LGS AO for LBT: possible Implementation Options

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LGS AO for LBT possible Implementation Options

• Arcetri
• Bologna
• Padova

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Talk Summary (after Nov06 Florence LBT LGS
workshop)

• Sodium Laser option
• A single WFS placed in the W unit provides
• NIR GLAO (ref. stars 2 radius)
• Single ref. star AO correction boosting Sky
  Coverage (SC)
• 4 ref. stars small sep. for high SC and SR
  (tomographic WFS)

• Raileigh/Sodium Laser option
• 3/4 WFSs placed outside of the W unit provide
• GLAO (ref. stars 4 radius)
• Single ref. star high SC AO correction

Instruments LUCIFER, MODS, LBTI (L-N)
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The Layer Oriented (LO) SH
The idea is to co-add spots coming from
different Sodium LGSs....
EspositoRagazzoni
The situation in one lenslet FoV of the LO SH
sensor
...a computed generated hologram split the
beam into 5 beams projected on a 2 arc min
diameter M. Lloyd-Hart, (MMT)
The effective spot dimension is approx.
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The concept of single/multi LGS SH WFS
Turbulent layer image
Collimator
Turbulent layer
Pupil image
Telescope
a qS fT/fC
fT
fC
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DT 8.2 m fT 123.0 m fC 0.2 m qS 2
arcmin nSUB 15 a 20.5 dPUP 13.3 mm dSH
0.88 mm fSH 4.8 mm
fSH 2 dSH / a 9.6 mm
Turbulent layer image
fSH n dSH / a q qS, qS/2, , qS/n
fSH dSH / a
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First Order optical design of SH lenslet array
Situation in Nov06
LO SH lenslet array
LBT F/15 focal plane
Collimator basics F 180mm Dcoll 160 mm Dpup
12mm
SH CCD plane
System collimator
A first optical design (E. Diolaiti) of the
collimator was showed during Florence Workshop.
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Some issues of LOSH...
Issues rised in Nov.06 and Dec06 meetings

• Rayleigh fratricide effect
• Optical design of collimator and SH spots imaging
  on WFS CCD
• NGS tip-tilt measurement error using the W unit
  WFS (CCD39)
• LGSs tracking
• Tip-Tilt from LGS

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Rayleigh fratricide effect
1 projector from the center of the
telescope (behind M2)
4 projectors from the side of the telescope
1 projector 2m apart from the side of the
telescope
Gating is not needed for the multi sources
Sodium system.
Tel. pup
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The collimator E. Diolaiti OAB
F/15 beam from LBT
Collimator basics F 180mm Dcoll 160 mm Dpup
12mm
length 250mm heigth 260mm
TL
L1
System focused at 90Km. Distance between
telecentric lens (TL) and L1 allows for 150 Km
LGS distance tracking or 53deg off Zenith.
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The lenslet array
SH spots have a width of 1, negligeable wrt spot
elengation effect
LO SH lenslet basics (15x15 sub.) F
21mm Dlenslet 0.8mm Llenslet 24mm F/
26.3 Pupil image diam.12mm
2arcmin
12mm
-2arcmin
CCD plane
Green and Red FoV are spaced by 1 arcsec CCD
plate scale 1arcsec/pixel or 1arcsec/50 micron
Scale can be increased by a factor 2
(0.5/pix) if needed
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A critical issue the CCD
R. Hartmann, SDW 2005 Taormina. 21.06.2005
CCD characteristics
MPI - Halbleiterlabor für Physik und für
extraterrestrische Physik
active pixels gt 256x264 Pixel size gt 51 mm CCD
image area gt 13.0x13.5 mm QE gt 0.8 _at_ 589nm
SH main optical parameters 15x15 subaps 1/pix,
lenslet FoV /-8.5 0.5/pix, lenslet FoV /-4.3
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Limiting mag for NGS tip-tilt
D. Sandler et al., JOSA 1994
Comparison of tip tilt measurement on a NGS using
the W unit WFS CCD39 with an APDs based
system Same QE is assumed for both units. WFS 6x6
on chip binning use 78 pixels.
?star 38mas 2DL R band FWHM CCD RON 3e-
(400kpix/s) Sky bkg 20 mag / arcsec2 Sensor
FoV diameter 3 Int. Time 1ms
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SR loss vs. NGS magnitude
Black CCD39(PNRONsky) Red APD (PNsky) Light
blue (PN, Phot. Noise)
SR loss 0.8 0.4
CCD / APD CCD / APD J gt 17.5 18.1 18.6
19.2 H gt 17.8 18.5 18.9 19.6 K gt
18.2 18.8 19.1 19.8
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Star counts Sky Coverage
CCD magSC magSC J band 17.50.11
18.60.20 H band 17.80.13 18.90.22 K band
18.20.16 19.10.24
SR_att 0.8 0.4
CCD / APD CCD / APD J gt 17.5 18.1
18.6 19.2 H gt 17.8 18.5 18.9
19.6 K gt 18.2 18.8 19.1 19.8

