Atmospheric
turbulence causes a severe degradation in astronomical images acquired
with large ground-based telescopes. The turbulence is known to take the
form of refractive index fluctuations, thought to exist in thin layers
at varying heights within the atmosphere.
The Problem
Light from an astronomical source traverses the large
distance to the earth's atmosphere practically undeviated; before entering
the earth's atmosphere its wavefront is essentially flat (due to the large
distances involved).
At this stage, a good telescope (such as an idealised
telescope in space) should be able to form an almost perfect (or diffraction-limited)
image.
However, the refractive index
inhomogeneities introduce random phase delays
into different parts of the wave, resulting in a distorted wavefront.
This wave cannot now be focussed to form a diffraction-limited image.
The image formed by a large telescope breaks up into
a number of speckles; for this reason it is known as a speckle
pattern.
This pattern fluctuates rapidly as the refractive
index distribution changes with a typical correlation time of a few milliseconds.
To see Speckle Movies
The measurement of the shape of the atmospherically distorted wavefront or the evaluation of its statistical properties is known as wavefront sensing.
Kolmogorov
See LINKS According
to Kolmogorov's energy
The behaviour of the atmospherically-distorted wavefront
at the telescope pupil is normally modelled in terms of Kolmogorov theory,
which predicts the statistical properties of the refractive index fluctuations
and leads to equations which describe the propagation of light from the
top of the atmosphere to the telescope aperture.
The theory predicts that the form of the statistics
of the phase fluctuations is known and that they are simply weighted by
a single time-dependent parameter, known as r0.
This parameter can be thought of as the maximum diameter
of telescope which can support diffraction-limited imaging under the prevailing
atmospheric conditions, or `seeing'. It is a measure of how strong the
turbulence is.
At a good observing site, r0
can reach 20cm under conditions of good seeing; under strongly turbulent
conditions it may be much less than this.