DfA Garching 2009-10-14

1. DfA Garching 2009-10-14 NIR wavefront sensing 1 We were commissioned to perform a detector selection study for TMT IRIS OIWFS. This presentation stems from the work performed as part of that study.We were commissioned to perform a detector selection study for TMT IRIS OIWFS. This presentation stems from the work performed as part of that study.

2. 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 DfA Garching 2009-10-14 NIR wavefront sensing 2 First Background. NFIRAOS has 6 LGS so why NGS? LGS unable to correct for tilt anisoplanatism. -- reciprocity Primary motivation is to sharpen TT guide stars ��use of single LGS provides no measure of wavefront tilt � use of multiple LGS also unable to measure tilt anisoplanatism TTFA: lenslet array. 2*TT: mono-lens 4.4arcsec probe FOVFirst Background. NFIRAOS has 6 LGS so why NGS? LGS unable to correct for tilt anisoplanatism. -- reciprocity Primary motivation is to sharpen TT guide stars ��use of single LGS provides no measure of wavefront tilt � use of multiple LGS also unable to measure tilt anisoplanatism TTFA: lenslet array. 2*TT: mono-lens 4.4arcsec probe FOV

3. 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 DfA Garching 2009-10-14 NIR wavefront sensing 3 Primary Goal & Metric of study = Sky Coverage Increase in number of red (K,M) stars in NIR improves sky coverage Primary Goal & Metric of study = Sky Coverage Increase in number of red (K,M) stars in NIR improves sky coverage

4. 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 NIR wavefront sensing 4 EBCCD: they may be working on improving dark current, but no promises. -Currently DC ~ 10,000e-/pix/sec at photocathode -RN small only for unity gain (where Intevac doesn�t recommend operating)EBCCD: they may be working on improving dark current, but no promises. -Currently DC ~ 10,000e-/pix/sec at photocathode -RN small only for unity gain (where Intevac doesn�t recommend operating)

5. 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 5

6. 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 DfA Garching 2009-10-14 NIR wavefront sensing 6 May have to reposition probe on final guide windowMay have to reposition probe on final guide window

7. Why Such a Big Detector? (� just to replace a quad cell !) Big telescope aperture makes tiny diffraction core: FWHM = ?/D = 0.008 arcsec at 1.2�m 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 7 On-chip dithering has also been discussedOn-chip dithering has also been discussed

8. 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) �s = 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 8 We need to locate �blob� and start shrinking the window ASAP to reduce window / increase frame rate Once we get to desired frame rate (100Hz or 800Hz) then multiple samplingWe need to locate �blob� and start shrinking the window ASAP to reduce window / increase frame rate Once we get to desired frame rate (100Hz or 800Hz) then multiple sampling

9. Noise & Frame Rate During Zoom DfA Garching 2009-10-14 NIR wavefront sensing 9 These noise contours are actual measurements. The �bright� splotch in the lower right corner will be explained shortlyThese noise contours are actual measurements. The �bright� splotch in the lower right corner will be explained shortly

10. Noise & Frame Rate During Zoom DfA Garching 2009-10-14 NIR wavefront sensing 10 Cut for time (similar to prev slide anyway)Cut for time (similar to prev slide anyway)

11. Readout Mode Fowler sampling usually incurs a loss of duty cycle. To avoid this, reset only occasionally, synthesizing fowler sampling by coadding and differencing non-destructive reads in what might be called �differential multi-accumulate�. Final samples for one exposure serve as the initial samples for the next, so duty cycle is 100%. All the photons are used! For fluxes where read noise matters resets are only needed every 100 to 1000 frames. �. interpolate over gaps. DfA Garching 2009-10-14 NIR wavefront sensing 11 Cut for time (summary of next few slides)Cut for time (summary of next few slides)

12. 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 DfA Garching 2009-10-14 NIR wavefront sensing 12 We all know these, but to put our readmode into perspective�We all know these, but to put our readmode into perspective�

