d. golimowski2010 stsci calibration workshop1 acs after sm4: new life for an old workhorse david...
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D. Golimowski 2010 STScI Calibration Workshop 1
ACS after SM4:New Life for an Old Workhorse
David GolimowskiSpace Telescope Science Institute
22 July 2010
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ACS Chronology QuickTime™ and aTIFF (Uncompressed) decompressor
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• March 2002: ACS installed in HST during Servicing Mission 3B (STS-109)
• June 2006: Failure of Side 1 LVPS. Switch to redundant Side 2 electronics. All camera operations restored.
• January 2007: Failure of Side 2 LVPS or APB. Loss of WFC and HRC operations; SBC still operational.
• May 2009: ACS-R hardware installed during Servicing Mission 4 (STS-125). WFC and SBC pass Aliveness and Functional Tests; HRC not recovered.
STS-125 astronauts Drew Feustel and John Grunsfeld during EVA-3
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The ACS Repair (ACS-R) QuickTime™ and aTIFF (Uncompressed) decompressor
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Four major components of ACS-R:• CCD Electronics Box Replacement (CEB-R)• Low Voltage Power Supply Replacement (LVPS-R)• Power Intercept Element (PIE)• Power Output Element (POE)
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The CEB-R QuickTime™ and aTIFF (Uncompressed) decompressor
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Main CEB-R features:
• Teledyne SIDECAR* ASIC** permits optimization of WFC performance via adjustment of CCD clocks, biases, and pixel transmission timing
• Built in oscilloscope mode (O-mode) that allows sensing of analog signal from each output amplifier
* System for Image Digitization, Enhancement, Control, and Retrieval
** Application Specific Integrated Circuit
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WFC Optimization Campaign (1) QuickTime™ and aTIFF (Uncompressed) decompressor
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Ground testing:• Verified performance of CEB-R with flight-spare WFC, but not actual flight WFC
SITe CCD performance highly variable
• Revealed non-ideal transient settling behavior in external preamp Preamp not replaced by ACS-R, so behavior with CEB-R not verifiable
• Revealed noise dependence on timing of data transmission from CEB-R to MEB
On-orbit testing:• To satisfy requirement that WFC perform at least as well as before, CEB-R must
accommodate above uncertainties via flexible, programmable settings
• Need iterative campaign to optimize CCD read noise, gains, linearity, full well depth, CTE, cross-talk
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WFC Optimization Campaign (2) QuickTime™ and aTIFF (Uncompressed) decompressor
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Strategy:
• Perform up to 8 iterations of comprehensive CCD performance tests and exploratory adjustments of bias voltages, clock rails, and data transmission timing via uplinked commands to ASIC
• Start with pre-failure default CCD voltages and timing patterns as baseline, then vary appropriate values to investigate specific conditions
• Converge to optimal settings; truncate Optimization Campaign if possible
• Select default CDS mode (dual-slope integrator or clamp-and-sample)
• Modify flight software and assembly code to conform to optimal settings
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OC Results QuickTime™ and aTIFF (Uncompressed) decompressor
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• OC began 28 May 2009 with pre-SM4 default (old CEB) voltage & timing settings
• Initial performance matched or exceeded projections and expectations. Dark current, CTE, and hot pixels consistent with prolonged radiation exposure.
• Dual-slope integrator (DSI) selected as default CDS mode
• Gradient of 5-10 DN caused by slow drift of bias reference voltage after readout of each row of pixels. Gradient is stable and removable.
• Low level (±1 DN) bias stripes caused by 1/f noise generated by ASIC during bias voltage offset.
• OC ended 3 Jun 2009 due to anomaly in WFC2 for non-default VOD. Pre-SM4 default settings adopted for all post-SM4 operations.
