Telescope Errors for NGAO

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

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Overview Slide/Outline

• Telescope Dynamic Errors
• Segment tilt
• Full aperture tilt
• Static Errors
• Segment figure
• Segment phasing

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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 -gt integration 1
  frame delay (readout), pure integrator
• A standard SVD-based wavefront reconstructor was
  used.

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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)

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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

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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

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Simulation results for dynamic segment tip tilt
errors
Adjusted loop gains for optimal results NGAO
June proposal allocation was 23 nm Last column
infinite bandwidth, no WF sensing/reconstruction
error
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Snap shot of simulation time series shows AO
amplification
Top input Bottom residual

• Left to right 0.5x,1x,2x,4x baseline

0.5x 1x
2x 4x
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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
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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
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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

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Static segment figures errors

• CfAO poster results
• UFS segment reconstruction

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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)

100nm rms input
Residual Input

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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

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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 scent
  FWHM/SNR
• Limiting magnitude 14, full aperture tilt
• Requires 20 arc minute FOV

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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

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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

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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.