Thursday, March 20, 2008

The effect of twinkling star

Theoretically, the giant Hale telescope at Mount Palomar is capable of a spectacular angular resolution of a .02” (or 20 milliarcseconds); that would be its resolution in the absence of complicating factors like the earth’s atmosphere. In actual practice, it has a resolution of about 1”. The source of this limit is related to the reason that stars twinkle. The earth’s turbulent atmosphere stands between the telescope’s gigantic primary mirror and the stars, smearing the image just as it sometimes causes starlight viewed with the naked eye to shimmer and twinkle.

If you took a still photograph of a twinkling star through a large telescope, you would see not a pinpoint image, but one that had been smeared over a minute circle of about 1” (1 arcsecond). This smeary circle is called the seeing disk, and astronomers call the effect of atmospheric turbulence seeing. When weather fronts are moving in (even if the skies appear clear), or have just moved out, the seeing can be particularly bad.

High, dry locations generally have the best seeing. To achieve resolutions better than about 1” from the surface of the earth is possible, but it requires a few tricks.

Adaptive optics, for example, are being increasingly employed on new research telescopes. This method allows a mirror in the optical path to be slightly distorted in real time (by a series of actuators) in order to compensate for the blurring effects of the atmosphere. Of course, much higher resolutions are possible at other wavelengths. As we will see, radio interferometers regularly provide images with resolutions better than 0.001” (or 1 milliarcsecond).

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