13. 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. DfA Garching 2009-10-14 NIR wavefront sensing 13

14. 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. DfA Garching 2009-10-14 NIR wavefront sensing 14

15. 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 15

16. 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. DfA Garching 2009-10-14 NIR wavefront sensing 16

17. 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� DfA Garching 2009-10-14 NIR wavefront sensing 17

18. 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 Garching 2009-10-14 NIR wavefront sensing 18

19. Noise vs. Frame Rate measured for various frame sizes Desired 100Hz operating point gives <3e- read noise for 4�4 window. DfA Garching 2009-10-14 NIR wavefront sensing 19 Note also I optimized the waveforms in the DSP in order to get the same CDS noise at 6us pixel time as Teledyne�s test data showed with 10us pix time. (Roger should have already discussed this in the morning)Note also I optimized the waveforms in the DSP in order to get the same CDS noise at 6us pixel time as Teledyne�s test data showed with 10us pix time. (Roger should have already discussed this in the morning)

20. �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 20 Either mux glow or self heating, this is NOISE PER READ, not per unit time. At first we thought this was MUX glow but then a closer look revealed what looks like self-heating (see noise maps)� So now let�s look at the Noise vs. Frame Rate again � Either mux glow or self heating, this is NOISE PER READ, not per unit time. At first we thought this was MUX glow but then a closer look revealed what looks like self-heating (see noise maps)� So now let�s look at the Noise vs. Frame Rate again �

21. �Dark Signal� Effects 8�8 dark image after 160,000 frames (75.5s), showing central peak, suggesting self heating profile� 10e-/sec dark current at peak is >1000 times normal DfA Garching 2009-10-14 NIR wavefront sensing 21 From Roger�s talk this morning, here as backupFrom Roger�s talk this morning, here as backup

22. �Dark Signal� Effects 32�32 dark image taken immediately following 8�8 shows remnant hot spot at location of 8�8� DfA Garching 2009-10-14 NIR wavefront sensing 22 From Roger�s talk this morning, here as backup From Roger�s talk this morning, here as backup

23. Noise vs. Frame Rate measured for various frame sizes DfA Garching 2009-10-14 NIR wavefront sensing 23 This is slide 19 again, but now if we subtract in quadrature the contribution from so-called �dark signal� effect/glow �This is slide 19 again, but now if we subtract in quadrature the contribution from so-called �dark signal� effect/glow �

24. �Dark Signal� Effect on Measured Noise DfA Garching 2009-10-14 NIR wavefront sensing 24 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 Accuracy of Still have unexplained increase for the 64^2 window, And the �bump� in the 2^2 window � but the next slide reveals the solutionAccuracy of Still have unexplained increase for the 64^2 window, And the �bump� in the 2^2 window � but the next slide reveals the solution

25. 2�2 Window DfA Garching 2009-10-14 NIR wavefront sensing 25

26. 64�64 Window DfA Garching 2009-10-14 NIR wavefront sensing 26 On the order of 10 hot pixels seen in noise map (large bar in histogram is centered on 5e-) Form a couple of regions which avoid these hot pixels, 4,8,16 ^2 regions, All in the upper left corner. IMPORTANT TO PICK WHERE GUIDE BOX LANDS!On the order of 10 hot pixels seen in noise map (large bar in histogram is centered on 5e-) Form a couple of regions which avoid these hot pixels, 4,8,16 ^2 regions, All in the upper left corner. IMPORTANT TO PICK WHERE GUIDE BOX LANDS!

27. Tip-Tilt Wavefront Error Median WFE 26.3 nm DfA Garching 2009-10-14 NIR wavefront sensing 27 Median WFE from collection of 500 randomly generated star fieldsMedian WFE from collection of 500 randomly generated star fields

28. Sky Coverage Analysis DfA Garching 2009-10-14 NIR wavefront sensing 28

29. Let�s party ! DfA Garching 2009-10-14 29 NIR wavefront sensing