Title: Adaptive Optics Nicholas Devaney GTC project, Instituto de Astrofisica de Canarias
1Adaptive OpticsNicholas DevaneyGTC project,
Instituto de Astrofisica de Canarias
- 1. Principles
- 2. Multi-conjugate
- 3. Performance challenges
2Overview
- Overview of current AO systems and Instruments
- Measures of performance
- Challenges for current systems
- Challenges for the future
3AO systems on 8-10m telescopes
4AO Systems on 3-8m Telescopes
5Measures of performance
- Image quality
- Strehl ratio and fwhm
- Astronomy
- Results
- Publications
- Citations
- Efficiency
- Correction achieved vs. Possible
- Use of observing time
6Wavefront correction Quality
Ref Rigaut et al. In High-resolution imaging by
interferometry, ESO conf. 1991
7Image quality
Ref Roddier Rigaut in Adaptive Optics in
Astronomy
8Image fwhm
Ref Roddier Rigaut in Adaptive Optics in
Astronomy
9AO 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,
10Compensation Efficiency of some systems
Roddier (PASP, 110, 1998)
11Images !
University of Hawaii AO http//www.ifa.hawaii.edu
/ao/
12Faint companion detection
University of Hawaii AO http//www.ifa.hawaii.edu
/ao/
13Keck I AO
http//www2.keck.hawaii.edu3636/realpublic/inst/a
o/ao.html
14Galactic center with Keck AO
15Astronomical publications based on AO in refereed
journals
http//www2.keck.hawaii.edu3636/realpublic/inst/a
o/ao_sci_list.html
16Efficiency
- 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.
17Challenges
- For Current Systems
- Characterise and Improve correction efficiency
- Improve Observing efficiency
- Improve astronomical productivity
- Prototype development
- MCAO for 8-10m
- Future
- AO for ELTs
18AO 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 -
19AO 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
20Scale 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
21Scaling 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 !
22MCAO 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)
23AO on Euro50
24Detection 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
25Other 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