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LGS at the LBT-. A Road Map to GLAO and Upgrades. Sebastian Rabien. Science cases from this morning. Laird: Deep fields, faint targets The ‘20% seeing case’ Big field of view ‘Planets’, binarys Sky coverage at high strehl. Frank Highest angular res: Interferometry

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LGS at the LBT-

A Road Map to GLAO and Upgrades

Sebastian Rabien

Science cases from this morning


Deep fields, faint targets

The ‘20% seeing case’

Big field of view

‘Planets’, binarys

Sky coverage at high strehl


Highest angular res: Interferometry

But fringe tracker limited

Z~2 dynamics

Large samples needed





Jets, stellar discs DL

SN z~1 DL



Optically faint galaxies GLAO

High-z cases GLAO

Mass-metallicity relation DL

Merging history GLAO


Thermal capabilities DL?

Goals of a Laser Guide Star System at the LBT

Phased approach

  • Provide as soon as possible moderate

  • correction with laser guiding over large field for:

  • Lucifer Spectroscopy

  • Lucifer Imaging

1st Step

Enhance the observing efficiency and sensitivity

2nd Step

  • High Strehl on Axis

  • Large field high Strehl?

First estimations of system performance

Cn2 profile from S. Egner

7 layers

x fudge factor 2

h km

7 layers at: 0.5, 0.8, 2.5,5, 10, 15, 20 km

Stars distributed over 300’’ field

X 1 laser center

Δ 3 lasers R=2‘

ם 5 lasers R=2‘

Without: fitting error, S/N from star,

AO bandwidth


Low altitude guide stars can provide a good ground layer correction

‘Rayleigh’ 6km

‘Rayleigh’ 6km ‘worse seeing’


  • 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

  • Further Studies needed for detailed decisions, like:

  • How many stars?

  • Where best in the field?

  • Detection technology?

Should be answered in Phase A study


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

Upgrade Road Provision:

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

  • 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

Modeling of system performance

Comparison with science goals

  • 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

Technology study

Laser system and type

Detail Launch concept

WFS layout/ optics

WFS Detector & Gating type (electronic shutter/EO-shutters)

Calibration source

Mechanical constrains/ layout

Mechanical analysis (flexure, etc)

Electronics needs (motorization, control loops, etc)

Operational scenario/ installation scenario

Observatory constrains, definition of requirements to LBT

Computational needs

Software needs

Impact on observations (Installation, commissioning)

Timescales for implementation


Manpower needs

Availability of people


Upgrades planning

Laser issues

Best candidates

Pulsed green systems:

Jdsu YAG

  • Available commercial

  • Good beam quality

  • Easy operation

  • 532nm Nd or 515nm Yb feasible

ELS, disc

589nm lasers

Dye lasers

Proven technology

20W demonstrated


Chemicals needed

Maintenance intense

Medium costs

Nd-YAG sum frequency

Proven technology

13 (50) W demonstrated


Solid state

Medium maintenance

High costs

Fiber lasers

Less mature

5W demonstrated

Very compact

Solid state

Low maintenance

Low costs

Currently no ‘easy’ option available

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

laser platform

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 pixel

2 e- noise

256x256 pixel

Cost and FTE expectations

Example done for a multi Rayleigh system




Preliminary design

Today’s meeting

Design review

Test review


Phase A

Concept comparison

Performance calculation

Technology evaluation

Preliminary design

Phase B


Phase C


Phase D



Phase A: 6 month

Phase B: 1 year

Phase C: 1 year

Goal: Operation 2010

A LGS facility is a must to keep LBT competitive

A phased approach leads to an early implementation

  • GLAO first

  • High strehl next

We have to start immediately