Seeing, in astronomy, is a measure of the image quality obtainable
during a telescopic observation. It is measured
by the width (in arcseconds) of an unresolved point source's
image.
Under ideal conditions, a point source will look like an
Airy pattern --
a central bright circle, surrounded by a series of
increasingly fainter circles.
This pattern is caused by Fraunhofer diffraction by the aperture of the
telescope. The distance between the center of the bright circle and the
first dark ring of the Airy pattern is the resolution of the telescope,
known as the Rayleigh criterion. The value of the Rayleigh
criterion is
θ (radians) = 1.22 &lambda / D
θ (arcseconds) = 206265 × 1.22 λ / D
where θ is the angular distance, &lambda
is the wavelength of the light you're observing, and D is the
diameter of the telescope. For visible light (average wavelength of 5000
angstroms) and a 1-meter telescope, the theoretical resolution
is about a tenth of an arcsecond.
If you've ever used a telescope, even one much smaller than one meter in size,
you probably know the resolution doesn't come close to that. This is because
the Earth's atmosphere is a soggy, blustery, turbulent mess
of eddies and currents. These disturbances in the atmosphere change
its refractive index as functions of time *and* location, which wind up
blurring any starlight passing through the air.
Seeing is the observed resolution limit for a given observation, a quantity that changes from site to site, telescope to telescope, and night to night. When astronomers discuss the quality of a given observation, seeing is one of the quantities they state.
In practice, seeing is usually limited to no better than
half an arcsecond, regardless of the size of your telescope. This
means that the smallest details you can make out in a large telescope
are about half an arcsecond across. This is quite a nuisance if, for example,
you're trying to observe both stars in a close binary star system, looking
for extrasolar planets, or trying to study the
morphologies of
galaxies halfway across the
universe. If two points were closer together than about half an arcsecond,
they would be blurred together into a single, amorphous blob. For major observatories, "sub-arcsecond" seeing is
considered good, and seeing of 0.5 arcseconds or less is
generally considered superb.
You can get partly around this by placing telescopes on top of high
mountains like Mauna Kea, or
Cerro Tololo, because you are
putting the telescope above a large mass of air that light would otherwise
have to pass through. But even then, seeing can still vary from hour to hour,
night to night, and season to season, depending upon the state of the
upper atmosphere. Another way around it is to try and measure the distortions
of the light caused by turbulence in the atmosphere, and then adjust the optics
of your telescope to cancel them out in real time. This technique is called
adaptive optics, and it has been used in some of the larger observatories
around the world for the past decade.
The ultimate way to avoid seeing problems altogether is of course to put
the telescope outside of the Earth's atmosphere. The Hubble Space Telescope
has nearly diffraction-limited resolution (about 0.02 arcseconds with a
2.5-meter mirror). As a result, it has produced some of the finest optical
images in history. Of course, it cost 100 times more than a
ground-based observatory, but who's counting?