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4389 - 42 Laser guide star adaptive optics at the Keck Observatory Adam R. Contos, Peter L. Wizinowich, Scott K. Hartman, David Le Mignant, Christopher R. Neyman, Paul J. Stomski, Jr., and Douglas Summers W.M. Keck Observatory SPIE Astronomical Telescopes and Instrumentation Conference 23 August 2002, Waikoloa, HI Talk Outline Overview Sky Coverage for LGS and NGS Modes Optical Path New LGS AO Hardware System Upgrades & Issues Laser Guide Star Future Summary Overview Traditional AO with NGS – Bright star in close proximity ~ 1% coverage – Sodium LGS for nearly complete coverage • Still require an NGS, but can be much dimmer Keck II System Upgrades for LGS AO – – – – Low Bandwidth Wavefront Sensor (LBWFS) Tip/Tilt Sensor (TTS), using quad-cell APDs Software control Laser system (described in following talk) Scheduled for integration in 2003 Sky Coverage: NGS and LGS 1Bahcall & Soneira, 1981; 2Keck Report 208, 1996. Sky Coverage vs. NGS Limiting Magnitude (Varying Galactic Lat. and Field of View) 22 Reference Star Magnitude (R-band) 21 20 LGS 19 18 17 16 15 14 NGS 13 12 30'' FOV @ 45º lat 11 60'' FOV @ 45º lat 10 60'' FOV @ 0º lat 9 8 0.10% 1.00% 10.00% Sky Coverage 100.00% Optical Path Highlighted AO Bench Elements: 1-Image Rotator 2-Tip/Tilt Mirror 4-Deformable Mirror 6-IR Dichroic 12-Sodium Dichroic 13-Field Steering Mirrors 14, 15-Wavefront Sensing Camera and Focus Camera Stage 17-Beamsplitter 18-Low Bandwidth Wavefront Sensor, Tip/Tilt Sensor & Stage 19-Acquisition Camera LBWFS & TTS Description Low Bandwidth Wavefront Sensor – – – – Shack-Hartmann lenslet array sensor 304 subapertures (240 active) Photometrics 512x512 CCD detector Readout every few sec. to few min. Tip/tilt Sensor – Quad-lens – Fiber-fed APDs Mounted on 3-axis tip/tilt stage Beamsplitter cube – 90% to TTS, 10% to LBWFS System Upgrades & Issues (1) 1 no bright NGS nearby LGS 2 3 4 5 laser spot moving wavefront changing WFS to deformable rapidly mirror loop laser spots elongated & distorted in WFS sodium distance changing recalibrate using LBWFS centroid & gains move WFS (focus) 6 7 WFS to Up-Tip/Tilt (laser pointing) sodium in science camera pupil size changes IR transmissive dichroic & science filters motorized pupil relay optics System Upgrades & Issues (2) no tip/tilt info; limited 8 focus info; no 'truth' calibrator 9 10 Dim NGS LGS wavefront will drift w.r.t. NGS LGS provides no tip/tilt information LGS focus will drift 11 w.r.t. NGS LBWFS to determine avg. wavefront (truth) quad cell TTS w/ APDs LBWFS to determine best focus System Upgrades & Issues (3) LBWFS/TTS co-located on x,y,z TSS stage when move TSS, LBWFS looses 14 registration with deformable mirror sodium trasmissive see excess sodium dichroic & sodium light in long 13 rejection filter for exposures TTS/LBWFS NGS not co-located 12 w.r.t. LGS LBWFS lenslet mounted on x, y stage 1st Laser Propagation (12/01) Laser power ~ 17 W FWHM ~ 1.4” (seeing ~ 1”) Magnitude ~ 9.5 Future Milestones and Deliverables First corrected images on NIRC2 with laser LGS AO facility operable by AO experts First LGS AO engineering science LGS AO observing strategy decision First shared-risk science First light engineering science paper LGS AO facility operable for science by OA and one expert First queue/non-shared science Target Date Spring 2003 Summer 2003 Fall 2003 Fall 2003 Winter 2004 Spring 2004 Fall 2004 Winter 2005 Summary of LGS AO at Keck Achieved first laser propagation AO upgrades being incorporated for LGS Integration milestones throughout 2003 – LGS AO on world’s largest optical telescope will become incredible tool for astronomy – First operational LGS on an 8-10 m telescope System Flow & Control