guide star AO with dynamical refocus Sebastian Rabien Fernando QuirosPacheco Enrico Pinna Lorenzo Busoni Simone Esposito ELTs Multiple sodium ID: 384045
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Slide1
Na- Laser
guide star AO with dynamical refocus
Sebastian
Rabien
,
Fernando
Quiros-Pacheco
,
Enrico
Pinna
,
Lorenzo Busoni, Simone
EspositoSlide2
ELT‘s…
Multiple sodium guide stars seems to
be
one
of the major ingredient to make it work…
EELT
TMT
GMTSlide3
Current planningMultiple 20W class
cw lasersSide launchSH detectors
Multiple 20W
long
pulse,
or
cw
lasers
Center
launch
SH
detectorsSlide4
Perspective (z) elongation at ELT scales
Side
image
of
the
laser beaconSlide5
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 telescopeEasy
integration
of
‚real‘
optics
raytrace later on into the AO modelUse of ‚geometric‘ imaging to retrieve intensity distributionsSlide6
Image of the Na-layer beacon
Laser
beacon
at
90 km
above
telescope
90km ELT
Focal
plane,
~7m after
the
infinite
focus
80km
85km
9
0km
95km
100km
1.8m
80km
85km
90km
100km
95kmSlide7
Flux distribution in the 90 km focal
plane
Pointsource
1‘‘
laser
spot
1.5‘‘
1‘‘Slide8
Simple zmx
SH setup‚
collimator
‘
Mirror
at
pupil imagelocation
Pupil
s
teering
mirror
Lenslet
CCD
Na-light
From
ELT
90km
focus
Re-imagingSlide9
Scanning the Sodium layer on a SH
sensor
~8
arcsec
80-100km
-> ~4
arcsec
10km FWHMSlide10
SH
Centroiding error
In a 12
pix
square
subaperture
0.5‘‘
per
pixel
With
3 e
-
RON Photon noise
onlySlide11
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‘‘ spots6 pixel for
round12 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 Slide12
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
)Slide13
Spot elongation on SH sensor
Required number of photons in the outer subapertures multiplies by (at least) 4Measurement accuracy in the elongation
direction
does
not
improve
with seeing! Large subapertures requiredLarge detector required
(80x80-> 1024pix at least!), or
special format detectors
required
Special
treatment
for
calibration requiredChanges in the Na-layer height distribution migrate into sensing errors-> very frequent
truth sensing requiredSlide14
Pyramid Sensitivity with Refocused LGS spot
LGS spot extension on sky:LGS spot size: 0.8” in diameter 90±3km Gaussian Intensity distribution
Pyramids
@ELT:
Fernando
Quiros-Pacheco
Thursday 15:00
Gain in sensitivity on this
scale
should
be
similar to SHBut: much smaller detectors, only 4 pixel per subaperture, ->less noise in the subaperture.
Net gain for various
settings:!Work in progress!
Refocussed
ElongatedSlide15
Dynamical refocusUtilizes
pulsed lasersAdjusts 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
(
A
ngel et al.)
Incoming LGS lightPupil imagelocation
Variable curvaturemirror
Collimated output
Pupil
plane
dynamical
refocusSlide16
Dynamical refocusDemonstrated
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…Slide17
Dynamical refocus with an oscillating
membrane
principle
1kHz
2
kHz
3
kHz
-33µs
33µs
0µs
Min
surface
curvatureSlide18
Oscillation
matching to optics needsOptimize
curvature
for
each
LGS distanceVCM
500mm
collimator
->25mm
membrane
->0.18mm
amplitude
@2kHz
oscillationSlide19
Testing membranes
Time
sequence
Al-
coated
Nitrocellulose
membranes
under test
Acoustically driven
closed
Helmholtz
cavity
for
efficient and quiet operationMembrane
Acousticcavity
Window
LoudspeakerSlide20
Membrane Curvature
50mm
diam
25mm
diam
ELT
need
Minimum
curvature
at
the
oscillation
extremes
C
urvature
over
time
25mm
membraneSlide21
Limits
Higher
Stroke
,
higher
frequency
Onset
of
higher
order
surface modesSlide22
LasersRequired
laser parameters for dynamical refocus are:Pulsed operation~1.5kHz pulse repetition rate (1 pulse in the air)~3kHz (2 pulses in the
air
)
~<
3
µs 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.)
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]
Spectral
optimization
,
backpumping
or
chirping can
improove here!Slide23
Uplink correctionNice
possibillity to further increase the SNR on the LGS detectionRecent studies
:
->
Gavel
et al.,
Villages
(SPIE 2008)->Gavel (NGAO trade study)
gain in spotsize with correcting
the
uplink
Plus
static aberation correction!-> factor 2 less photons required for a given σ
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
:Slide24
SummaryUsing pulsed
lasers:Enables to gate out the Rayleigh scattering i.e. removes the fracticide effectAllows 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 scaleEnables the use of small detectors
Membrane VCMs:Can be acoustically
driven at the desired
frequenciesHave shown in
the
laboratory
sufficient
stroke
for
the
use
at
an
ELT
Uplink
correction
Can
provide
a
gain in spotsize (plus static
error & flexure correction)
Total
Gain: 4…10 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…