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LGS-AO Performance Characterization Plan

The Keck II LGS Adaptive Optics (AO) characterization plan aims to demonstrate and optimize the LGS AO system's performance for scientific observations in collaboration with the LGS and science AO community. Key products include a comprehensive LGS AO error budget, accurate planning information for users, and new data on sodium layer structure and density variations. Goals include detailed characterization of on-axis and off-axis performance metrics such as Strehl ratio, FWHM vs. magnitude, anisoplanatism, and overall system throughput by 2004.

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LGS-AO Performance Characterization Plan

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  1. LGS-AO Performance Characterization Plan AOWG meetingDec. 5, 2003A. Bouchez, D. Le Mignant, M. van Damfor the Keck AO team

  2. 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.

  3. 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.

  4. 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

  5. Performance error budget (on-axis)

  6. 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

  7. NGS Strehl vs. magnitude

  8. 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

  9. 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.

  10. 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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