The typical size of a turbulence cell in the Earth's atmosphere is such that from the ground it subtends a few arc seconds. Stars twinkle because these cells change the light path of the light from the star. For planets, the angular sizes of planets are much larger than stars, and are 10's of arcseconds. This is much larger than the size of the turbulent cells (an arcsecond or so) so any intensity changes due to turbulence are averaged over the disk of each planet and yield a more or less constant brightness to the eye. Even though the eye cannot resolve a planet, we can still deduce that the disks of planets are larger than stars in the sky because they do not twinkle.
Since the early 1980's, astronomers have been experimenting with 'speckle interferometry' and can now remove most of the twinkling of specific stars being observed through the telescope. This lets them take full advantage of the maximum possible resolving power of their telescopes, and in cases such as the Keck Telescopes, actually surpass the resolution of the Hubble Space Telescope itself.
Because of the turbulence in the upper layers of the atmosphere, a star's image will shift position hundreds of times each second, over a spot size in the telescope that is about one arcsecond in diameter. Even though a large telescope is capable of seeing details less than 0,1 arcsecond, the atmosphere limits the typical seeing to much worse than this. By photographing the star hundreds of times each second with a fast electronic camera, the individual image frames can be realigned so that all of the star images are once again on top of each other...canceling the atmospheric jitter. The resulting image is not only brighter but is as sharp as the theoretical limit of the telescope itself!
In another technique, the secondary mirror in the telescope is actually a pliable surface whose shape is changed thousands of times a second. The way the surface is altered is determined by measuring the properties of a bright reference spot...usually an artificial star created by a laser nearby...and the light from this reference spot is used to actively deform the secondary mirror.
This has resulted in a healthy competition between space-based and ground-based telescopes, with much significant research now being reclaimed by giant ground-based telescopes over 2-4 times the diameter of the Hubble, and equipped with atmosphere compensation technology. For example, here is a speckle image of a star before atmosphere compensation, and after. Note how clean and sharp the image now is after the atmosphere's twinkling effect has been removed!
The picture at the top of this page is of the center of the globular cluster M13 in Hercules, before and after correction for atmospheric effects using a 'sodium beacon' at the Center for Astronomical Adaptive Optics. Using a 6.5-meter telescope. According to their commentary:
These pictures are shown on the same gray scale, which is actually logarithmic to help show more of the dynamic range, which is more than 25000. The imaging camera was a 256 x 256 NICMOS3 array with 0.093 arc sec pixels, giving a field of view of 24 arc seconds, and the exposure time in each case is 15 s.
At left is the raw uncompensated image; the point-spread function has a FWHM of 0.74 arc sec, which indicates that seeing conditions at the time were very close to median for the site. The compensated image is shown after post-processing with the iterative blind deconvolution algorithm in the right-hand panel. For this exposure, the laser was aimed at the center of the field of view (the beam is of course invisible to the infrared camera), and global tilt information was derived from a natural star separated from the laser by about 40 arc sec, outside the field of view to the upper left. In this image, the adaptive optics system succeeded in reducing the FWHM of the star images to 0.53 arc sec, which has been further reduced to about 0.35 arc sec by the deconvolution.
Copyright 1997 Dr. Sten Odenwald
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