The twinkling of the stars is a source
of inspiration to poets, but it presents a serious obstacle to
astronomers’ observations. The flickering is caused by turbulence
in the atmosphere, which prevents large earth-bound telescopes
from providing pictures with the degree of sharpness that would
theoretically be possible. One way out of this dilemma is to use
space telescopes, such as the Hubble Space Telescope. However,
observatories in orbit around the earth are not only expensive
systems, but they are also very difficult to handle.
Since the start of the 1980’s, a technology
has been developed which makes it possible to correct the image
distortions caused by turbulence (known as »seeing«
in astronomical jargon) while the observation is still in progress.
This method, known as adaptive optics, will be of crucial importance
on the large telescopes of the new generation, such as the Very
Large Telescope (VLT), or the Large Binocular Telescope (LBT,
cf. Chapter I). Interferometry, which is to be operated in conjunction
with the VLT, the Keck telescopes and the LBT, will also benefit
decisively from adaptive optics. It brings more light to the interference
and therefore makes it possible to observe fainter objects.
In collaboration with colleagues at the
MPI für extraterrestrische Physik / MPI of Extra-Terrestrial
Physics in Garching, astronomers and technical experts at the
MPIA have developed and built an adaptive optical system for use
at the Calar Alto Observatory. In addition to this, a laser system
has been set up which creates an ‘artificial star’ in
the night sky. The adaptive optics system uses this as a bright
star for purposes of comparison during the image correction. This
system, known as ALFA, was tested successfully for the first time
at the end of 1997, and it places the MPIA at the very forefront
of research: in the astronomical sector, there are currently only
two other instruments of this sort anywhere in the world.
The Principle
of Adaptive Optics
In theory,
the resolution of a telescope (that is to say, its ability to
show separate images of two objects that are located close to
one another) depends exclusively on the diameter of the main mirror
and the wavelength of the light. In the visible range (wavelength
of about 550 nm), a 3.5-metre telescope has a theoretical resolution
capaci
ty – also known as the diffraction
limit – of 0.04 seconds of arc; at 2.2
mm, the figure is 0.16, or four times
less. In practice, however, the turbulence of the air blurs the
received image so heavily that the typical resolution is only
one second of arc. This means that every earth-bound telescope,
no matter how large it may be, will only attain the same resolution
as can already be achieved by any 15 centimetre telescope!
The light from a star may be imagined as
a spherical wave which spreads out in space at the speed of light.
If a wave of this sort encounters the earth’s atmosphere,
it is almost perfectly flat, on account of the huge distance from
the source (Figure II.1). As it passes through the various layers
of the air, however, this wave will experience disturbances which
are variable in terms of space
