Star Ware: The Amateur Astronomer's Guide to Choosing, Buying ...

So You Want to Buy a Telescope! 31
These two important pluses are frequently enough to sway amateurs in
favor of a reflector. They feel that although there are drawbacks to the design,
these are outweighed by the many strong points. But just what are the problems
of reflecting telescopes? Some are peculiar to certain breeds, while others
affect them all.
One shortcoming common to all telescopes of this genre is the simple fact
that mirrors do not reflect all the light that strikes them. Just how much light
is lost depends on the kind of reflective coating used. For instance, most telescope
mirrors are coated with a thin layer of aluminum and overcoated with a
clear layer of silicon monoxide for added protection against scratches and pitting.
This combination reflects about 89% of visible light. But consider this:
Given primary and secondary mirrors with standard aluminum coatings, the
combined reflectivity is only 79% of the light striking the primary! That’s why
special enhanced coatings have become so popular in recent years. Enhanced
coatings increase overall system reflectivity to between 90% and 96%. Some
say, however, that enhanced coatings scatter light, in turn decreasing image
contrast. If so, the decrease is minimal.
Reflectors also lose some light and, especially, image contrast because of
obstruction by the secondary mirror. Just how much light is blocked depends
on the size of the secondary, which in turn depends on the focal length of the
primary mirror. Generally speaking, the shorter the focal length of the primary,
the larger its secondary must be to bounce all of the light toward the eyepiece.
Tradition has it, however, that the central obstruction is expressed in terms of
the percentage of aperture diameter, not area. Therefore, a reflector with a
10% central obstruction by area is referred to as having a 16% obstruction by
diameter. Both refer to an obstruction that measures 1.27 inches across. Primary
mirrors with very fast focal ratios can have central obstructions in the
neighborhood of 20%, even 25%. The only reflectors that do not suffer from
this ailment are referred to as off-axis reflectors. Classic off-axis designs
include the Herschelian and members of the Schiefspiegler family of instruments.
Since the idea of a telescope that uses mirrors to focus light was first conceived
in 1663, different schemes have come and gone. Today, two designs continue
to stand the test of time: the Newtonian reflector and the Cassegrain
reflector. Each shall be examined separately.
Newtonian reflectors. For sheer brute-force light-gathering ability, Newtonian
reflectors rate as a best buy. No other type of telescope will give you as large an
aperture for the money. Given a similar style mounting, you could buy an 8inch
Newtonian reflector for the same amount of money needed for a 4-inch
achromatic refractor.
Newtonians (Figure 3.3) are famous for their panoramic views of star
fields, making them especially attractive to deep-sky fans, but they also can be
equally adept at moderate- to high-powered glimpses of the Moon and the
planets. These highly versatile instruments come in a wide variety of styles.
Commercial models range from 3 inches to more than 2 feet in diameter, with