In practice, however, both atmospheric and telescope
errors distort the spherical wavefront, creating phase errors in
the image-forming ray paths. Even at the best sites, ground-based telescopes
observing at visible wavelengths cannot, because
of atmospheric turbulence alone, achieve an angular
resolution better than telescopes of 10- to 20-cm diameter. For
a 4-m
telescope, atmospheric distortion degrades the spatial
resolution by more than one order of magnitude compared with the diffraction
limit, and the intensity at the center of the star image is lowered by
a factor of 100 or more. The cause is random spatial and temporal wavefront
perturbations induced by turbulence in various layers of the atmosphere;
one of the principal
reasons for flying the Hubble Space Telescope was
to avoid this image smearing.
In addition, image quality is affected by permanent
manufacturing errors and by long time scale-wavefront aberrations introduced
by mechanical, thermal, and optical effects in the telescope, such as defocusing,
decentering, or mirror deformations generated by their supporting devices.
The earth's surface is covered by a layer of atmosphere
consisting of a mixture of gases
and other solid and liquid particles.
The gaseous materials extend to several hundred
kilometers in altitude, though there
is no well defined boundary for the upper limit of the
atmosphere.
The first 80 km of the atmosphere contains more than
99% of the total mass of the earth's
atmosphere.
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Robert T MacIntyre RIT/CIS
10/31