Requirements for AO-ELT
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Requirements for AO-ELT operation and AO site monitor. Andrei Tokovinin NOAO/CTIO. Three ways to schedule AO. CS – classical scheduling (fixed nights) SQS – standard queue scheduling (on seeing) AQS – advanced queue scheduling (on relevant parameter).

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Requirements for AO-ELT

operation and AO site monitor

Andrei Tokovinin


Three ways to schedule ao
Three ways to schedule AO

  • CS – classical scheduling (fixed nights)

  • SQS – standard queue scheduling (on seeing)

  • AQS – advanced queue scheduling (on relevant parameter)

ELT and its AO instruments are such a huge investment

that even a small gain in performance is important.

Optimum scheduling will increase AO science output

Which parameter is relevant
Which parameter is relevant?

  • AO with NGS, on-axis: r0, 0

  • LGS and wide-field: 0 , 0

  • Laser tomography: K , 0

At large telescopes, dominant AO error comes

from the high-altitude turbulence

Case 1 vlt gemini lgs ao
Case 1: VLT/Gemini LGS AO

  • D=8m, one sodium LGS at 90km

  • One tip-tilt star at 30”

  • Radial order 30 (240 modes, 0.27m)

  • Very fast AO loop

  • Imaging in H band (1.65 m), 20” field

Fourier code to compute PSF (fast, 0.2s/profile!)

Median on-axis Strehl: 0.95 (NGS), ~0.2 (LGS)

Cone effect and tilt anisoplanatism dominate

Atmospheric data
Atmospheric data

  • Cerro Tololo MASS-DIMM, 4.8years (from S.Els)

  • 433161 coarse profiles (7 layers), 0

  • Use every 50th profile (8517)

Similar data exist for:

6 TMT sites

[ArXiv:0904.1183 Schoeck et al.]

Paranal, Vizcachas

Cerro Pachon, etc.

Relevant parameter
Relevant parameter?

On-axis Strehl ratio


Free seeing

Isoplanatic angle

Efficiency of queue scheduling
Efficiency of queue scheduling

Best 25% nights

on selected



CS: 0.25

SQS: 0.5

AQS: 1.0

Queue scheduling on 0 gives nearly guaranteed Strehl

Conclusion for lgs ao operation
Conclusion for LGS AO operation

  • Do not use seeing for queue scheduling!

Nights with good 0 also have “slow” seeing

Case 2 e elt
Case 2: E-ELT

The worst offender is

high-altitude turbulence,

not seeing

  • D=42m, one LGS @ 90km

  • Actuators 0.16m

  • One tip-tilt star at 30”, 20” field

  • Imaging in K-band, Strehl ~10%

Laser tomography will be used almost always.

This case is sensitive to high-altitude turbulence,

also relevant to tomography

Queue scheduling ao@elt
Queue scheduling [email protected]

On-axis Strehl, best 25% of nights

0 is not as good as

in the VLT case, but

better than other


Requirements for elt site monitors
Requirements for ELT site monitors

  • External site monitor (outside the dome)

  • Internal seeing monitor (part of the telescope)

External site monitor
External site monitor

100% time coverage

Real-time data

  • Seeing at upper dome level

  • Isoplanatic angle

  • AO time constant

  • + Cn2(h) profile, resolution TBD

  • + Outer scale

  • + Na profile

  • + Extinction


    on top of the dome


  • FADE

  • G-Scidar

  • Other

Path to selecting the site monitor
Path to selecting the site monitor

  • Select few representative science programs and instruments for ELT

  • Determine relevant atmospheric parameters or their proxies for each case (few!)

  • Determine resolution of turbulence profile needed to measure these parameters

  • Select or develop suitable instruments

  • Compromise, second iteration

Internal seeing monitor
Internal seeing monitor

  • Seeing through telescope (active optics)

  • Internal seeing (laser beams)

  • Cn2(h) from laser tomography

Two (or more) collimated 5-cm laser beams

from top-end to focus

will measure (r0, L0) of internal turbulence without

perturbing normal telescope operation

Internal seeing at soar telescope
Internal seeing at SOAR telescope

  • Laser interferometer 0.5m, λ=532nm

    (fringe period 0.22”)

  • Camera 30 FPS, 15mas pixels

  • RMS fringe motion 33-66 mas


  • Simple queue-scheduling (SQS) is not optimal even for 8-m telescopes

  • Need two seeing monitors for ELT: external & internal

  • Develop AQS and internal monitor on 8-m telescopes