• NGS AO
• K band 16.5
• SC 0.06
• SR10-15
• LGS SR 0.6

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NGS Tilt from LGS
LGS camera FoV
To the sodium layer
The uplink tilt first derivative is measured.
Integration of this quantity allows to retrieve
the NGS tilt by sutraction from LGS tilt measured
on the full aperture
Red vector is proportional to the uplink
tilt time derivative.
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Tip-Tilt from LGS (laser tracking camera)
Beam splitter
LGS1
LGS2
A FoV of 240 at 1 pixel sampling requires a
256x256 A single CCD could be used.
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Covered configurations

• GLAO with ref. star at /-2 arcmin (full LUCIFER
  FoV)
• Single star, SR 0.24, SC 0.24 (maybe with a
  different lenslet)
• Small FoV tomography with stars at 30
• (need of 2nd WFS and optics)

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GLAO Raileigh option....
Basic geometrical elements -12 Km Raileigh
beams -100m range gating (0.33ms) - 3/4 stars /-
4 off-axis

• Raileigh beams in focus at 125.04m (6635mm)
• NGS in focus at 123.75m (5345)
• Raileigh Focus difference
• wrt NGS1290mm (1421 ZEMAX)
• M1 axis to focus 5345mm
• Front surface of derotators structure
• to NGS focal plane 585
• Front surface of derotators structure
• to Raileigh focal plane 14215852006
• From FM to Raileigh focus 2006
• From FM center to structure corner 1289
• Sodium Focus difference 173mm (ZEMAX)
• Front surface of derotators structure
• to Sodium focal plane 173585758

2000mm
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GLAO RB arrangement
outer edge of derotator structure
2000
Folding mirror footprint
4x4FoV
Mirrors at 4.5 from center. Some vignetting
close to the FMs of the 4x4 arcmin FoV. NGS
beam width is 39mm.
Total track for RB 2006mm F/16.6 Total track for
SB 758mm F/15.2 Total track for NGS 585mm F/15
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Upgrade to Sodium Beacon (SB)
outer edge of derotator structure
Mirror diameter for SB 50mm
Minimum distance for SB 126mm or 3.5
Mirrors at 4.5 from center. No vignetting of the
4x4 arcmin FoV. NGS beam width is 39mm.
Total track for RB 2006mm F/16.6 Total track for
SB 758mm F/15.2
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GLAO the WFS
16x16 subaps.
total track 182mm
3mm
LBT F/16.6 Focal plane
Collimating lens f 51mm
11 reimaging lens f40mm
Lenslet array 16x16 F/20 dsh 0.19mm fsh 4mm
add a pockel cell in collimated beam
The same WFS optics can be used with minor
adjustment (collimator lens) for the RB and for
the SB (F/ change).
CCD (EEV) 24 mm pixel 0.53 /pixel
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High SC upgrade
NIR GLAO with 4 radius ref. star A single GLAO
WFS placed inside the W unit. The NGS WFS used
for tip tilt star measurement
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Covered configurations

