Na Laser Guide Stars for CELT

1. Na Laser Guide Stars for CELT CfAO Workshop on Laser Guide Stars 99/12/07 Rich Dekany

2. California Extremely Large Telescope (CELT) • Early concept • 30 m diameter, filled, segmented hyperbolic primary mirror • Alt/az mount (fully steerable) • To be built at excellent site • Extensive instrumentation to exploit various AO modes • Individually or in combination

3. CELT AO performance requirements • Undefined, but strong function of science requirements • For example, what are sufficient statistics for studies of galaxy formation in early Universe? • Possible AO modes: • High order AO (HOAO) corrected field (1’ - 2’ diameter) • Strehl > 50%+- 5% at 1mm (l/D = 6.9 mas) • Requires MCAO • Low order AO (LOAO) corrected field (2’ - 6’ diameter) • MCAO tradeoff between uniformity of correction and rolloff with field) • Full sky coverage with 2 m AO subapertures • Classic NGS (no MCAO) • Implemented w/ deformable secondary to minimize # of surfaces • Active control (ACS) only

4. CELT AO preliminaries • Assumptions • Mauna Kea seeing profile (Keck AO Set B, 0.55mm) r0 = 0.2 m Q0 = 4 arcsec Qk = 160 arcsec t0 = 3 ms tk = 140 ms d0 = 7 m (Assume Na LGS’s only) L0 = 50 m • No use of NGS’s for high order wavefront information • NGS stars for tip/tilt determination utilize the entire 30m aperture

5. CELT LGS issues • CELT goal is total wavefront error of 133 nm rms • Since LGS’s remain expensive, allow a generous 75 nm for laser specific errors • Assume FA dominates LGS errors • Focal anisoplanatism error = efa = (D/d0)5/6 [rad] --> D < 6 m • This calls for ~ 24 Na LGSs • Other multiple LGS errors to worry about • Coma anisoplanatism, propagation of conic tilt errors, beacon position uncertainty • Tip/tilt NGS’s • Down to mV = 20 implies N = 2-3 stars per square arcmin

6. CELT LGS issues (continued) • Other error budget terms • Keeping focal anisoplanatism as the performance driver requires • Small bandwidth and measurement errors: • 1.5 kHz high order sampling (to get 75 Hz -3db bandwidth) • ~ 10 W average power per beacon, each delivered into ~ 1 arcsec2 focus • Yields ~ 100 photons per measurement (0.41 cm subaps, 0.3 total transmission) • Result is ~50 nm rms measurement error (Sandler, et. al.) • Fitting error • N ~ 10,000 efficient d.o.f. correction for highest order mirror • Other MCAO mirrors may be ~ 1,000 actuator range • Compute speed > 20x performance of highest-order DoD AO system demonstrated to date

7. CELT LGS open issues • Entire LGS implementation dependent on capability to accurately describe, model, and understand MCAO • For example, what is the role of NGS in high order MCAO? • Can lesser, but sufficient, sky coverage may be available with fewer (or even no) LGS beacons • Probability of finding N=5 guides stars of mV < 14 within 6 arcmin diameter is ~ 10% • Can MCAO be extended to exploit such large area? • What is degree and uniformity of correction? • What information beyond tip/tilt can be gained from NGS’s • What can be gained using variable integration times for each NGS? • For example, what is performance of an AO system which slows as much as necessary to get good SNR on brightest 5 stars in field? • How can all this be modeled (requires supercomputing resources)?

8. CELT LGS issues (continued) • AO design strategy • To assure technology readiness, encourage Na LGS development today • Discussion this afternoon: “How to do this?” • Resolve major sky coverage issues ASAP • LGS issues permeate nearly all design issues • For example, site selection • Latitudinal/seasonal/diurnal variations in column density and height, frequency of ‘enhanced Na layers’, infrastructure, safety, neighborly relations, etc. • Emphasize modeling effort in near term • Validate models • Multiple NGS wavefront sensing • Gemini MCAO system