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Requirements for AO-ELT operation and AO site monitor

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

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  1. Requirements for AO-ELT operation and AO site monitor Andrei Tokovinin NOAO/CTIO

  2. 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

  3. 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

  4. 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

  5. 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.

  6. Relevant parameter? On-axis Strehl ratio Seeing Free seeing Isoplanatic angle

  7. Efficiency of queue scheduling Best 25% nights on selected parameter Strehl>0.33: CS: 0.25 SQS: 0.5 AQS: 1.0 Queue scheduling on 0 gives nearly guaranteed Strehl

  8. Conclusion for LGS AO operation • Do not use seeing for queue scheduling! Nights with good 0 also have “slow” seeing

  9. 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

  10. Queue scheduling AO@ELT On-axis Strehl, best 25% of nights 0 is not as good as in the VLT case, but better than other parameters

  11. Relevant parameters for ELTqueue-scheduling

  12. Requirements for ELT site monitors • External site monitor (outside the dome) • Internal seeing monitor (part of the telescope)

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

  14. 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

  15. 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

  16. 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

  17. 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

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