The Newtonian Reflector, also known as catoptrics, is a telescope which uses a spherical or concave parabolic primary mirror to collect, reflect and focus the light onto a flat secondary mirror (diagonal). This secondary mirror in turn reflects the light out of an opening in the side of the tube and into an eyepiece for focus and magnification.



Advantages

Lowest cost per inch of aperture compared to refractors and Catadioptrics since mirrors can be produced at less cost than lenses in medium to large apertures.
Reasonably compact and portable up to focal lengths of 1000mm.
Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters due to the generally fast focal ratios (f/4 to f/8).
Reasonably good for lunar and planetary work.
Good for deep sky astrophotography (but not as convenient and more difficult to use than Catadioptrics).
Low in optical aberrations and deliver very bright images.

Disadvantages

Requires regular alignment (collimation) of optics in order to perform at its best. Badly aligned optics can make the image quality suffer quite dramatically.
Primary mirror may require re-coating (usually after years of service).
The open nature of the tube assembly could mean more complicated cleaning of mirror surfaces when compared to telescopes of other designs.
Generally not suited for terrestrial applications.
Slight light loss due to secondary (diagonal) obstruction when compared with refractors.