1. Telescope Errors for NGAO Christopher Neyman & Ralf Flicker W. M. Keck Observatory Keck NGAO Team Meeting #4 January 22, 2007 Hualalai Conference Room, WMKO

2. 2 Telescope Wavefront Errors WBS elements Changes in scope denoted in red 3.1.1.1.2 Telescope Dynamic Performance Data –Improve/document our understanding of the actual primary mirror (telescope) wavefront errors. (set spec from simulation) 3.1.1.1.3 Telescope Static Wavefront Errors –Improve/document our understanding of the actual primary mirror (telescope) wavefront errors. (set spec from simulation) 3.1.2.1.9 Telescope Wavefront Errors –Review new data on the telescope static and dynamic wavefront errors. Determine how and whether NGAO can correct for these errors. Determine the performance benefit of a large LOWFS patrol field to enable use of the brightest possible NGS (Pending Results of LOWFS study). Consider whether a separate sensor outside the NGAO FOV would be useful for measuring/correcting the telescope errors. Complete when impact on current Keck LGS AO system understood and impact on NGAO reviewed

3. 3 Overview Slide/Outline Telescope Dynamic Errors –Segment tilt –Full aperture tilt Static Errors –Segment figure –Segment phasing

4. 4 AO simulation approach Simulate each effect individually, as opposed to “all in” numbers Useful for later parameterization in wavefront error budget Typical parameters: –NGS AO simulation –48x48 SH (4x4 pix per sub-ap, 0.5" pixel size), –49x49 actuators (Fried geometry) –PZT modeled influence functions –No turbulence –No noise –WFS integration time = 1ms -> integration + 1 frame delay (readout), pure integrator –A standard SVD-based wavefront reconstructor was used.

5. 5 Dynamic segment errors: composite of several data sets Segment motion datasets –Primary actuator control system (ACS) fast data capture Erm (TMT) used ACS FDC to estimate segment motions Highly correlated motion, looks like focus mode Power at 29 Hz ~10-20 mas rms motion Historical problems reconstructing wavefronts with this system –Issues documented in KAON 199 –IF accelerometers on 3 M1 segments High noise Some uncorroborated peaks Calibration was uncertain 29 Hz power similar to ACS –F. Dekens thesis Optical measurement: 15 mas rms Partially correlated motion Vibration environment improved during intervening time (circa 1997)

6. 6 Dynamic: Segment Errors Adopt following baseline: –Uncorrelated segment motions –3 peaks at 28.3, 29.06, 29.68 Hz –Total tilt 0.015 arc seconds rms –Consistent with Dekens, ACS, IF

7. 7 Dynamic: Segment Errors Adopt following baseline: –Uncorrelated segment motions –3 peaks at 28.3, 29.06, 29.68 Hz –Total tilt 0.015 arc seconds rms –Consistent with Dekens, ACS, IF

8. 8 Simulation results for dynamic segment tip tilt errors Uncorrected Tilt (rms mas) Uncorrected wavefront (rms nm) Corrected wavefront 1000 Hz (rms nm) Corrected wavefront 500 Hz (rms nm) Optimal act. fitting (rms nm) 11.8 (0.5x)31162310 15.7 (1x)44243214 22.2 (2x)62425020 31.4 (4x)87929328 Adjusted loop gains for optimal results NGAO June proposal allocation was 23 nm Last column infinite bandwidth, no WF sensing/reconstruction error

9. 9 Snap shot of simulation time series shows AO amplification Left to right: 0.5x,1x,2x,4x baseline Top: input Bottom: residual 0.5x 1x 2x 4x

10. 10 AO reconstruction of segmented wavefront amplifies error Dekens conjectured that AO will increase the errors in a segmented wavefront AO sensor/reconstructor assumes continuous wave front AO correction results in tiled wavefront Amplification of coherent tilts

11. 11 AO reconstruction of segmented wavefront amplifies error Dekens conjectured that AO will increase the errors in a segmented wavefront AO sensor/reconstructor assumes continuous wave front AO correction results in tiled wavefront Amplification of incoherent tilts

12. 12 Tip/tilt errors Same frequencies seen throughout observatory ~29 Hz –IF Accelerometers on M2, M3 High noise Some uncorroborated resonance's Calibration –AO bandwidth error NGS data (van Dam et al.,Applied Optics 2004) STRAP telemetry? (Future) NGWFC telemetry? (Future) –AO simulation No significant rejection below 300 Hz sampling Poor performance on faint NGS for tracking

13. 13 Static segment figures errors CfAO poster results UFS segment reconstruction

14. 14 Static segment piston errors PCS phasing errors –60nm (nominal) –10nm (best algorithm) Did not include interaction with segment figures –makes actual wavefront error higher than phasing error alone AO stack algorithm (future) Uncorrected Piston (rms nm) Uncorrected wavefront (rms nm full aperture) Corrected wavefront (rms nm) Optimal act. fitting (rms nm) 108.532 2017.564 4034138 6052.51912 807022.516 1008833.520 Residual Input 100nm rms input

15. 15 Summary of simulation results Full aperture tip tilt errors could dominate tip/tilt error budget –Poor sky coverage –“Encircled Energy Science” might be impacted less Segment motion –Acceptable error, comparable to NGAO proposal Segment figures –Acceptable error, already included in NGAO proposal Segment phasing –Small, interaction with figure errors not tested

16. 16 NGAO: instrument system for image stabilization Idea: scale up commercial image stabilization lens by 100x How: use several uncorrected NGS outside NGAO field to stabilize telescope Conventional system requires –300-600 Hz sampling: factor 10x-20x for f -3db ~30Hz –Large SNR for seeing limited spots  cent ~ FWHM/SNR –Limiting magnitude ~14, full aperture tilt –Requires 20 arc minute FOV

17. 17 NGAO: instrument system for image stabilization Assume IF KAT parametric oscillator as typical of improvement –good rejection at 50-100 Hz sampling Limiting magnitude ~16, full aperture tilt Requires 5 arc minute FOV Consider possibility of extended field LOWFS (ExLOWFS) Could be key for NGAO meeting its error budget

18. 18 NGAO: PCS all the time Current PCS subapertures would provide low sky coverage Full aperture solutions for phasing telescope Phase Discontinuity Sensing (PDS) - Chanan Donut – Tokovinin (Neyman MAGIQ guider upgrade) Limiting magnitude ~15 R mag with minute exposure times Not needed NGAO appears to cope with current errors Sample PDS images: Left: Phased Right: One Segment Poked

19. 19 References T. Erm, “Report of the Keck mission March 10-19 Part 1. Vibrations,” Caltech, 3-31-2004 T. Erm, “Analysis of Keck vibration data from 4/16/04, 4/29/04, 5/1/04,” Caltech, 4-16-2004 F. Dekens, “Atmospheric characterization for adaptive optics at the W. M. Keck and Hale telescopes,” PhD thesis, UCI, 1999 G. Chanan, et al., “The W. M. Keck Telescope phasing camera system”, SPIE 2198, 1994 G. Chanan, et al., “Phasing the mirror segments of the W. M. Keck Telescope II: the narrow-band phasing algorithm”, Applied Optics, 2000 van Dam, et al., “Performance of the Keck Observatory adaptive- optics system”, Applied Optics, 2004 E. Johansson and G. Chanan, “Summary of WFS/FDC Vibration Tests”, KOAN 199, 2000.