• GLAO with RB/SB ref. stars at /-4 arcmin (full
  LUCIFER FoV)
• Single star, SC 0.24 with single WFS in W unit

3 stars can be used
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Resuming results
LOSH WFS inside W unit GLAO 4x4 arcmin star at
/-2 arcmin (FW ok, UNIF, MORF?) Single star AO
(SR0.6, SC0.25_at_GP) (FW ok, UNIF no) Small FoV
tomography with stars at 30 (TBI)
Multi-WFSs on derotator structure GLAO with RB
12km at 4 (FW ok, UNIF ok, MORF ok) GLAO with
SB at 4 (FW ok UNIF ok MORF ok) Single star SB
with same WFS into W unit (SR OK, UNIF no)
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LGS configurations

• A single laser with a single on axis LGS,
  LUCIFER, LBTI
• -boost of SC, good SR in H and K.

• A single laser but multi LGSs asterism
• -Large asterism for GLAO 2-4 arcmin
• LUCIFER(NIR)
• MODS, multi-object spectrograph (VIS)
• -Small size asterism for high SR (FA) and larger
  SC,
• LUCIFER, LBTI (Keck-like)

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Tomography on small FoV
50mm diam
outer edge of derotator structure
mirror position for GLAO
WFS
1414
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164/126 4.5/3.5
mirror position for small FoV tomography
50 from FoV center (30mm)
WFS
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unvignetted FoV is 10x10mm or 16.7x16.7 arcsec
WFS
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Then we considered the possibility to provide
with the same WFS a GLAO WFS for LUCIFER
Could we retrofit with LGS more then one
instrument ? e.g., LUCIFER, MODS, LBTI (?
Keck-mode) Is it a common upgrade possible for
the different instruments ? yes, use their own
existing AGW units. What are the performance
? to be computed, some initial results for NIR
GLAO
Instruments LUCIFER, MODS, LBTI
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Tilt from the LGS
LGS camera FoV
To the sodium layer
The uplink tilt first derivative is measured.
Integration of this quantity allows to retrieve
the NGS tilt by sutraction from LGS tilt measured
on the full aperture
Red vector is proportional to the uplink
tilt time derivative.
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Conclusion
Rayleigh fratricide effect and laser beam
launching geometry It can be reduced a factor
8/10 using off-axis launch WRT center launch.
GLAO Performance then not limited by fratricide.
(L. Busoni) First Order optical desig of SH
lenslet array -A complete design for the
collimator is done with 180mm focal lenght and
12mm pupil image. -A first design of a SH lenslet
array is done prociding 15x15 sub. F/ is 26.3,
pitch is 0.8mm. Plate scale is 1/pix (1/50
micron) -The optics (including CCD) fit in the W
unit volume NGS tip-tilt measurement error using
the W unit WFS (CCD39) Limiting mag for NGS are
17.4,17.9,18.2 (CCD39). APD gain 0.7 mag LGSs
tracking camera Could be placed at the launching
telescope. NGS tip tilt measurement LGS
tracking camera could track for NGS tip tilt
looking at the LGS spot displacement
Ragazzoni96
LUCIFER FoV vignected by LGS pick-up mirror LGS
spot FWHM vs. subapertures
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LBT instruments
LUCIFER
LBTI
MODS

• Imaging across the 330-1100nm band,
• 6x 6, 0.15 arcsec/pix
• MOS mode 25-position mask cassette,
• slitlets over 4x 4

Small FoV High SR
LBTI
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The Layer Oriented (LO) SH
EspositoRagazzoni
On sky asterism
The situation in one lenslet FoV of the LO SH
sensor
The effective spot dimension is approx.
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LGS configurations

• A single laser with a single on axis LGS,
  LUCIFER, LBTI
• -boost of SC, good SR in H and K.

