How AO Helps:
Adaptive optics uses deformable mirrors (DMs) to compensate for the
effects of the Earth's atmosphere, resulting in a
pristine image. Light from a reference beacon such as a laser beam is analyzed to determine how the atmosphere is
moving and hence causing distortions. This information is used to determine how a
deformable mirrorA mirror used in adaptive optics that changes shape in order to correct for atmospheric effects on light rays.
should be reshaped to compensate, and a series of
actuators (pistons) are then used to deform the mirror
on extremely tiny length scales. This must take place several hundred times per second
in order to negate atmospheric effects!
A mirror bent in the same shape as the light reflects back a flat wavefront
(the light has been corrected).
Image Credit: Introduction to Adaptive Optics presentation by Glen Herriot (NRC-CNRC)
A great way to see how much adaptive optics can improve observing is
by looking at its effects on the imaging of a star. You can do this by watching this movie of an AO system
named Hokupa'a which means "immovable star" in Hawaiian. This system was first operated in 1997, and moved to Gemini North in 1999. In the video, lab-generated turbulence is corrected by the Hokupa'a AO system
and the accompanying audio track is by Buzz Graves, a member
of the Hokupa'a instrument team from the University of Hawaii.
Using AO will allow the TMT to operate at the diffraction limit,
improving the resolution of a seeing-limited observation by a factor
of 10-30. In addition to this, since extinction is no longer an issue, astronomers can see objects that are farther
away and fainter. However, the AO system used by the TMT requires the use of a deformable mirror not yet in existence. This DM will need a large number of actuators and a large deformation stroke
and is currently being developed
by our affiliates at the University of Victoria.
The University of Victoria is collaborating with the NRC Herzberg Institute of Astrophysics
to develop an AO Test Bed which is expected to become a major research facility in
the field. In addition to this, it is carrying out the Woofer-Tweeter experiment. This is an attempt to determine
if using two DMs -one with few actuators and large strokes and the other with lots of actuators producing small
strokes - is a viable alternative to one DM.
The Center for Adaptive Optics website has more AO information.