Laser Launch System for the Laser Launch System for the LBTLBT

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 UseLaunch Systems in Use

Mt Wilson 100”

pulsed Rayleigh laser;launched from Primary mirror;focussed at 18km

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).

sketch of beam path directing the beam onto the coude axis

Launch Systems in UseLaunch Systems in Use

Keck, Lick, ALFA, SOR

master laser on dome floor & amplifier on M1 cell: dye tubes & fibre in cable wrap;side launch: simple beam relay but elongated spots;

launch lens at top of beam tube

LGS spots on Keck WFS

Sketch of Keck Telescope

Launch Systems in UseLaunch Systems in Use

VLT, Subaru

laser on Nasmyth platform (constant gravity vector);photonic crystal fibre as relay (only possible for cw lasers);central launch telescope

Subaru fibre

Subaru launch telescope (small secondary)

Sketch of Subaru Telescope

Launch Systems in UseLaunch Systems in Use

Gemini North (& South), Palomar 200”, MMT

laser at M1 cell;actively controlled mirror relay;central launch telescope

Gemini North beam relay

Gemini North

5mm beam fits behind narrow support fin

Launch Systems in UseLaunch Systems in Use

WHT

laser mounted on top ring leads to minimalist beam relay;central launch telescope

single laser for GLAO

Launching Multiple Laser BeamsLaunching Multiple Laser Beams

VLT (for HAWK-I & MUSE)4 launch telescope around M1 cellneeded for cw lasers to avoid fratricide

MMT1 central launch telescope;split beam holographically;for pulsed lasers, gating avoids fratricide

Gemini South1 central launch telescope;multiple beams directed into LT; pointings can be controlled independently

Laser Launch Telescope for Gemini Laser Launch Telescope for Gemini South 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 VLTLaser 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;

• on-axis design with 46 cm diameter, F/1.8 primary

• Installed behind secondary

• LT Transmission 87%

• open structure

Laser Launch Telescope for Palomar Laser Launch Telescope for Palomar 200” 200”

Basic Design ConsiderationsBasic Design Considerations

fibre – not compatible with pulsed lasers.(e.g. VLT) 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 - long focal length expander is part of beam relay.(e.g. Keck) scattered light may be an issue if beam is centrally launched

beam relay

launchinglaunch telescope(s) distributed

(top ring? space around M1 cell?)

central(headroom? weight limit?)

on axis or off axis beam expander?(e.g. VLT) (e.g. Gemini)

(light loss, manufacturing, alignment)

Distributed Launch Telescopes not Distributed Launch Telescopes not possiblepossible

space around M1 cell blocked by M3 arm

space around M1 cell blocked by walkway

View of LBT M1 mirror cell

Laser mounted on platform above Laser mounted on platform above LuciferLucifer

Two 1m electronics cabinets

laser heads

diagnostics & alignment optics

platform removable in 2 halves

identical installation here

Access to LUCIFER auxiliary cryostatsAccess to LUCIFER auxiliary cryostats

LUCIFER auxiliary cryostats

laser platform

laser platform

Top View between mirror cells

laser platform

LUCIFER main cryostatsTop View between mirror cells

Access to LUCIFER main cryostatsAccess to LUCIFER main cryostats

Launch Telescope mounted behind M2Launch Telescope mounted behind M2

narrow (~10cm) beam relay

launch telescope & beam expander

laser platform

non-rigid connection

Central launch, with expander built into beam relay Central launch, with expander built into beam relay

flat folding mirror launches beam upwards

laser platform

open air propagation from here

beam expanded by lenses in wide (~45cm) relay

Attachment of beam relay cover tubesAttachment of beam relay cover tubes

Pads already available on telescope structure

Beam relay cover

Things to think about when launching laser Things to think about when launching laser beamsbeams

aircraft avoidance systemrequirements from FAA & Space Commandhuman spottersall-sky cameraradar information from airportsbore-sight camera (infrared/optical)

interlockstemptation is to make these too complex

laser traffic control systemprocure tool used at Mauna Kea & Cerro Pachon?own development?

Proposed Design for the LBT LGSFProposed Design for the LBT LGSF

beam expanded by lenses in wide (~45cm) relay

flat folding mirror launches beam upwards

open air propagation from here

laser platform