• A single laser but multi LGSs asterism
• -Large asterism for GLAO 2-4 arcmin
• LUCIFER(NIR)
• MODS, multi-object spectrograph (VIS)
• -Small size asterism for high SR (FA) and larger
  SC,
• LUCIFER, LBTI (Keck-like)

Initial phase with multiple NGS ? TBC !!
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GLAO case a 4 FoV Collimator
Currently available space on W
E. Diolaiti (OAB) (adapted from LN high
altitude WFS)
NGS/LGS light from telescope
W unit auxiliary units bench.
128x128 LLLCCD Ronlt1e-
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Conclusion

• The concept of a multi source SH WFS has been
  given.
• It is based on the use of a single Sodium
  laser.
• A first sketch of the optical elements fits into
  the
• present AGW unit empty space.
• Several configurations of asterisms are possible
  to manage
• different operating modes for 3 LBT
  instruments
• LUCIFER single LGS (boost SC)
• LUCIFER, NIR GLAO
• MODS, VIS GLAO
• LBTI, small FoV boost SC
• Initial simulations of a 4x4 arcmin NIR GLAO
  system shows that 8GS gives a good compromise
  between SR value and FoV uniformity.

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Some considerations more...
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A monochromatic solution
Length 180 mm
Monochromatic ? 0.5876 ?m F/0.93, f 140
mm Pupil image diameter d ? 9 mm Non-telecentricit
y lt 10 arcmin
20?20 sub-apertures ? RMS blur ? 1/20 sub-ap.
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A critical issue the CCD

• Lenslet array coupling with optical beam.
• -Beam footprint after collimator is
• about 9mm diam.
• -Spot pattern after lenslet aray is
• about 9mm diam.
• Centroid measurements
• -Spot pattern reimaged on a standard size CCD
• -Spot pattern created on a suitable CCD (10mm)

NGS/LGS light from telescope
MPI - Halbleiterlabor für Physik und für
extraterrestrische Physik
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Some additional issues

• Dichroic window creates astigmatism.
• LGS sources are fixed so they rotates with repect
  to any structure attached to the derotator.
• Actuator on the secondary rotates with respect to
  the lenslet array.
• LGS focus change with zenith angle.

• Use some reflective patches as an anulus placed
  in the reflected beam.
• The problem is solved using acomplete anulus. LGS
  could be counter rotated at the level of LGS
  projection system (rotating the hologram, the
  pyramid....).
• The lenslet array and the detector need to be
  rotated. Alternatively, current IM can be
  interpolated using SW.
• LGS focus has to be tracked translating the whole
  unit (collimator, lenslet and detector). A travel
  range of 160mm is needed for 60deg of z.

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The reflective window
Three geometry for Reflecting window
As far as the reflected field displacement is
different from the position of a given guide
star WRT the others no LGS obscuration happens.
15deg LUCIFER WINDOW
Reflected FoV
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The present point

• After a brief investigation multi source LGS WSFS
  for LBT seems feasible in the W units, as showed
  in Florence LGS workshop
• Some issues arises after a second look at the
  problem. However
• The 256x256 CCD with 50micron pixel do solve most
  of the optical design issues.
• The use of an anular (or smarter) window allows
  correct reimaging of reflected LGS
• Focus traking fo LGS possible in W unit available
  space.
• Pupil rotation issue can be attacked in at least
  two ways.

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DT 8.2 m fT 123.0 m fC 0.2 m qS 2
arcmin H 1000 m nSUB 15 a 20.5 dPUP
13.3 mm dSH 0.88 mm fSH 4.8 mm
Turbulent layer image
The beams identify a lower layer
fSH dSH / a
fSH 2 dSH / a
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Statistical distribution of seeing at LBT