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  General Why AO? How AO Helps The Diffraction Limit

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How AO Helps

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.

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