Astronomy Principles and Practice Fourth Edition.pdf

268 The optics of telescope collectors
(a)
(b)
Figure 16.31. (a) One of the four 8·1 m telescopes making up the VLT array on the Paranal Mountain. (b) The
panoramic disposition of the VLT array. (Photographs by courtesy of ESO.)
prior to the optical working, is more critical for an objective than for a mirror. Large homogeneous
disks of glass are very difficult to manufacture. This is one of the reasons why all the very large
telescopes are reflectors.
Despite these advantages, some experienced amateur visual observers prefer to use refractors for
their measurements and it appears that refractors do perform better under the actual working conditions
in the telescope dome. Perhaps the chief reason for this is that, in general, the optics of the refractor are
less sensitive to the changes in temperature which occur during the night. A fall in temperature causes
the usual optical materials to contract, thus causing the optical surfaces to change shape. In the case of
refractors, any warping produced in the front surface is largely cancelled out by the changes in the rear
surface of the lens. The images produced by an objective are usually affected only slightly by changes
of temperature within the dome. However, in the case of a mirror, its optical and rear surfaces are not
exposed in identical ways and, therefore, suffer different rates of temperature changes. Some of the
smaller reflectors are susceptible to heat which is generated by the observer at the telescope. Strain is
set up within the mirror with consequent warping of the optical surface. Under severe circumstances,
some reflecting telescopes produce multiple images, each image being produced by a different zone in
the optical surface. The mirror material must have a small thermal coefficient of expansion. Ordinary
plate glass is poor in this respect. Pyrex, which has been used in the past, has an expansion coefficient of
about one-third of that for plate glass. Many of the new larger telescope mirrors are made of materials
such as Cer-Vit, with an extremely small thermal expansion.
All large telescopes, i.e. greater than 1 m (∼40 inches) in diameter, are reflectors. The mechanical
and optical problems involved in the design of a large telescope are more easily overcome in a reflector
than in a refractor. It is obvious, for example, that the weight of the collector increases as the telescope