APERTURE AND FOCAL LENGTH OF TELESCOPES
The width of the optical tube is known as its aperture. Technically,
it is the area of the optics that determines how much light is gathered,
but convention uses the width to describe aperture. This is what is referred
to when you hear the term “8-inch scope” or “20-inch
scope”. Every telescope’s primary optics, whether lens or
mirror, has an eventual focal point. The distance from the Objective (the
light gathering primary optical component) to the final focal plane of
the telescope is called its Focal Length. In general, for a given aperture,
longer focal lengths produce narrower fields of view, but higher magnifications,
while shorter focal lengths produce wider fields of view but lower magnifications.
The ratio of the focal length to the aperture is known as f/Ratio, and
you will often see this quoted for a particular scope. F/2 to f/5 are
considered short f/Ratios; f/6 to f/10 are considered medium f/Ratios,
while f/12 to f/20 are considered long f/Ratios. It is the medium f/Ratios
that are considered best for general use, where Moon, planets, stars,
star clusters, nebulae, and galaxies are all going to be observed with
the same instrument.
The longer the focal length of a scope, the larger and heavier the telescope
will be. The exception is the catadioptric telescopes, where a longer
focal length is compressed in a shorter tube. As an extra, catadioptric
telescopes often come with computerized mounts, making finding objects
very easy and simple.