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Laser Launch System for the LBT
Richard Davies
Sebastian Rabien
Max Planck Institute for Extraterrestrial Physics
 Approaches of other observatories
 Trade-offs that should be considered
 Possibilities for LBT
SOR, Calar Alto, Lick, MMT, Keck, VLT, Subaru, Gemini North, WHT, Palomar 200”, Mt Wilson 100”
Launch Systems in Use
Mt Wilson 100”
sketch of beam path
pulsed Rayleigh laser;
launched from Primary mirror;
focussed at 18km
directing the beam onto the coude axis
need to sharing telescope optical system
(block optical path while laser fires);
control alignment of laser on optical axis
not an option for ‘classical’ LGS on 8-m telescope; but would be used for some novel
techniques such as laser phase shifting interferometry (Rabien et al. 2006).
Launch Systems in Use
Sketch of Keck Telescope
Keck, Lick, ALFA, SOR
launch lens at top of beam tube
master laser on dome floor & amplifier on M1 cell: dye
tubes & fibre in cable wrap;
side launch: simple beam relay but elongated spots;
LGS spots on Keck WFS
Launch Systems in Use
VLT, Subaru
Sketch of Subaru Telescope
Subaru fibre
Subaru launch telescope
(small secondary)
laser on Nasmyth platform (constant gravity vector);
photonic crystal fibre as relay (only possible for cw
central launch telescope
Launch Systems in Use
Gemini North
Gemini North (& South), Palomar 200”, MMT
Gemini North beam relay
laser at M1 cell;
actively controlled mirror relay;
central launch telescope
5mm beam fits
behind narrow
support fin
Launch Systems in Use
single laser for GLAO
laser mounted on top ring leads to minimalist beam relay;
central launch telescope
Launching Multiple Laser Beams
4 launch telescope around M1 cell
needed for cw lasers to avoid fratricide
1 central launch telescope;
split beam holographically;
for pulsed lasers, gating avoids fratricide
Gemini South
1 central launch telescope;
multiple beams directed into LT;
pointings can be controlled independently
Laser Launch Telescope for Gemini South
• afocal 60:1 expander,
projecting 45cm
collimated beam to sky
• installed behind secondary
• off-axis parabola design
• designed to launch 5 beams of
10W each
• care needed for glass & coating
resistance to laser power
density 500-1000W/cm2
• aluminium construction with
carbon fibre thermal rod
to maintain distance
from expander to
primary mirror
Laser Launch Telescope for VLT
• afocal 12.5:1 expander (36cm beam size) with 50cm primary mirror
• on-axis with small secondary
• diagnostics in situ (but not used)
• suitable for launching multiple beams
• ~F/1 (due to space restrictions), so thermally sensitive
• enclosed structure filled with N2; totally surrounded by wind baffle;
Laser Launch Telescope for Palomar 200”
• on-axis design with 46 cm
diameter, F/1.8 primary
• Installed behind secondary
• LT Transmission 87%
• open structure
Basic Design Considerations
fibre –
(e.g. VLT)
beam relay
not compatible with pulsed lasers.
alignment into small fibre can be problematic
throughput an issue for high power, long fibres,
and small bending radii.
mirrors - complex if many must be servoed.
(e.g. Gemini) in a static relay, only flexure need be compensated.
throughput good for short relays with few optics.
lenses (e.g. Keck)
long focal length expander is part of beam relay.
scattered light may be an issue if beam is centrally launched
(top ring? space around M1 cell?)
(headroom? weight limit?)
on axis or off axis beam expander?
(e.g. VLT)
(e.g. Gemini)
(light loss, manufacturing, alignment)
Distributed Launch Telescopes not possible
View of LBT M1 mirror cell
space around M1 cell
blocked by M3 arm
space around M1 cell
blocked by walkway
Laser mounted on platform above Lucifer
diagnostics & alignment
Two 1m
laser heads
in 2 halves
installation here
Access to LUCIFER auxiliary cryostats
Top View between mirror cells
LUCIFER auxiliary cryostats
Access to LUCIFER main cryostats
Top View between mirror cells
LUCIFER main cryostats
Launch Telescope mounted behind M2
launch telescope &
beam expander
narrow (~10cm)
beam relay
Central launch, with expander built into beam relay
open air propagation
from here
flat folding
mirror launches
beam upwards
beam expanded by
lenses in wide
(~45cm) relay
Attachment of beam relay cover tubes
Pads already available
on telescope structure
Beam relay cover
Things to think about when launching laser beams
aircraft avoidance system
requirements from FAA & Space Command
human spotters
all-sky camera
radar information from airports
bore-sight camera (infrared/optical)
temptation is to make these too complex
laser traffic control system
procure tool used at Mauna Kea & Cerro Pachon?
own development?
Proposed Design for the LBT LGSF
open air propagation
from here
flat folding
mirror launches
beam upwards
beam expanded by
lenses in wide
(~45cm) relay