Transcript - Lorentz Center

University of Durham
Centre for Advanced
Instrumentation
Rayleigh Laser Guide Stars on the WHT
Tim Morris, Durham University, UK
GLAO Workshop, Leiden 26-28/04/05
University of Durham
Centre for Advanced
Instrumentation
Talk Overview
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GLAO overview
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Durham GLAO System
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System Performance
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Ongoing Work
University of Durham
Centre for Advanced
Instrumentation
GLAO with LGSs
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Atmospheric tomography requires multiple high power lasers, laser launch
systems and multiple wavefront sensors
A low-altitude LGS requires one laser and WFS only
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acceptable performance on a 4m telescope
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Approach taken by Durham with LGS demonstrator and SOAR
Atmospheric Tomography with
multiple reference sources
Low-altitude LGS
University of Durham
Centre for Advanced
Instrumentation
Laser Launch System
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Laser Head
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BLT Enclosure
positioned in GRACE Nasmyth
platform on WHT optical axis
Secondary baffling
Light-proof
Tubing
Uses dielectric relay mirrors to
direct light to the top end of the
WHT
Secondary mirror
Beam Launch Telescope (BLT)
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Focus Lens
Laser Launch System (LLS)
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Relay Mirror 2
Positioned behind WHT secondary
mirror
Photon Return
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Observed at GHRIL Nasmyth
Focus
Cylindrical
Lenses
Return light
to GHRIL
Laser Head
Nasmyth Flat
Relay Mirror 1
Nasmyth turret
University of Durham
Centre for Advanced
Instrumentation
Laser
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5W Frequency doubled Nd:YLF DPSS laser
523nm light
7Khz Pulse Rate
M2 < 1.3
University of Durham
Centre for Advanced
Instrumentation
Laser Launch System
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Laser aligned to optical axis
of telescope
Relay mirrors directs light to
BLT at top end of WHT
through light-tight system
No active steering
components for correction of
telescope vibration/sag
Operation limited to
elevation angles between 60
and 80 degrees
University of Durham
Centre for Advanced
Instrumentation
Beam Launch Telescope
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300mm diameter, 1.83m
focal length primary mirror
25mm diameter secondary
mirror
3 motors on primary mirror
for fine gimbal tip/tilt and
focus adjustment
Simple box construction
with cross-bracing cables
to increase structural
stiffness
Measured 64% of output
laser power to sky
Light-proof
material
Secondary (fold)
mirror
Tension Cables
Primary Mirror
Focus Lens
University of Durham
Centre for Advanced
Instrumentation
AO System
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97 actuator continuous phase sheet DM (Xinetics)
37 actuator electrostatic DM for non-common path error
removal
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10 x 10 subaperture Shack-Hartmann WFS
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8 x Parallel Texas Instruments C40 DSP control system
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In-house built fast steering mirrors for tip/tilt correction
University of Durham
Centre for Advanced
Instrumentation
GLAO Design
Off-axis Toroidal Mirror
Parabolic Mirror
NGS FSM
AO System Output
Flat mirrors with 20mm
central aperture
Infinity
Focus
Input from
WHT
Dichroic
Beamsplitter
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Xinetics DM at NGS and
LGS pupil plane
Parabolic Mirror
Electrostatic DM
LGS FSM
LGS Focus
A large difference in position of infinity and LGS foci exists when using a lowaltitude LGS
Requires either oversized optics, or a reconjugation system to match the science (@
infinity) and LGS (@4.5km) pupil sizes on the DM
Reconjugation system allows variable height LGS to take advantage of changes in
higher layer turbulence altitude
University of Durham
Centre for Advanced
Instrumentation
GLAO Bench
WHT
Laser dichroic
beamsplitter
LGS focus
Infinity focus
Tip/Tilt mirror
To DM
Electrostatic DM
University of Durham
Centre for Advanced
Instrumentation
Range Gate
• Optical baffling and Pockels cells used to reject most
unwanted light from Rayleigh plume
University of Durham
Centre for Advanced
Instrumentation
LGS Performance
University of Durham
Centre for Advanced
Instrumentation
LGS Performance
Off-axis monitoring of Rayleigh plume
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Images taken using 16” Meade telescope 350m off-axis
82° =
2.483km
85° =
3.989km
83° =
2.842km
84° =
3.320km
86° =
4.990km
87° =
6.659km
University of Durham
Centre for Advanced
Instrumentation
LGS Performance
1000 frames, 30Hz Frame rate, 1ms
exposure, no range gate, 15” box size
Mean FWHM = 2.45”
No Range Gate With Range Gate
Large FWHM Smaller FWHM
University of Durham
Centre for Advanced
Instrumentation
LGS Performance
The magnitude of spot motion measured is important to
determine e.g. WFS subaperture FOV
Histogram showing instantaneous LGS position
1
2
4
3
250
200
150
100
50
Offset from mean position (arcsec)
1.16
1.11
1.06
1.01
0.96
0.91
0.86
0.81
0.76
0.71
0.66
0.61
0.56
0.51
0.46
0.41
0.36
0.31
0.26
0.21
0.16
0.11
0.06
0
0.01
Occurrence of measured offset
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University of Durham
Centre for Advanced
Instrumentation
LGS Performance
• As WHT tracks an object, the top end of the WHT sags,
causing the LGS to move on-sky
• Time elapsed between initial and final image = 7 minutes
LGS Centroid position
118
117
Y-pixel centroid
Run 1
116
Run 2
Run 3
115
Run 4
Run 5
Run 6
114
Run Mean Position
113
112
90
92
94
96
X-pixel centroid
98
100
102
University of Durham
Centre for Advanced
Instrumentation
AO Performance
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A quadrant of the WFS failed whilst on the run.
