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

ELT‘s… GMT EELT Multiple sodium guide stars seems to be one of the major ingredient to make it work… TMT

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

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

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

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

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

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

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

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

SH Centroiding II 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 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 Round 1.5‘‘, 6 pixel elliptical 1.5x4‘‘, 12 pixel 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….

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

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

Pyramid Sensitivity with Refocused LGS spot LGS spot extension on sky: LGS spot size: 0.8” in diameter 90±3km Gaussian Intensity distribution 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. Elongated Refocussed Net gain for various settings: !Work in progress! Pyramids @ELT: Fernando Quiros-Pacheco Thursday 15:00

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 Collimated output Pupil image location Variable curvature mirror Incoming LGS light Image plane dynamical refocus (Angel et al.) Pupil plane dynamical refocus

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

Dynamical refocus with an oscillating membrane 3kHz 2kHz 33µs Min surface curvature principle 1kHz

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

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

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

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

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) ~<3µs pulse duration as a compromise between saturation avoidance and illuminated column ~10…15% less photon return than a cw format (e.g. Rampy et al.) Spectral optimization, backpumping or chirping can improove here! Pulsed laser systems Lick/Keck II (dye, 150ns) Gemini (qcw mode locked) TIPC (pulsed 100µs) LLNL fibre sum frequency (200ns lab, 30µs goal) Starfire 840Hz laser Injection mode locked laser, 800Hz, 1-3µs (Munch et al.) Average laser power [W]

Uplink correction Nice possibillity to further increase the SNR on the LGS detection 2 gain in spotsize with correcting the uplink Plus static aberation correction! -> factor 2 less photons required for a given σ ~5x5 subappertures on a 0.5m telescope -> DMs available -> Lasers available -> Sensors available Recent studies: ->Gavel et al., Villages (SPIE 2008) ->Gavel (NGAO trade study) Green or UV Rayleigh laser and wavefront sensor Close to what is needed: 589nm laser DM Launch telescope

Summary Total Gain: 4…10 times less photons required? 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 Total Gain: 4…10 times less photons required? Worth to be looked at much more carefully !Work In Progress! Pyramids @ ELT Fernando, Thursday Membrane VCMs: Can be acoustically driven at the desired frequencies Have shown in the laboratory sufficient stroke for the use at an ELT More detailed simulations: To appear in the paper… Uplink correction Can provide a 2 gain in spotsize (plus static error & flexure correction)