The term "Newtonian reflector" is derived from the 17th century astronomer and scientist Isaac Newton, who is more famous for getting beaned with an apple than for his association with telescopes. Although he did not invent the reflector, (despite persistent rumor to the contrary) he had a great deal to do with building and perfecting the design already created by James Gregory.
Unlike refractors, which make use of optical lenses, Newtonian reflectors utilize mirrors to do their work. Light from the image is directed to the bottom of a tube, where a precision-ground "primary" mirror is mounted. It bounces the image back up to a small "secondary" mirror which is mounted at an angle to redirect the image to the eyepiece. The observer turns a knob on a focuser to move the eyepiece inward or outward until correct focus is achieved.
This simple arrangement is very efficient, and allows relationships between diameter and length to be manipulated into all sorts of various configurations. This accounts for the existence of "short and fat" and "long and skinny" Newtonians, all depending on users' preferences for different observing agenda.
Newtonian reflectors are the easiest to fabricate, and some extremely high quality examples of homemade designs are almost always in attendance at star parties. Parts to make reflectors are available from numerous suppliers, and are very affordable. Building a homemade telescope is an excellent way to become fully involved in astronomy, and can eventually lead to a full time business endeavor.
Reflectors are immune from chromatic aberration, and very few disadvantages are associated with them. One minor issue is coma, an effect that causes concentric spiked rings to appear around bright stars. There are coma filters available to correct it.
The most significant disadvantage with a Newtonian reflector was always considered to be maintenance. Not too long ago, mirrors had to be painstakingly realigned, (collimated). Recently though, special collimating kits and lasers have been developed to make this chore a simple do-it-yourself task. Virtually all good Newtonian reflectors are now equipped to facilitate it with relative ease.
Newtonian reflectors are the most popular variety of telescope used by amateur astronomers - for some very good reasons. Number one, reflectors (including Dobsonians) are by far the least expensive per inch of aperture than any other type of telescope. They can also be very portable, and easy to get started with. Number two, they yield excellent images, and with good quality eyepieces, good Newtonians can provide the user with magnificent celestial views. Excellent examples of quality Newtonian reflectors are available from Meade, Vixen, and Orion. A typically orthodox specimen might be Orion's 6-inch AstroView Newtonian on their EQ-3 mount. Smaller Newtonians can be supported by Alt-Az mounts (see a description of Alt-Az mounts below*). Larger ones are found associated with equatorial mounts (EQ mounts). The very large tubes require Dobsonian support.
|* Alt-Az is an abbreviation for "Altitude-Azimuth". It is the type of mount found on all Dobsonian telescopes and some small and medium refractors. One axis swivels "horizontally" (side-to-side – Azimuth) while the other works "vertically" (up and down – Altitude). With the two working simultaneously, the Alt-Az mount can guide a telescope in any direction across the sky, and track with any celestial object. Good ones (like TeleVue's Gibraltar) are easy to guide manually, and are handy for "star-hopping" techniques. The Alt-Az mounts found on cheap telescopes are NOT easy to use for celestial viewing. Alt-Az type mounts and Dobsonian telescopes are a perfect combo, for smooth, precise operation regardless of aperture size.