The effect of telescope sag caused more light to pass through the
range gate system than was expected. This made WFSing difficult.
AO loop was closed, but on a very poor WFS image. No
correction observed
Demonstrated a viable Rayleigh LGS can be created on the WHT
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FWHM and spot motion acceptable
LGS performance analysis gave valuable data for GLAS system
modelling
University of Durham
Centre for Advanced
Instrumentation
Ongoing LGS Work
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GLAS
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A Rayleigh LGS upgrade for WHT NGS AO System
(NAOMI)
LGS concept demostrators
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P4
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SPLASH
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PIGS (collaboration with MPIA Heidelburg and INAF)
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4 LGS GLAO (Gemini and ESO geometry)
University of Durham
Centre for Advanced
Instrumentation
LGS Concept Demonstrators
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GLAS will provide a 30W laser and launch system
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capable of creating approx V=9 LGS @ 20km
LGS distance ranges from 5km to infinity
High laser beam quality gives sub-arcsecond 1/e2 diameter
LGS
WHT has an empty Nasmyth focus for visiting
instruments/experiments
Part of the GLAS system requirements is that the laser
can be removed from launch system
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Allows possibility of launching the laser from the full aperture
of the WHT (shared launch)
University of Durham
Centre for Advanced
Instrumentation
LGS Concept – P4
• Collimated beam launched
from full aperture
• Turbulence will induce
intensity fluctuations in
beam
• Dynamic refocus
mechanism coupled to an
APD array can track
intensity changes along
beam to determine
wavefront
University of Durham
Centre for Advanced
Instrumentation
LGS Concept - SPLASH
• Requires full aperture shared
launch and return
• Shack-Hartmann spot pattern is
created in the sky
• Turbulence is sensed on upwards
path through the atmosphere
• Spots experience individual
tip/tilt on uplink, but global
tip/tilt on return path
– Wavefront can be determined by
imaging spot pattern
University of Durham
Centre for Advanced
Instrumentation
Conceptual Design
• Uses shared full-aperture launch
– Beam requires multiplexing
– Spinning shutters/mirrors
Spinning Mirror
M2
Beam
from
WHT
Minimum Beam Diameter
• LGS/NGS AO system
M1
– Xinetics 97 DM
– 2+ arcmin FOV
– 300Hz+ loop speed
• Modular PC (with FPGA) based
control system
• Dynamic refocus system
(birefringent lenses with
FLC/Pockels cell)
• Large bench space remaining for
LGS/NGS monitoring equipment
DM
LGS Focus
LGS/NGS
Beamsplitter
NGS Focus
University of Durham
Centre for Advanced
Instrumentation
Conclusion
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GLAO demonstrator
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An upgraded versatile AO system is being designed
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Fully engineered laser launch system will greatly simplify
system setup
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System provided valuable data and experience for GLAS
design
LGS Concept Demonstrator
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Already had successful first on-sky test of PIGS WFS
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Upgraded AO system is being designed to allow on-sky testing
of (almost) every LGS concept/beacon geometry
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ING Director is very, very nice and loves to see lasers being
launched from the WHT
University of Durham
Centre for Advanced
Instrumentation
Aligning optics with a 5W laser!
Shared launch light scatter
WHT