Transcript Remote sensing of extrasolar planets

Na Laser Guide Stars for CELT
CfAO Workshop on Laser Guide Stars
99/12/07
Rich Dekany
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
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
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
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
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
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)?
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