Post on 20-Mar-2016
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DfA Garching 2009-10-14
NIR wavefront sensing
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Low-Noise IR Wavefront Sensingwith a Teledyne HxRG
David HaleGustavo Rahmer & Roger Smith
Caltech
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Why a Natural Guide Star for Laser AO?
• Wavefront tilt is not seen by a laser guide star, since the laser light retraces its outward path, so…
• The TMT/IRIS OIWFS will use three natural guide stars to measure tilt, rotation and scale changes.
• The brightest guide star will pass through a 2×2 Shack-Hartmann sensor to measure focus and astigmatism.– Each of three probes can be reconfigured on the fly to be
either TT only, or TTFA
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Why the NIR ?
Goal: increase fraction of sky over which adequate AO performance is achieved.
• Need to guide on AO corrected image to close the tilt loop. – Need NIR to get good Strehl and thus adequate tilt sensitivity.
• The most common stars are brightest in the NIR.
• Diffraction core of 30m telescope is so small that background per pixel is negligible in J+H band
– Although Strehl is better for K band, sky is much brighter and diffraction core is larger
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Some Detector Options Considered• Intevac: electron bombarded CCD with InGaAs photocathode.
– Dark current way too high and uncontrolled– >100 Hz frame rates not available (until CCD upgraded to CMOS imager)
due to ROI overheads– Current format not ideal (1024×256)
• HgCdTe APD arrays:– Attractive promises, but not ready enough yet
• HxRG– Well understood– Large format– High QE.– Noise on recent devices good enough after multiple sampling.
DfA Garching 2009-10-14
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Format (not to scale)
• Current baseline is ~ 1K×1K operable region within H2RG rather than H1RG, since Teledyne advises this will be no more expensive and that the H1RG may be discontinued.
– Size of capture region is set by seeing and probe positioning accuracy
• Spot will be small when high order correction is turned on, moving on scale of seeing profile until low order loop closed.
• Final guide window size may be as large as 14×14 pixels to handle impulse perturbations.
DfA Garching 2009-10-14
NIR wavefront sensing
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H2RG
2048
2048
Capture region ~ 1k × 1k
4×4 guide window 2 mas/pix
2 arcsec
14×14 recapture window
Zoom to Capture
• Start with seeing limited image (>1/4 detector area) – Big, fuzzy, low contrast. – Move to center by adjusting probe position or telescope pointing.
• Turn on high order correction. – Tiny spot scribbles lines over the seeing profile. Not much change
seen in long exposure.
• Window down on seeing limited image.– Faster frame rate makes wiggly line shorter.
• Close loop at low gain to drive centroid towards center of window. – Steadily reduce window size to increase frame rate and loop gain. – Servo keeps spot within shrinking window.
• Zoom in to 4×4 window– avoid bad or noisy pixels
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Why Such a Big Detector? (… just to replace a quad cell !)
• Big telescope aperture makes tiny diffraction core:
FWHM = /D = 0.008 arcsec at 1.2µm
• Quad cell requires ~0.004 arcsec/pixel
• Prefer 0.002 arcsec/pixel so that positioning on pixel boundary not required to maximize centroiding sensitivity.
• 2 arcsec field of view needed to capture seeing profile. We could go larger to aid acquisition. Thus need >1K2
• Freebie: science image can be acquired around guide star, since H2RG allows nested windows and independent reset.
DfA Garching 2009-10-14
NIR wavefront sensing
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Maximizing Frame Rate
• The spot is compact throughout the zoom, since laser guide star sensor is blind to tip tilt, but it is smeared by image motion. – Our problem is to locate it (short frame time) and– Measure tilt accurately (low noise) to feed back to servo.
• Flux per pixel depends on image motion rather than exposure time, so maximize frame rate.– Pixel time has been minimized.– For window >64 pixels: 32 ch readout, skipping unwanted lines.– For window <64 pixels: single channel, window mode.
• By 64×64, frame rate = 50Hz … tilt error already << window size.– Readout time = (5.16*N2 +10.33*N + 5.28) µs = 21.8ms
• For fainter guide stars final rate ~100Hz. For frames <44×44, can use multiple sampling to reduce the read noise below the ~11 e- for CDS.
• For brighter guide stars final rate ~800 Hz, with less noise averaging.
DfA Garching 2009-10-14
NIR wavefront sensing
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Noise & Frame Rate During Zoom DfA Garching
2009-10-14NIR wavefront sensing
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Start zoom
Big frames, CDS noise
Faint stars
Bright stars
4x4 1024x1024
NIR wavefront sensing
Begin multiple sampling here
Correlated Double Sampling
• Exposure delay = p dummy reads for constant self heating
• Subtract first frame from last frame
• Equivalent to Fowler sampling with m = 1
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NIR wavefront sensing
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Ignore p scans
e = 1 = number of exposures to do …. not shown herer = number of reset scans between exposuresm = 1 = number of scans to coadd then store.p = 10 = number of dummy scans between coadded groupsk = 2 = number of store cycles per exposure
At least one reset between frames
Reset while idling Initial
scanFinal scan
Exposure timeFrame time
Let’s review common readout timing options….
Fowler “m”
• Exposure delay is in units of full scan ties but need not be multiple of m.
• Subtract mean of first group from mean of last group.
