ACS Status Update
Norman Grogin (TIPS/JIM 20 Aug 2009)
Outline
• WFC Post-SM4 Performance Recap
• Day 174 Suspend : Determinations and Corrective Actions
• Spotlight : WFC Bias Striping (Characterization & Mitigation)
• Review of Latest SMOV Findings
TIPS/JIM 20 Aug 2009
1
WFC Performance Metrics
January 2007
(measured)
May 2009*
(projected)
Problematic*
Read noise
(e-; gain = 2)
C&S: 5.5
DSI: 4.0
> 10
DSI: 3.9-4.7
C&S: 4.4-5.7
Dark current (e-/pix/hr)
10.7
15
> 100
20-25
Hot pixels (%)
0.68
1.1
> 1.5
1.1
Full well depth (e-)
84,000
84,000
< 40,000
~80,000
Non-linearity (%)
< 0.1
< 0.1
> 0.5
< 0.2
Metric
Post SM4
(measured)
(exp time > 2 s)
CTE (@ 1620 e-)
0.999949
0.999921
< 0.9999
0.99989
Cross-talk
4x10-5
4x10-5
> 4x10-4
1x10-4
Bias shift** (%)
0.02
< 0.1
> 0.2
0.02-0.30
(before correction)
* Projected and problematic values from Gilliland et al. 2008 (TIR ACS 2008-04) .
**Residual bias shift after analytical correction of signal-dependent effect.
TIPS/JIM 20 Aug 2009
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Day 174 Suspend: Revisited
•
ACS suspended on 23 June as result of 2 Error Status Buffer messages:
 ACS 955: CEB Command Max Retries Exceeded
 ACS 1002: NED ASIC Not Alive
•
Status at time of suspend:
 No observations executing or lost
 All monitored currents and voltages normal
 ASIC & FPGA properly initialized & functioning after SMS 173 reboot
 Error occurred during NED collection cycle (CS to ASIC comm broken)
 HST orbital position far from SAA
•
Tiger Team formed on 24 June and identified 2 possible causes:
 ASIC Status Line raised (informs FPGA of bit anomaly or comm error)
 FPGA reset in response to SEU of power regulator
TIPS/JIM 20 Aug 2009
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Day 174 Suspend: Back-story
Initial assessment (presented by D. Golimowski at 16 Jul 2009 TIPS):
•
ASIC Status Line scenario not favored because it produces only ACS 955
ESB message, does not explain ACS 1002 (“ASIC Not Alive”) failure
•
Radiation test reports now available show ambient heavy-ion flux sufficient
to upset power regulator once every ~100 days
 SEU causes small voltage drop that triggers Power On Clear circuit
 FPGA and ASIC both reset; only FPGA reboots
 Scenario explains both ACS 955 and ACS 1002 ESB messages
Plausible, but this outcome of has 0.1% probability after ion hit because it
occurs in small (2 ms) window of 2 s NED collection cycle.
TIPS/JIM 20 Aug 2009
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Day 174 Suspend: Resolution
Revised assessment (subsequent to TIPS presentation on 16 Jul 2009):
•
Further investigation of FSW and ESTIF testing revealed scenario in which
raised Status Line produces both 955 and 1002 error messages with
reasonable probabilities.
•
GSFC subsequently discovered that cause of raised Status Line was singlebit comm error, which is automatically corrected by hardware. Status Line
supposed to be raised only for double-bit comm errors. Incorrectly set bit in
error response register  register now patched
•
MO and Project considering proposal to alter FAC and FSW to allow “slow
suspend” of ACS after future occurrences of 955 errors to enable dumps of
FPGA and ASIC memory before power shutdown.
Estimated delivery time: 2.5 months.
TIPS/JIM 20 Aug 2009
6
WFC Bias “Striping”
•
•
•
•
Bias striping pattern varies between WFC chips and between frames
1/f noise (1mHz to 1Hz) generated with ASIC during bias voltage offsetting
Striping appears constant across rows of CCD : ~removable in S/W
No measurable effect on ~bright-src photometry, but could be worrisome for
studies at low surface brightness (and low background)
TIPS/JIM 20 Aug 2009
7
De-striping the BIAS: Before/After
ACS/WFC Bias Image: before correction
TIPS/JIM 20 Aug 2009
ACS/WFC Bias Image: after correction
8
Superbias Quality
• Striping averages away at nearly N1/2
• Stripe-mitigated superbias only marginally noisier
than pre-failure superbias
• Superdark similarly amenable to stacking
• Testing underway for end-user app to de-stripe
science images (similar to the WFPC2 wdestreak)
TIPS/JIM 20 Aug 2009
9
ACS News Roundup
• WFC crosstalk (A. Suchkov):
– Pre-failure: Avg. ghosting on victim amps is ~1-2 e-/pix.
– Post-repair: Visually indistinct but still present for high fluxes
• WFC DARK & BIAS monitoring (T. Desjardins, P.-L. Lim):
– Now “warm” pixels = 0.04 ≤ DC < 0.08 e-/s; +40 hot&warm pix/day
– Bias level drifting ~linearly at 0.02 e-/day
• SIAF (C. Cox): New aperture definitions installed.
• Pixel area map (A. Fruchter):
– Two new header keywords for Multidrizzle (no end-user impact).
• Pipeline calib. files (R. Bohlin, C. Cox, T. Desjardins, R.
Lucas, J. Mack, A. Maybhate, M. Mutchler, et al.)
– WFC Flatfields, Gain ratios coming soon (old Flatfields are ~same)
– Plan is for all calib. files to be in place for post-ERO operations
TIPS/JIM 20 Aug 2009
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Summary
•
All SBC and WFC SMOV programs have been executed and data analysis is
nearly complete. No significant post-SM4 performance issues discovered.
•
Day 174 Suspend traced to FAC bug, now patched. Update to FAC/FSW
planned to implement “slow suspend” for memory dumps before shutdown.
•
Low-level bias striping in WFC is being characterized & mitigation strategies
being tested. “Clean” superbias/superdark will be in pipeline as of 9 Sep 09.
•
Both SBC and WFC have had successful and spectacular “first light”
external observations. (Pretty pictures still embargoed -- sorry!)
 ACS now proceeding with WFC and SBC GO Science
 Farewell to Tyler Desjardins (RIA in AWT), departing STScI for
grad school. You will be sorely missed!
TIPS/JIM 20 Aug 2009
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