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

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Adaptive OpticsNicholas DevaneyGTC project,
Instituto de Astrofisica de Canarias

• 1. Principles
• 2. Multi-conjugate
• 3. Performance challenges

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Overview

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

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AO systems on 8-10m telescopes
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AO Systems on 3-8m Telescopes
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Measures of performance

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

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Wavefront correction Quality
Ref Rigaut et al. In High-resolution imaging by
interferometry, ESO conf. 1991
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Image quality
Ref Roddier Rigaut in Adaptive Optics in
Astronomy
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Image fwhm
Ref Roddier Rigaut in Adaptive Optics in
Astronomy
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AO Compensation Efficiency

• Roddier (PASP, 110, 1998) defined compensation
  efficiency based on the following argument
• Gmax1.6 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,

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Compensation Efficiency of some systems
Roddier (PASP, 110, 1998)
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Images !
University of Hawaii AO http//www.ifa.hawaii.edu
/ao/
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Faint companion detection
University of Hawaii AO http//www.ifa.hawaii.edu
/ao/
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Keck I AO
http//www2.keck.hawaii.edu3636/realpublic/inst/a
o/ao.html
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Galactic center with Keck AO
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Astronomical publications based on AO in refereed
journals
http//www2.keck.hawaii.edu3636/realpublic/inst/a
o/ao_sci_list.html
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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 lt 5s
• Extra overheads for AO include closing the loop
  and optimization (typ. 5 minutes), centering
  coronographic masks.
• Loose time if loop opens during integration.

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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

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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

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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 gt 16.4 MHz pixel rate
• Computing power 10 Gflop

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Scale to OWL

• If we scale the same system to OWL...
• 35000 actuators
• 512x512 CCD
• 1kHz sampling gt 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

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Scaling issues

• Deformable mirrors
• current piezomirros cost 1k per actuator
• 7mm per actuator gt 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 !

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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 b50? for multi-fov LO on OWL (Marchetti
  et al., Venice 2001)

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AO on Euro50
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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 m4
  stars with 3.5 hour integration
• XAO for OWL will require 100k DM

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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