Newtonian telescope
The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727), using a parabolic primary mirror and a flat diagonal secondary mirror.
Advantages of the Newtonian Design
- Newtonians are usually less expensive than comparable telescopes of other types.
- A short focal_ratio can be obtained, leading to wide field, bright views.
- Long focal_ratio Newtonian telescopes can give excellent planetary views.
- There is no chromatic_abberation caused by lenses as in a refractor
- The eyepiece is at the top end of the telescope, allowing for a shorter and more stable mount.
- The heavy primary mirror is at the bottom allowing for large portable scopes. Dobsonian mounted Newtonians can have a 40" primary mirror.
- Collimation is relatively easy.
Disadvantages of the Newtonian Design
- Newtonians have coma, which is bluring of images away from the optical center. This problem is worse the shorter the focal length. Newtonians with a focal ratio of f/6 or higher are considered to have little to no coma. Newtonians of a focal ratio of less than f/4 are often considered to have too much coma.
- Newtonians have a central obstruction due to the secondary mirror in the light path. This obstruction and the diffraction spikes caused by the support of the secondary mirror reduces contrast.
