Reflecting telescopes use a system of mirrors to gather and focus the incoming light for magnification by the eyepiece. They can be configured in a number of different ways, but they all, almost invariably, employ a parabolic primary mirror in combination with a secondary mirror which may be flat or curved. The image formed by reflecting telescopes is inverted, but from an astronomical perspective, this is of little importance.
Newtonian telescopes are by far the simplest of all astronomical instruments. Light is reflected by the parabolic primary mirror to form an image at the mirrors focal plane. A secondary mirror, in this case an optically flat diagonal, is placed just before the focal plane to reflect it at right-angles to, and outside, the original light path. The secondary mirror is normally supported by four spoke-like vanes attached at the top of the telescope, an arrangement called the spider.
Both the primary and secondary mirrors should have provision allowing for their re-alignment or collimation, when necessary. The primary mirror should be cradled and supported in a mirror cell which has an arrangement of three adjusting screws and three locking screws built in. The secondary should have a similar arrangement for collimating it. Provision may also be made to allow the spider to be adjusted to laterally align the diagonal mirror with the optical axis of the primary.
Newtonians are arguably the most popular of all amateur astronomical instruments. The simplicity of their design translates into a relatively low cost of manufacture compared to other instruments and therefore a lower retail price tag. This means that relative to other designs, the Newtonian telescope represents good value, either allowing you to save money or to upgrade to a larger aperture.
In Cassegrain telescopes, incoming light is reflected by a perforated parabolic primary mirror to form an image at the mirrors focal plane. A hyperboloidal secondary mirror is located just before the focal plane of the primary to reflect the light forming the image back along the primary optical axis and through the perforation to the eyepiece.
Because the light path of Cassegrain telescopes is turned back along its optical axis, effectively halving the length of the telescope tube required, they produce a larger primary image than a Newtonian of the same aperture and tube length, however the field of view is smaller and the image fainter.
The classic Cassegrain telescope design has become the basis for a new generation of amateur telescopes combining lenses and mirrors, the catadioptrics.