transcript
- Slide 1
- Na- Laser guide star AO with dynamical refocus Sebastian
Rabien, Fernando Quiros-Pacheco, Enrico Pinna, Lorenzo Busoni,
Simone Esposito
- Slide 2
- ELTs Multiple sodium guide stars seems to be one of the major
ingredient to make it work EELT TMT GMT
- Slide 3
- Current planning Multiple 20W class cw lasers Side launch SH
detectors Multiple 20W long pulse, or cw lasers Center launch SH
detectors
- Slide 4
- Perspective (z) elongation at ELT scales Side image of the
laser beacon
- Slide 5
- 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
- Slide 6
- 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
- Slide 7
- Flux distribution in the 90 km focal plane Pointsource 1 laser
spot 1.5 1
- Slide 8
- Simple zmx SH setup collimator Mirror at pupil image location
Pupil steering mirror Lenslet CCD Na-light From ELT 90km focus
Re-imaging
- Slide 9
- Scanning the Sodium layer on a SH sensor ~8 arcsec 80-100km
-> ~4 arcsec 10km FWHM
- Slide 10
- SH Centroiding error In a 12 pix square subaperture 0.5 per
pixel With 3 e - RON Photon noise only
- Slide 11
- 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
- Slide 12
- 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)
- Slide 13
- 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
- Slide 14
- 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
- Slide 15
- 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
- Slide 16
- 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
- Slide 17
- Dynamical refocus with an oscillating membrane principle 1kHz
2kHz 3kHz -33s 33s 0s Min surface curvature
- Slide 18
- Oscillation matching to optics needs Optimize curvature for
each LGS distance VCM 500mm collimator ->25mm membrane
->0.18mm amplitude @2kHz oscillation
- Slide 19
- 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
- Slide 20
- Membrane Curvature 50mm diam 25mm diam ELT need Minimum
curvature at the oscillation extremes Curvature over time 25mm
membrane
- Slide 21
- Limits Higher Stroke, higher frequency Onset of higher order
surface modes
- Slide 22
- 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:
- Slide 24
- 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