The Need for Adaptive Optics
Astronomers have long been plagued by a major limitation to their observations: atmospheric aberration. Telescopes on Earth must look at the stars through the atmosphere, which distorts the incoming light waves and imposes a limitation on the resolution of these telescopes.
The resolution of optical systems is measured in minutes of arc. An arcminute is 1/60 degree, and an arcsecond is 1/60 arcminute. According to the Center for Adaptive Optics, the 10 meter Keck Telescope should have the exceptionally fine resolution of 0.013 arcseconds, but because of atmospheric aberration, its actual resolution is only 0.5 to 1.0 arcseconds.
One solution has been to place telescopes in space, the most famous of which is the Hubble Space Telescope. Unfortunately, space telescopes are expensive to deploy and maintain. A relatively new technology has been developed to compensate for atmospheric aberration in astronomy: adaptive optics.
Adaptive Optics Illustrations
Left: Diagram demonstrating how adaptive optics technology works. The incoming light waves are distorted by the atmosphere, a phenomenon called wavefront aberration. The distortion is detected by the wavefront sensor and the deformable mirror changes shape to correct for the distortion. The outgoing light waves are "straightened."
Right: The left image is of a star viewed without adaptive optics. The resolution is 1.02 arcseconds. The right image is of the same star viewed with a laser guided optics system. The resolution is only 0.16 arcseconds.
How Adaptive Optics Technology Works
Adaptive optics technology requires three components: a wavefront sensor, which detects the atmospheric distortions and blurring in light coming from space; a computer to receive and process this input; and a deformable mirror, controlled by the computer, that corrects the distortions. An adaptive optics system must update the mirror’s shape several hundred times per second.
The most basic adaptive optics sytems uses a bright star as a reference beacon; the light from this star is used as the wavefront sensor’s input. This imposes a significant limitation on the use of this technology: it is only useful for observing objects that happen to be within a few arcseconds of a star bright enough to be a reference beacon.
A possible solution is to use multiple guide stars to examine the tomography (sections) of the atmospheric distortion of the sky. This method allows astronomers to calculate the aberration in the section of sky of the object they are observing.
A more sophisticated adaptive optics technology provides its own reference data using a laser. For example, the Mount Palomar Observatory’s laser guided optics system uses two polarized lasers with wavelengths of 589 nm (yellow) and 660 nm (red). The lasers’ energy excites sodium atoms in a particular part of the atmosphere (the "sodium layer"). The sodium atoms give off light that is detected by the wavefront sensor, allowing the adaptive optics sytem to be used on any part of the sky.
References and Image Credits
- Center for Adaptive Optics — https://cfao.ucolick.org/
- "Adaptive-optics corrections available for the whole sky." R. Ragozzoni, E. Marchetti, G. Valente. Nature 2000 Jan 6;403(6765):54-6. https://www.ncbi.nlm.nih.gov/pubmed/10638747?dopt=Abstract
- Mount Palomar Adaptive Optics technical information — https://www.oir.caltech.edu/twiki_oir/bin/view/Palomar/PalmLGS/WebHome
Adaptive optics diagram: Bob Tubbs (https://commons.wikimedia.org/wiki/File:Adaptive_optics_correct.png)
Star images: Mount Palomar Observatory, from JPL website (https://ao.jpl.nasa.gov/Palao/PalaoIndex.html)