All
of the refracting telescopes we have discussed in the previous topics share a
common limitation – the f/number of the telescope is limited by the size of the
objective lens, and it is difficult and expensive to build high-quality lenses
with large diameters. In addition, the weight of the lens increases at least as
the square of its diameter, so that supporting a large lens at its edge is a
hard job. Since the ray-tracing rules for lenses and mirrors are very similar,
any optical design that can be realized with lenses can usually be realized
with mirrors as well.
For
example, an astronomical telescope constructed using two positive lenses that
are separated by the sum of their two focal lengths can also be constructed
using two concave mirrors with the same separation. This design is usually
called a “Gregorian” configuration as shown in the following figure.
The
basic idea is the same: the first mirror forms a real, inverted image near its
focal point, and the rays from this image strike a second mirror where they are
directed to the eye of the observer. Sometimes a simple magnifier lens is
inserted into this path. Note that the second mirror partially blocks the field
of view, but this is usually a small obstruction relative to the size of the
main mirror. This configuration also requires drilling a small hole in the main
mirror.
In
addition to being easier to construct and support front-surface mirrors have no
chromatic aberration, which is an advantage in many astronomical applications.
Most large telescopes use mirrors rather than lenses for these two reasons.
In
addition to the “Gregorian” design shown in the figure above, there are a
number of other configurations, which differ primarily in the shape and
location of the second mirror. Instead of using two concave mirrors as above, a
“Cassegranian” design replaces the second small mirror with one that is convex.
This concave-convex design is similar in concept to the Galilean telescope,
which uses a positive-negative lens combination. There are also designs which
use a combination of a large mirror to form the primary image and secondary
lenses to focus this image into the eye or onto a sheet of film.
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