Newtonians are a type of reflecting telescope. A concave parabolic primary mirror at the back of the telescope collects and focuses incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image out of an opening at the side of the front end of the main tube and into the eyepiece. (For large f/ratio Newtonians, the primary mirror often has a spherical shape.)
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 as and more difficult to use than catadioptrics).
- Low in optical aberrations and deliver very bright images.
Disadvantages
- Generally not suited for terrestrial applications.
- Secondary (diagonal) mirror and its supporting structure obstruct some of the incoming light, leading to slight light loss and scattering (diffraction of light) when compared with refractors.