LGS at the LBT-

A Road Map to GLAO and Upgrades

Sebastian Rabien

Science cases from this morning

Laird:

Deep fields, faint targetsThe ‘20% seeing case’Big field of view

‘Planets’, binarysSky coverage at high strehl

Filippo

Jets, stellar discs DLSN z~1 DLGRB DLAGN’s DLOptically faint galaxies GLAOHigh-z cases GLAOMass-metallicity relation DLMerging history GLAO

Frank

Highest angular res: InterferometryBut fringe tracker limitedZ~2 dynamics Large samples needed

Roger:

Thermal capabilities DL?

DL

GLAOGLAO

Goals of a Laser Guide Star System at the LBT

Provide as soon as possible moderate correction with laser guiding over large field for:

•Lucifer Spectroscopy•Lucifer Imaging

•High Strehl on Axis•Large field high Strehl?

Enhance the observing efficiency and sensitivity

Phased approach

1st Step

2nd Step

System possibilities under discussion

SR-LGS MR-LGS S-LGS MS-LGS

Single Rayleigh LGS: gated at low altitude

Multiple Rayleigh stars

Single Sodium

Multiple Sodium

On-axis performance

Medium Medium High High

Homogenity Medium High Low High

Tech. Risk Optics/ Detectors

Optics/ Detectors

Laser Laser

Cn2 profile from S. Egner

x fudge factor 2

h km

7 layers

First estimations of system performance

X 1 laser centerΔ 3 lasers R=2‘

5 ם lasers R=2‘

‘Sodium’

‘Rayleigh’ 6km ‘Rayleigh’ 6km ‘worse seeing’

7 layers at: 0.5, 0.8, 2.5,5, 10, 15, 20 kmStars distributed over 300’’ field

Without: fitting error, S/N from star, AO bandwidth

Low altitude guide stars can provide a good ground layer correction

•Low altitude guide stars can provide a good ground layer correction•Multiple stars provide a more homogeneous PSF over the field•A single sodium provides high on-axis strehl

Theses

Further Studies needed for detailed decisions, like:

•How many stars?•Where best in the field?•Detection technology?•…

Should be answered in Phase A study

Constrains:

No easy sodium laser currently available.

Proposal for a staged approach

Start with multi Rayleigh low altitude gated system

•Provides homogeneous GLAO•Could be implemented fast•Mostly commercial components can be used•Leave the current NGS sensor in place•Can be used with co-adding and separate spot detection

Design the launch system to be suitable for general purpose

Leave space for additional (yellow) laser

Design of the high strehl (yellow? Cw, pulsed?) WFS to be foreseen in the system

Upgrade Road Provision:

•Provides the high strehl on axis correction•Leaves tomography option open•Single high altitude+ multiple low altitude stars could be a nice path towards MCAO

Phase A study includes:

Modelling of system performance:

•N-Rayleigh guide stars at x-altitude•Compare with Sodium option

Develop roadmap of upgrades

Technology study

Preliminary design

Phase A Performance Study of LGS Systems:

•Single low altitude guide star•Multiple low altitude stars•Single high altitude•Multiple high altitude stars•Modeling of S/N for laser type•Modeling of fitting and bandwidth

Including: good/medium/bad conditions

Modeling of system performanceComparison with science goals

Technology study

Laser system and typeDetail Launch conceptWFS layout/ opticsWFS Detector & Gating type (electronic shutter/EO-shutters)Calibration source

Mechanical constrains/ layoutMechanical analysis (flexure, etc)Electronics needs (motorization, control loops, etc)

Operational scenario/ installation scenarioObservatory constrains, definition of requirements to LBT

Computational needsSoftware needs

Impact on observations (Installation, commissioning)

Timescales for implementationCostingManpower needsAvailability of peopleUpgradeability

Upgrades planning

Laser issues

Pulsed green systems:

•Available commercial•Good beam quality•Easy operation•532nm Nd or 515nm Yb feasible

ELS, disc

Jdsu YAG

Best candidates

Nd-YAG sum frequency

Proven technology13 (50) W demonstratedBulkySolid stateMedium maintenanceHigh costs

Fiber lasers

Less mature5W demonstratedVery compactSolid stateLow maintenanceLow costs

Dye lasers

Proven technology20W demonstratedBulkyChemicals neededMaintenance intenseMedium costs

Currently no ‘easy’ option available

589nm lasers

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

WFS remarks and questions

Low altitude detection system is not straightforward•Space constrains•Large backfocal distance

New or separate dicroic needed?

Detectors

Single detector preferable (cost)Optically switching preferred (best CCD can be used)Leaves upgrade to cw/ sodium detection

50μm pixel2 e- noise256x256 pixel

Cost and FTE expectationsKomponents Hardware Costs

Laser Sum 1300 k€

Electronics and Computers 100 k€

Lasers 800 k€

Optics und Mechanics 400 k€

Else 100 k€

Projektion Sum 1300 k€

Electronics and computers 200 k€

Beam transport 300 k€

Teleskopes 500 k€

Aircraft avoidance 100 k€

else 100 k€

Wellenfrontsensor Sum 800 k€

Computers and electronics 200 k€

Detectors and electronics 300 k€

Optics and mechanics 200 k€

else 100 k€

Installation Sum 300 k€

Change to telescope, infrastructure

100 k€

Transport / Travel 100 k€

Installation at telescope 100 k€

Continguency (10%) Sum 400 k€

Total 4100 k€

Aufgabe FTE‘s

Management 2

System Engineering 4

Construction 4

Elektronics 4

Software 4

Mechanical workshop 6

Electronics workshop 2

Integration and Test 8

Continguency (10%) 4

Sum 38

With 50 k€ / Personenjahr

= 1900 k€

Example done for a multi Rayleigh system

Timeline

Today’s meeting

TechnologyDecisionPreliminary design Design review Test review Commissioning

Phase A

Concept comparisonPerformance calculationTechnology evaluationPreliminary design

Phase B

Design

Phase C

Manufacturing

Phase D

ShippingInstallation

Goal: Operation 2010Phase A: 6 monthPhase B: 1 yearPhase C: 1 year

We have to start immediately

A LGS facility is a must to keep LBT competitive

A phased approach leads to an early implementation

•GLAO first•High strehl next