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Na- Laser guide star AO with dynamical refocus Sebastian Rabien, Fernando Quiros-Pacheco, Enrico Pinna, Lorenzo Busoni, Simone Esposito

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ELTs Multiple sodium guide stars seems to be one of the major ingredient to make it work EELT TMT GMT

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Current planning Multiple 20W class cw lasers Side launch SH detectors Multiple 20W long pulse, or cw lasers Center launch SH detectors

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Perspective (z) elongation at ELT scales Side image of the laser beacon

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Image of the Na-guide star Using a zmx optics model for the 5 mirror EELT to create the Na laser Image (still at 42m) Scanning the Na layer: Placed the guide star from 80-100km above the telescope Easy integration of real optics raytrace later on into the AO model Use of geometric imaging to retrieve intensity distributions

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Image of the Na- layer beacon Laser beacon at 90 km above telescope 90km ELT Focal plane, ~7m after the infinite focus 80km 85km90km95km 100km 1.8m 80km 85km 90km 100km 95km

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Flux distribution in the 90 km focal plane Pointsource 1 laser spot 1.5 1

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Simple zmx SH setup collimator Mirror at pupil image location Pupil steering mirror Lenslet CCD Na-light From ELT 90km focus Re-imaging

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Scanning the Sodium layer on a SH sensor ~8 arcsec 80-100km -> ~4 arcsec 10km FWHM

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SH Centroiding error In a 12 pix square subaperture 0.5 per pixel With 3 e - RON Photon noise only

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SH Centroiding II Round 1.5, 6 pixelelliptical 1.5x4, 12 pixel Round spot can be measured in a smaller subaperture!!! 0.5/pix 1.5 spots 6 pixel for round 12 pixel for the ellipse 3 e - RON The amount of photons needed to achieve a given sigma is ~4..10 times higher in the outer sub-apertures (ELT, center launch) While the central spots get smaller in good seeing conditions, the elongation stays constant! -> good seeing does not help. Round spot can be measured in a smaller subaperture!!! 0.5/pix 0.8 spots 4 pixel for round 12 pixel for the ellipse 3 e - RON

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Static modes & calibration error ->Could be useful to calibrate the AO with a z-elongated source ? Calibration spots On-sky spots ( Rayleigh guide star system 8m, 12 km 500m gating)

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Spot elongation on SH sensor Required number of photons in the outer subapertures multiplies by (at least) 4 Measurement accuracy in the elongation direction does not improve with seeing! Large subapertures required Large detector required (80x80-> 1024pix at least!), or special format detectors required Special treatment for calibration required Changes in the Na-layer height distribution migrate into sensing errors-> very frequent truth sensing required

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Pyramid Sensitivity with Refocused LGS spot LGS spot extension on sky: LGS spot size: 0.8 in diameter 903km Gaussian Intensity distribution Pyramids @ELT: Fernando Quiros-Pacheco Thursday 15:00 Gain in sensitivity on this scale should be similar to SH But: much smaller detectors, only 4 pixel per subaperture, ->less noise in the subaperture. Net gain for various settings: !Work in progress! Refocussed Elongated

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Dynamical refocus Utilizes pulsed lasers Adjusts the WFS optics in realtime to follow the pulse through the atmosphere (Sodium layer) Generates a constant divergence at the output Image plane dynamical refocus (Angel et al.) Incoming LGS light Pupil image location Variable curvature mirror Collimated output Pupil plane dynamical refocus

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Dynamical refocus Demonstrated for Rayleigh laser guide stars (MMT, Angel & Lloyd Hart) Extreme elongation, RLGS 20-30km Solid aluminum rod oscillator Difficult to move to an ELT scale due zto extreme low F# and oscillation requirements

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Dynamical refocus with an oscillating membrane principle 1kHz 2kHz 3kHz -33s 33s 0s Min surface curvature

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Oscillation matching to optics needs Optimize curvature for each LGS distance VCM 500mm collimator ->25mm membrane ->0.18mm amplitude @2kHz oscillation

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Testing membranes Time sequence Al-coated Nitrocellulose membranes under test Acoustically driven closed Helmholtz cavity for efficient and quiet operation Membrane Acoustic cavity Window Loudspeaker

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Membrane Curvature 50mm diam 25mm diam ELT need Minimum curvature at the oscillation extremes Curvature over time 25mm membrane

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Limits Higher Stroke, higher frequency Onset of higher order surface modes

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Lasers Required laser parameters for dynamical refocus are: Pulsed operation ~1.5kHz pulse repetition rate (1 pulse in the air) ~3kHz (2 pulses in the air) ~

Uplink correction Nice possibillity to further increase the SNR on the LGS detection Recent studies: ->Gavel et al., Villages (SPIE 2008) ->Gavel (NGAO trade study) Green or UV Rayleigh laser and wavefront sensor 589nm laser Launch telescope ~5x5 subappertures on a 0.5m telescope -> DMs available -> Lasers available -> Sensors available DM Close to what is needed:

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Summary Using pulsed lasers: Enables to gate out the Rayleigh scattering i.e. removes the fracticide effect Allows the use of a dynamical refocus Dynamical refocussing with a membrane VCM: Sharpens the spots on a SH sensor, removes the elongation Sharpens the spot in the focal plane, enables the use of pyramid sensors on ELT scale Enables the use of small detectors Membrane VCMs: Can be acoustically driven at the desired frequencies Have shown in the laboratory sufficient stroke for the use at an ELT Total Gain: 410 times less photons required? Worth to be looked at much more carefully !Work In Progress! Pyramids @ ELT Fernando, Thursday More detailed simulations: To appear in the paper