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Adaptive Optics Nicholas Devaney GTC project, Instituto de Astrofisica de Canarias

Adaptive Optics Nicholas Devaney GTC project, Instituto de Astrofisica de Canarias. 1. Principles 2. Multi-conjugate 3. Performance & challenges. Overview. Overview of current AO systems and Instruments Measures of performance Challenges for current systems Challenges for the future.

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Adaptive Optics Nicholas Devaney GTC project, Instituto de Astrofisica de Canarias

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  1. Adaptive OpticsNicholas DevaneyGTC project, Instituto de Astrofisica de Canarias 1. Principles 2. Multi-conjugate 3. Performance & challenges

  2. Overview • Overview of current AO systems and Instruments • Measures of performance • Challenges for current systems • Challenges for the future

  3. AO systems on 8-10m telescopes

  4. AO Systems on 3-8m Telescopes

  5. Measures of performance • Image quality • Strehl ratio and fwhm • Astronomy • Results • Publications • Citations • Efficiency • Correction achieved vs. Possible • Use of observing time

  6. Wavefront correction Quality Ref: Rigaut et al. In ‘High-resolution imaging by interferometry’, ESO conf. 1991

  7. Image quality Ref: Roddier & Rigaut in ‘Adaptive Optics in Astronomy’

  8. Image fwhm Ref: Roddier & Rigaut in ‘Adaptive Optics in Astronomy’

  9. AO Compensation Efficiency • Roddier (PASP, 110, 1998) defined compensation efficiency based on the following argument: Gmax=1.6 N at D/r0 = 2.4 N. At Gmax, S0.3 An AO system with N actuators behaves as an ideal system with Neff actuators compensation efficiency,

  10. Compensation Efficiency of some systems Roddier (PASP, 110, 1998)

  11. Images ! University of Hawaii AO http://www.ifa.hawaii.edu/ao/

  12. Faint companion detection University of Hawaii AO http://www.ifa.hawaii.edu/ao/

  13. Keck I AO http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao.html

  14. Galactic center with Keck AO

  15. Astronomical publications based on AO in refereed journals http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao_sci_list.html

  16. Efficiency • Marco et al. (PASP, 113, 2001) observing efficiency of ADONIS over 3 years • Efficiency = Science ‘shutter time’/ Available dark time = 10%-30% Other instruments = 50%-80% • Detector readout accounts for 5% of observing time; 60% of observations had exposure time < 5s • Extra overheads for AO include closing the loop and optimization (typ. 5 minutes), centering coronographic masks. • Loose time if loop opens during integration.

  17. Challenges • For Current Systems • Characterise and Improve correction efficiency • Improve Observing efficiency • Improve astronomical productivity • Prototype development • MCAO for 8-10m • Future • AO for ELTs

  18. AO Scaling laws • Recall wavefront fitting error In order to keep fitting error constant The number of pixels in the wavefront sensor will also scale as D2

  19. AO scaling laws • In order to maintain bandwidth the pixel readout rate also has to increase as D2. • Using a full matrix-multiply, the required computing power increases as D4 • Keck AO has 349 actuator; scale to 30m • 3000 actuators • on 128x128 if quad cell (just!) • 1kHz sampling => 16.4 MHz pixel rate • Computing power ~10 Gflop

  20. Scale to OWL • If we scale the same system to OWL... • 35000 actuators • 512x512 CCD • 1kHz sampling => 262 MHz pixel rate • computing power 103 Gflops !! • Even given Moores’ law, need to develop sparse matrix techniques • Note that noise propagation error increases as the ln(ndof) so need brighter guide stars Ref: Donald Gavel in ‘Beyond conventional Adaptive Optics’ 2001

  21. Scaling issues • Deformable mirrors • current piezomirros cost 1k$ per actuator • 7mm per actuator => 1.3m DM (ok) • MEMS promising but currently too small. • Stroke scales with D but outer scale will keep it to 5-10 m • Laser guide stars • Elongation • Optical errors due to finite distance (P.Dierickx) • Tolerances !

  22. MCAO on ELTs • For MCAO need 2-3 Deformable mirrors with similar number of actuators and 2-5 wavefront sensors • Sky coverage with natural guide stars may be sufficient • 42% at b=50 for multi-fov LO on OWL (Marchetti et al., Venice 2001)

  23. AO on Euro50

  24. Detection of exo-planetsXAO • Jupiter-Sun intensity ratio ~ 109 • Need very high order and very fast AO to suppress uncorrected halo. • Also need correction of scintillation. • Smooth optics • Sandler et al. Claim can detect Jupiter at m~4 stars with 3.5 hour integration • XAO for OWL will require 100k DM

  25. Other concepts • Ground-conjugate wide field AO • 1 DM conjugate to ground • 10-20´ field of view • improved fwhm rather than diffraction-limited • FALCON • Division of field of view into multiple areas • WFS/DM ‘buttons’ placed on guide stars around several objects in field • micro-DMs correct each object (low order correction) • Used in combination with Integral Field Spectroscopy

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