Adaptive Optics 1

John O’Byrne

School of Physics

University of Sydney

Adaptive Optics 2

What is AO?What is AO?

Adaptive Optics: fast image correction (f ³ 1 Hz), primarily to correct

atmospheric wavefront distortions

Active Optics: slow image correction (f £ 1 Hz), to correct mirror and

structural deflections

Adaptive Optics 3

Why do we need AO?Why do we need AO?

Scintillation - describes random amplitude fluctuations of wavefront (twinkling)

Seeing - describes random phase fluctuations of wavefront (image motion and blurring)

AO aims to correct seeing effects - i.e. sharpen images

Science objectives - e.g. GEMINIhttp://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/Science_drivers.html

Adaptive Optics 4

Where does Seeing arise?Where does Seeing arise?

Turbulence in the atmosphere leads to refractive index variations.Contributions are concentrated into layers at different altitudes.

Adaptive Optics 5

What can we expect from AO?What can we expect from AO?

Improvement depends on

Dtel relative to ro

AO is easier in the infrared ro is larger qo is larger to is longer

Also easier if H is lower Vwind is lower(R/Rmax is Strehl resolution normalised by

exposure resolution of an infinte aperture)

Adaptive Optics 6

Essentials of an AO systemEssentials of an AO system

Wavefront sensor Computer Phase modulator

Adaptive Optics 7

Phase ModulatorPhase Modulator

The phase modulators are always a deformable mirror

- usually tip-tilt and higher order separately.

Actuators used: piezoelectric (PZT) electrostrictive voice-coil electrostatic

But other technologies are possible Liquid Crystal phase screen devices

More actuators => better correction.

Adaptive Optics 8

Tit-tilt correctionTit-tilt correction

Tip-tilt mirror mounted on

4 piezoelectric stacks.

Segmented surface deformable

mirrors use tip-tilt on

individual segments

Adaptive Optics 9

Stacked-array MirrorsStacked-array Mirrors

Continuous faceplates

attached to

piezoelectric stacks

Visible on the edges of

each mirror are the PZT

actuators.

Adaptive Optics 10

Sample of an AO result - 1Sample of an AO result - 1

Adaptive Optics 11

Sample of an AO result - 2Sample of an AO result - 2

Core diameter is recovered with low order correction, but a surrounding halo remains

Adaptive Optics 12

AO limitationsAO limitations

AO systems have limitations (e.g. light loss, IR emissivity

driven by the large number of optical surfaces) but more

fundamental are limits imposed by the guiding star, which is

monitored by the wavefront sensor, and is likely to be

different from the science target

Adaptive Optics 13

Natural Guide Stars (NGS)Natural Guide Stars (NGS)

temporal anisoplanatism - delays introduced by the servo loop

angular anisoplanatism - NGS is usually offset from science target, but can't be too far away or it lies outside isoplanatic patch angle (qo) - can be improved by making the WFS conjugate to the primary turbulence layer (or multiple layers in multi-conjugate AO [MCAO])

WFS sensitivity limit => limited sky coverage

Adaptive Optics 14

Laser Guide Stars (LGS) - 1Laser Guide Stars (LGS) - 1

Use a laser to generate a ‘star’ in

the atmosphere, very close to the

science target’s light path through

the atmosphere. This may be a

Rayleigh guide star at 7-20 km

or a Sodium guide star at 90 km.

Overcomes NGS sky coverage limitation

Adaptive Optics 15

Laser Guide Stars (LGS) - 2Laser Guide Stars (LGS) - 2

Provides no tip-tilt information

Cost! Problem to other

telescopes on the site caused by back-scattered light

Sodium guide star and Rayleigh back-scatter

Adaptive Optics 16

Laser Guide Stars (LGS) - 3Laser Guide Stars (LGS) - 3

Focus anisoplanatism the laser does not fully

sample the stars light path through the atmosphere

worse for a Rayleigh guide star

provide multiple LGS?

Adaptive Optics 17

AO Projects - 1AO Projects - 1

Australian projects RSAA 2.3m tip-tilt system Anglo-Australian Telescope

International projects (e.g. see University of Durham list of links to other projects

http://aig-www.dur.ac.uk/fix/adaptive-optics/area_main_ao.html) GEMINI

http://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/AOIndex.html

AO at ESO / VLT http://www.eso.org/projects/aot/

Adaptive Optics 18

AO Projects - 2AO Projects - 2

Keck II and now Keck I http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao.html

University of Durham (UK) http://aig-www.dur.ac.uk/fix/adaptive-optics/area_main_ao.html

University of Hawaii most recently Hokupa’a on GEMINI

http://www.ifa.hawaii.edu/ao/

Earlier PUEO on CFHT http://www.cfht.hawaii.edu/Instruments/Imaging/AOB/

Adaptive Optics 19

Hohupa’a Images - 3Hohupa’a Images - 3

GEMINI

Adaptive Optics 20

KeckKeck

Keck I AO image in H band taken during the first Keck I AO night (Dec.12,2000).

Io angular size: 1.23 arcsecond Spatial resolution: 120 km

Adaptive Optics 21

Starfire Optical Range (SOR)Starfire Optical Range (SOR)

Adaptive Optics 22

ReferencesReferences

Information on AO projects can be obtained from their web sites or from the

Proceedings of the (all too frequent) AO conferences (e.g. SPIE, OSA or ESO).

A few other useful references:

Popular level: Sharper Eyes on the Sky - Sky & Space, 9, 30 (1996) Untwinkling the Stars - Sky & Telescope, 87, May 24 & Jun 20, (1994) Adaptive Optics - Scientific American, Jun (1994)

Reviews: Young, A.T. (1974), ApJ, 189, 587 Roddier, F. (1981), Progress in Optics, 19, 281 Coulman ARAA (1985), 23, 19 Beckers, J.M. (1993), ARAA 31, 13 Wilson, R.W.,Jenkins C.R. (1996), MNRAS, 268, 39