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Coadd m
Ignore p scans Coadd m
e = 1 = number of exposures to do …. not shown herer = number of reset scans between exposuresm = 3 = number of scans to coadd then store.p = 6 = number of dummy scans between coadded groupsk = 2 = number of store cycles per exposure
Exposure timeFrame time
Duty cycle < 1
Sample up the ramp.
• Store every scan (no real time coadd)
• Use post facto least squares fit to measure slope with best S/N;
• Effective exposure duty cycle due to weighting of shot noise by least squares ~ 90%; reduce this to include effect of the reset overhead.
• Equivalent MultiAccumulate with m=1.
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e = 1 = number of exposures to do …. not shown herer = number of reset scans between exposuresm = 1 = number of scans to coadd then store.p = 0 = number of dummy scans between coadded groupsk = 12 = number of stores per exposure
Multi-Accumulate (JWST terminology)
• Coadd in real time, store every m scans, total exposure duration is multiple of m scan times.
• Least squares fit of stored (coadded) scans is used to estimate noise.
• Advantage of coadd over single samples with gaps is lower noise and better cosmic ray detection ( which appears as jump in ramp).
• One or more reset scans between exposures.
DfA Garching 2009-10-14
NIR wavefront sensing
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Coadd m
Coadd m
Coadd m
Coadd m
Coadd m
Reset r scans
e = number of exposures to do …. not shown herer = 2 = number of reset scans between exposuresm = 3 = number of scans to coadd then store.p = 0 = number of dummy scans between coadded groupsk = 4 = number of stores per exposure
Proposed mode for OIWFS (used in the noise tests presented here)
Differential Multi-Accumulate
• Using previous frame as baseline for next frame (without reset) makes duty cycle ~100%, except for a gap when reset occurs.
• Gaps at time of reset can be reduced in duration by using single scan or significantly fewer scans to establish first baseline instead of averaging m scans. This will produce a noisier result instead of a missing result. Which is better?
• The reset scan and initial read scan can be combined so the reset time is hidden.
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NIR wavefront sensing
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Coadd m
Coadd m
Coadd m
Coadd m
Coadd m
Reset Coadd m
Difference = frame 1
Difference = frame 2
Difference = frame 3
Difference = frame 4
Occasional gap !
Exposure time
Nested Windows
• TMT operates at such a fine plate scale that there is concern over loss of lock of the tip tilt servo due to imperfections in the M3 bearing.
• Nested windows: multiple sample small guide window, then CDS read surrounding window during the exposure delay for the small window.
• Thus if the spot jumps out of the guide window we can get a snapshot just half a frame time later…
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Nested Windows
• Read a 4x4 window multiple times and coadd to beat down noise.
• Read surrounding, larger window once, then revert to small window.
• Calculate differences of (coadds of) small windows and differences of large windows separately.
• Exposure times for each window size are same (though offset by half an exposure time). Noise is lower for the central 4x4 window since it is sampled more often.
• Size of the large window depends on frame rate and fraction of time allocated to big window as opposed to beating down the noise in the 2x2 window.
• If 50% of time is allocated to the larger window at 100 Hz, it can be 31 pixels across with 11 e- read noise, or a 14 pixel window can be read five times reducing read noise to ~5e-.
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NIR wavefront sensing
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coadd coadd coadd coadd
subtract subtract subtract
subtract subtract subtract
Noise vs. Frame Rate measured for various frame sizes
Desired 100Hz operating point gives <3e- read noise for 4×4 window.
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Why this bump?
• Mean noise for all pixels in window.
• Negligible dark current at 80K.
Latest low noise 2.5µm recipeHigher CDS noise
Why these turn-ups?
NIR wavefront sensing
“Dark Signal” (…mentioned in Roger’s talk this morning)
Slope depends on number of reads per pixel, not time
DfA Garching 2009-10-14
NIR wavefront sensing
2020
0.0034 e- / read for 6µs pixel
Noise vs. Frame Rate measured for various frame sizes DfA Garching
2009-10-14NIR wavefront sensing
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Latest low noise 2.5µm recipe
NIR wavefront sensing
“Dark Signal” Effect on Measured NoiseDfA Garching
2009-10-14NIR wavefront sensing
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Latest low noise 2.5µm recipe
NIR wavefront sensing
Just 1/f noise
Still have these…
• The small turn-ups are caused by the “dark signal”
• Subtracting mean effect leaves us with 1/f noise
But…
• still have the “bump” and the unexplained large turn-up at very low frequencies
2×2 WindowDfA Garching
2009-10-14NIR wavefront sensing
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Latest low noise 2.5µm recipe Individual pixels in the 2×2 window
… mean was contaminated by noisy pixels
NIR wavefront sensing
CDS noise is poor predictor of final
noise floor !
64×64 WindowDfA Garching
2009-10-14NIR wavefront sensing
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Latest low noise 2.5µm recipe
NIR wavefront sensing
Hot pixels
Tip-Tilt Wavefront Error
• Median WFE 26.3 nm
DfA Garching 2009-10-14
NIR wavefront sensing
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Demonstrated:2.8e- @ 80Hz,4×4 window>80% QE
Read noise contours
• Zero read noise device needs QE ≥ 65% to be as good as H2RG
• Neither QE nor noise improvements offer significant WFE reduction in this regime
Sky Coverage AnalysisDfA Garching
2009-10-14NIR wavefront sensing
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Requirements:• 2mas jitter @ 50%
Exceeded, ~90% sky coverage
2mas
Let’s party !
The EndDfA Garching
2009-10-14
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NIR wavefront sensing