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WFC Performance Summary
CharacteristicJan 2007
(measured)May 2009* (projected)
Problematic*Post SM4
(measured)
Read noise (e–; gain = 2)
C&S: 5.5 DSI: 4.0 10DSI: 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
CTE (EPER; 1620 e–) 0.999949 0.999921 0.9999 0.99989
Cross-talk (50 ke– hot pixels)
4x10-5 4x10-5 4x10-4 (5±4)x10-5
Bias shift** (%) 0.02 < 0.1 < 0.20.02-0.30 (before correction)
* Projected and problematic values from Gilliland et al. 2008 (TIR ACS 2008-04) .**Signal-dependent shift due to incomplete settling of reference voltage in DSI high-pass filter. Effect can be corrected by software algorithm.
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WFC Dark Current History
12 hr anneal 6 hr anneal
-77 C -81 C
6 hr anneal
-77 C
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WFC CTE History
• EPER test shows worse than expected CTE degradation (0.99989 vs. 0.99992), but…
• Evolution of deferred-charge trails from hot pixels show degradation consistent with expectation
• Pre-SM4 correction formulae for WFC and HRC aperture photometry valid for post-SM4 data (Chiaberge ISR 2009-01)
• Empirical pixel-based CTE corrections now a reality ! (Next talk by Jay Anderson)
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WFC Bias and Dark Frames QuickTime™ and aTIFF (Uncompressed) decompressor
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Superbias - 1 full anneal cycle (DSI; 34 frames)
Superdark - 1/2 anneal cycle (DSI; 24 frames)
bias “gradient” (5-10 DN)
bias stripes (± 1 DN)
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Bias Stripe Effect QuickTime™ and aTIFF (Uncompressed) decompressor
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• Caused by 1/f noise (1 mHz to 1 Hz) in bias reference voltage after CDS (Talks by Bernie Rauscher and Markus Loose)
• Appears in both DSI and C&S frames
• Stripes vary by ~ 0.75 e–; negligible relative to read noise
• Noise is correlated; may affect photometry of the faintest sources
• Software developed to mitigate effect (Poster A2 by Norman Grogin)
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Bias Stripe Removal QuickTime™ and aTIFF (Uncompressed) decompressor
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(before) (after)
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Cross Talk QuickTime™ and aTIFF (Uncompressed) decompressor
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NGC 4701 (pre-SM4) NGC 6217 (post-SM4)
(See poster A3 and ISR ACS 2010-02 by Anatoly Suchkov, et al.)
Gain = 2 e–/DN
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Summary QuickTime™ and aTIFF (Uncompressed) decompressor
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• ACS record after SM4: 1 win (WFC), 1 loss (HRC), 1 tie (SBC)
• ACS-R Optimization Campaign showed that WFC read noise, linearity, full-well depth, and cross-talk match or exceed pre-SM4 levels
• CTE, hot pixels, dark current have degraded to levels consistent with prolonged exposure to HST’s radiation environment
• Bias gradient of 5-10 DN caused by a slow drift of the DSI reference voltage during and after the readout of each row of pixels; stable and removable
• Low level (1 DN) bias stripes caused by 1/f noise in bias reference voltage set after CDS stage of signal processing chain; not reproducible from frame to frame
• Algorithms for mitigating bias stripes and pixel-to-pixel CTE correction will be available to users very soon; STScI is investigating implementation into calacs.
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ACS-R Design and Optimization Teams
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PI: Ed Cheng (Conceptual Analytics)
STScI: George Chapman, Marco Chiaberge, Tyler Desjardins, Tracy Ellis, David Golimowski, Norman Grogin, Pey-Lian Lim, Ray Lucas, Aparna Maybhate, Max Mutchler, Merle Reinhart, Marco Sirianni (ESA/ESTEC), Linda Smith, Anatoly Suchkov, Alan Welty, Tom Wheeler
GSFC: Steve Arslanian, Kevin Boyce, Darryl Dye, Olivia Lupie, Kathleen Mil, Barbara Scott, Beverly Serrano, Augustyn Waczynski, Erin Wilson
Teledyne: Markus Loose (Markury Scientific), Raphael Ricardo