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LGS-AO Performance Characterization Plan. AOWG meeting Dec. 5, 2003 A. Bouchez, D. Le Mignant, M. van Dam for the Keck AO team. LGS-AO performance characterization effort. Purpose (from CfAO proposal):

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LGS-AO Performance Characterization Plan

AOWG meetingDec. 5, 2003A. Bouchez, D. Le Mignant, M. van Damfor the Keck AO team

lgs ao performance characterization effort
LGS-AO performance characterization effort
  • Purpose (from CfAO proposal):
    • Demonstrate and characterize the Keck II LGS AO system for science via collaboration with the LGS and science AO community.
    • Using the output from goal 1, optimize the Keck LGS AO system and feed back the lessons learned to future LGS AO systems.
  • Products:
    • Full LGS-AO error budget.
    • Accurate planning information for users.
    • New data on sodium layer structure and density variations.
lgs ao characterization plan
LGS-AO Characterization Plan
  • LGS-AO characterization effort is closely related to
    • LGS-AO development priorities
    • demonstration of LGS-AO observing modes.
  • Goals
    • On-axis performance (Strehl & FWHM vs. magnitude) fully characterized by Feb. 2004.
    • Off-axis performance (anisoplanatism, dither accuracy, etc.) documented by Aug. 2004.
error budgets
Error budgets
  • Not one single error budget!
    • Performance
      • Subsystems
      • LGS performance
      • Integrated system performance
    • Throughput
    • Field of view
      • off-axis FOV
      • dithering precision
    • Operational aspects
      • Overheads
      • Stability
sodium layer characterization
Sodium layer characterization
  • Continuously monitor mean altitude through WFS focus.
  • Continuously monitor sodium density through return power / projected power.
  • Nightly map of density structure with unstacked images
subsystem performance
Subsystem performance
  • Tip-tilt sensor stage - STRAP Residual tip-tilt as function of:
    • Star magnitude.
    • TT reference motion (asteroid, etc.)
    • Sky background (moonlight, raleigh).
    • Resolved sources (AGN, close binaries).
  • Low-bandwidth wavefront sensor - Precision of wavefront measurement as function of:
    • Star magnitude & integration time.
    • Limit due to aberrations in LBWFS.
    • Off-axis location of tip-tilt stage.
    • Pupil rotation rate
  • Tip-tilt sensor positioning accuracy
    • Repeatability
    • Accuracy
  • Effect of LGS spot elongation on WFS measurement
lgs performance
LGS performance
  • Laser spot size / elongation / power
    • Temporal stability
    • How it is affected by seeing
    • Correlation against laser table diagnostic measurements
  • LGS tip-tilt and jitter
    • Flexure compensation performance
    • Magnitude of atmosphere-induced jitter
    • Residual tip-tilt after up-tip-tilt implementation
  • Sodium layer
    • Track mean altitude
    • Power returned vs. power out (sodium density)
    • Layer structure from unstacked images.
integrated system performance
Integrated system performance
  • Strehl on target versus
    • Tip-tilt star magnitude.
    • Tip-tilt star separation.
    • Spot elongation / sodium altitude / elevation
    • Laser power / sodium density
    • Integration time
  • Anisoplanatism
    • Different from NGS?
  • Focal anisoplanatism
    • Predict from Cn2 profiles
    • Measure from total error budget
  • Operational aspects
    • Acquisition overhead
    • Dithering overhead
    • Mean time between failures
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