LGS-AO Performance Characterization Plan
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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

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Lgs ao performance characterization plan

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


Lgs ao performance characterization plan

Performance error budget (on-axis)


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


Ngs strehl vs magnitude

NGS Strehl vs. magnitude


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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