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Star testing is a very sensitive technique for checking
telescope optics. It is easily performed, but requires
careful examination to reach a correct interpretation.
When a point source is viewed through a telescope,
many optical aberrations can be determined by carefully
inspecting the patterns of the diffraction images. The
telescope is focussed at a very high magnification, if
possible at least 10x per cm of aperture. A good quality
eyepiece is used but it is best not to use a Barlow lens
since it might introduce its own aberrations. The
patterns are then observed both in focus and very
slightly inside and outside of the focal point. In a
perfect diffraction image, the focussed pattern has a
sharp, bright Airy disk, surrounded by increasingly
faint concentric circles, and what is more important,
the patterns at similar points inside and outside of
focus, are round and match each other exactly. The
perfect out-of-focus image for a refractor is a series
of alternating light and dark, concentric circles, while
that of a centrally obstructed telescope shows a dark
central shadow. Specific problems can be determined
from both the overall shape of a diffraction pattern and
by comparison of the pattern differences between
the inside and outside focus images.
The star test is usually carried out at night using a
bright star but it can be done in daylight using an
artificial star, such as the reflection off of a Christmas
tree ornament placed at some distance away. Before
trying to identify aberrations, the telescope must be
carefully collimated, and its temperature must be
allowed to come to equilibrium with the surrounding
air, to avoid tube currents. Testing must only be done
when atmospheric turbulence is minimal so that the
results are not masked. For the same reason it should
not be done while sighting over buildings or parking
lots where rising columns of air can interfere.
Some of the patterns will indicate things that you
should be able to correct. For example, severe
atmospheric turbulence means that you should try
another night, and the presence of tube currents
means that you should be patient and wait a bit
longer for your scope's temperature to come to
equilibrium. Poor collimation of a reflector, or of
a new Sky-Watcher refractor model which has
an adjustable objective-lens cell, means that you
should get out your tools. Similarly, coma may be
caused by poor collimation. However, if some
coma still exists around the edges of the field
after collimating, its effects should not be seen
at the field's centre. Any one of several "pinched
optics" patterns means that you should check
your system for sources of stress on the optics,
such as collimation screws which have been
over tightened, tube-ring clamps which are too
tight, and so on. Finally, astigmatism can have
many causes, but two of the most common are
poor collimation and a secondary mirror or a
diagonal mirror which is not perfectly flat. Some
aberrations result from poor shaping and polishing
during the mirror-making process and cannot be
readily corrected by the user. These include mirror
over-correction and under-correction (which can
result during the parabolizing process), some
causes of astigmatism, edge defects, zonal defects
and rough surfaces.
An excellent, advanced reference, for interpreting
the results from this technique, is "Star Testing
Astronomical Telescopes" by Harold Richard Suiter,
(Willmann-Bell, Inc., Richmond, VA, 1994). |
