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INTROUCTION
Adaptive
optics is a new technology which is being used now a days in ground based telescopes
to remove atmospheric tremor and thus provide a clearer and brighter view of stars
seen through ground based telescopes. Without using this system, the images obtained
through telescopes on earth are seen to be blurred, which is caused by the turbulent
mixing of air at different temperatures causing speed & direction of star
light to vary as it continually passes through the atmosphere
.
Adaptive
optics in effect removes this atmospheric tremor. It brings together the latest
in computers, material science, electronic detectors, and digital control in a
system that warps and bends a mirror in a telescope to counteract, in real time
the atmospheric distortion.
The advance promises to let ground based telescopes reach their fundamental limits
of resolution and sensitivity, out performing space based telescopes and ushering
in a new era in optical astronomy. Finally, with this technology, it will be possible
to see gas-giant type planets in nearby solar systems in our Milky Way galaxy.
Although about 100 such planets have been discovered in recent years, all were
detected through indirect means, such as the gravitational effects on their parent
stars, and none has actually been detected directly.
WHAT
IS ADAPTIVE OPTICS ?
Adaptive optics refers to optical systems which adapt to compensate for optical
effects introduced by the medium between the object and its image. In theory a
telescope's resolving power is directly proportional to the diameter of its primary
light gathering lens or mirror. But in practice , images from large telescopes
are blurred to a resolution no better than would be seen through a 20 cm aperture
with no atmospheric blurring. At scientifically important infrared wavelengths,
atmospheric turbulence degrades resolution by at least a factor of 10.
Under ideal circumstances, the resolution of an optical system is limited by the
diffraction of light waves. This so-called " diffraction limit " is
generally described by the following angle (in radians) calculated using the light's
wavelength and optical system's pupil diameter: a
= 1.22 ^
D Where
the angle is given in radians. Thus, the fully-dilated human eye should be able
to separate objects as close as 0.3 arcmin in visible light, and the Keck Telescope
(10-m) should be able to resolve objects as close as 0.013 arcsec.
In practice, these limits
are never achieved. Due to imperfections in the cornea nd lens of the eye, the
practical limit to resolution only about 1 arcmin. To turn the problem around,
scientists wishing to study the retina of the eye can only see details bout 5
(?) microns in size. In astronomy, the turbulent atmosphere blurs images to a
size of 0.5 to 1 arcsec even at the best sites.
Adaptive optics provides a means of compensating for these effects, leading to
appreciably sharper images sometimes approaching the theoretical diffraction limit.
With sharper images comes an additional gain in contrast - for astronomy, where
light levels are often very low, this means fainter objects can be detected and
studied
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