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

AO systems on 8-10m telescopes

AO Systems on 3-8m Telescopes

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

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

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

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

AO Compensation Efficiency Roddier (PASP, 110, 1998) defined compensation efficiency based on the following argument: G max =1.6 N at D/r 0 = 2.4  N. At G max, S  0.3 An AO system with N actuators behaves as an ideal system with N eff actuators compensation efficiency,

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

Images ! University of Hawaii AO

Faint companion detection University of Hawaii AO

Keck I AO

Galactic center with Keck AO

Astronomical publications based on AO in refereed journals

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.

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

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 D 2

AO scaling laws In order to maintain bandwidth the pixel readout rate also has to increase as D 2. Using a full matrix-multiply, the required computing power increases as D 4 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

Scale to OWL If we scale the same system to OWL... –35000 actuators –512x512 CCD –1kHz sampling => 262 MHz pixel rate –computing power 10 3 Gflops !! Even given Moores’ law, need to develop sparse matrix techniques Note that noise propagation error increases as the ln(n dof ) so need brighter guide stars Ref: Donald Gavel in ‘Beyond conventional Adaptive Optics’ 2001

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 !

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)

AO on Euro50

Detection of exo-planets XAO Jupiter-Sun intensity ratio ~ 10 9 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

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