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” 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 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 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 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 Use WHT laser mounted on top ring leads to minimalist beam relay; central launch telescope single laser for GLAO

Launching Multiple Laser Beams VLT (for HAWK-I & MUSE) 4 launch telescope around M1 cell needed for cw lasers to avoid fratricide MMT 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 W/cm 2 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 N 2 ; 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 200”

Basic 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 launching launch 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 possible 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 Lucifer Two 1m electronics cabinets laser heads diagnostics & alignment optics platform removable in 2 halves identical installation here

Access to LUCIFER auxiliary cryostats LUCIFER auxiliary cryostats laser platform Top View between mirror cells

laser platform LUCIFER main cryostats Top View between mirror cells Access to LUCIFER main cryostats

Launch 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 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 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) interlocks 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 beam expanded by lenses in wide (~45cm) relay flat folding mirror launches beam upwards open air propagation from here laser platform