Adaptive Optics
Written by Jean Tate
"Twinkle, twinkle little star", how maddening! Adaptive optics (AO) to the rescue.
Down here, under a sea of air, stars dance around and flash with different colors when viewed at high magnification through a telescope. That's because the path of light rays through the air is not straight and not constant, due to the air being in motion and of uneven density; turbulence. Astronomers call this 'seeing'.
Seeing can be reduced by keeping the telescope and its equipment (and dome) at the same temperature as the air, by getting above as much of the atmosphere as possible (on mountaintops, for example), and by choosing sites where the air is unusually steady (on only some mountaintops).
Better still would be to find a way to cancel out the distortions introduced by the air, and to make the image that falls on a detector such as a camera as close to that you'd get if the telescope were above the atmosphere. That's what adaptive optics does.
In a nutshell, a number of wavefront sensors measure how distorted the light entering the telescope is (the wavefront), calculate how to warp ('deform') a bendy mirror in the light path so as to make it undistorted, and do so … hundreds or thousands of times a second. There are several different technologies for doing this, and the simplest can be found on some amateur telescopes (the 'tilt-tip mirror').
Adaptive optics became possible only with the advent of computers both cheap and fast enough to calculate the wavefront distortions in real time (the air is still on timeframes of milliseconds); the principles of AO were known several decades before they were implemented on astronomical telescopes in the 1990s. Today, just about every new large telescope comes with AO instruments, capability, or plans for such.
In general, AO only works over a relatively small field within the image plane of a telescope, and for it to be successful on faint objects, there needs to be a bright one nearby to serve as a guide. Initially, these were stars; today several observatories are equipped with lasers which can create an artificial 'star' a dozen or so km up in the atmosphere (they often operate in pulsed mode, in synch with the detector, to keep the artificial 'starlight' from spoiling the image of the faint astronomical source).
With AO, a large telescope can achieve a resolution close to its theoretical maximum, which can be as low as a few hundredths of an arcsecond (as good as, or better than, the Hubble Space Telescope!).
The University of California's Center for Adaptive Optics has a good set of material for further exploration of the technical aspects.
Universe Today has lots of stories on observations made using AO; for example, Adaptive Optics Reveal Massive Star Formation, Best Ground-Based Image of Jupiter – Ever!, Closest Ever Look at Betelgeuse Reveals its Fiery Secret, and Titan's Desert Sports a Surprising, Powerful Storm. Enhanced Vision for the Subaru Telescope, and Keck Uses Adaptive Optics for the First Time cover the AO techniques themselves.
Astronomy Cast has an episode on Adaptive Optics; The Rise of Supertelescopes explains how important AO has become, and Optical Astronomy gives the general background.
Filed under: Astronomy
Related stories on Universe Today
- Podcast: Adaptive Optics
- The Clear Skies Above Paranal
- Deep Impact: Before and After
- Enhanced Vision for the Subaru Telescope
- Asteroid Davida
