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

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

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

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

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 km above the telescope
Easy integration of ‚real‘ optics raytrace later on into the AO model
Use of ‚geometric‘ imaging to retrieve intensity distributions

6. 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

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

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

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

10. 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

11. 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….

12. 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 ?

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

14. 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!
Fernando Quiros-Pacheco Thursday 15:00

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

16. 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

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

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

19. 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

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

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

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)
~<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]

23. 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

24. 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!
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)