Requirements for AOELT

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Requirements for AO-ELT operation and AO site
monitor
Andrei Tokovinin NOAO/CTIO
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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
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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
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Case 1 VLT/Gemini LGS AO

• D8m, 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
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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
ArXiv0904.1183 Schoeck et al. Paranal,
Vizcachas Cerro Pachon, etc.
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Relevant parameter?
On-axis Strehl ratio
Seeing
Free seeing
Isoplanatic angle
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Efficiency of queue scheduling
Best 25 nights on selected parameter
Strehlgt0.33 CS 0.25 SQS 0.5 AQS 1.0
Queue scheduling on ?0 gives nearly guaranteed
Strehl
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Conclusion for LGS AO operation

• Do not use seeing for queue scheduling!

Nights with good ?0 also have slow seeing
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Case 2 E-ELT
The worst offender is high-altitude
turbulence, not seeing

• D42m, one LGS _at_ 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
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Queue scheduling AO_at_ELT
On-axis Strehl, best 25 of nights
?0 is not as good as in the VLT case, but better
than other parameters
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Relevant parameters for ELTqueue-scheduling
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Requirements for ELT site monitors

• External site monitor (outside the dome)
• Internal seeing monitor (part of the telescope)

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External site monitor

• Seeing at upper dome level
• Isoplanatic angle
• AO time constant
• Cn2(h) profile, resolution TBD
• Outer scale
• Na profile
• Extinction

100 time coverage Real-time data

• MASS-DIMM
• on top of the dome
• SLODAR
• FADE
• G-Scidar
• Other

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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

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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
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Internal seeing at SOAR telescope

• Laser interferometer 0.5m, ?532nm
• (fringe period 0.22)
• Camera 30 FPS, 15mas pixels
• RMS fringe motion 33-66 mas

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Conclusions

• 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