Low-Noise IR Wavefront Sensing with a Teledyne HxRG

DfA Garching 2009-10-14

NIR wavefront sensing

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Low-Noise IR Wavefront Sensingwith a Teledyne HxRG

David Hale

Gustavo 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.



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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.



5 5

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.



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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.



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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


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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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 time

Frame 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 time

Frame 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.



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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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Coadd m

Coadd m

Coadd m

Coadd m

Coadd m

Reset Coadd m

Difference = frame 1




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-.

DfA Garching2009-10-14


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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.

DfA Garching2009-10-14


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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?


“Dark Signal” (…mentioned in Roger’s talk this morning)

Slope depends on number of reads per pixel, not time



2020

0.0034 e- / read for 6µs pixel

Noise vs. Frame Rate measured for various frame sizes DfA Garching


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Latest low noise 2.5µm recipe


“Dark Signal” Effect on Measured NoiseDfA Garching


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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


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Latest low noise 2.5µm recipe Individual pixels in the 2×2 window

… mean was contaminated by noisy pixels


CDS noise is poor predictor of final

noise floor !

64×64 WindowDfA Garching


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Hot pixels

Tip-Tilt Wavefront Error

• Median WFE 26.3 nm



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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


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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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Low-Noise IR Wavefront Sensing with a Teledyne HxRG

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