Project
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There are two ways to mitigate CTE.
 Lower operating temperature to immobilize traps.
 Partially fill the wells so that the fractional loss is minimized.
- Pre-flash does this with light, but introduces shot noise.
- Charge injection is an electrical method that is potentially noise-free.
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Even before the observed anomaly, the charge injection method has
been discussed.
 Implemented in WFC3 with a special charge injection structure.
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This observed effect appears to be charge injection.
 We have analyzed its noise to see if it can potentially be used for CTE mitigation.
 May provide better performance than pre-flash.
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VERY PRELIMINARY!
 Still many issues with stability and quantitative usefulness that need to be addressed
since ACS was not designed to operate in this manner.
 Will probably result in a proposal to perform some controlled testing that targets this
feature.
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HST Project
Noise In Anomalous Quadrant Is
Less Than Poisson
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Goddard Space Flight Center
To Do
HST Project
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Understand the anomalous sensitivity to VOD.
Plan observing proposal to investigate charge injection for CTE
mitigation.
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