Newtonian telescope - Definition and Overview

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 high quality primary mirror can be made by hand by amateur astronomers

• A short focal length can be obtained, leading to wide field, bright views.

• Long focal length Newtonian telescopes can give excellent planetary views.

• There is no chromatic aberration 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 length of f/6 or higher are considered to have little to no coma. Newtonians of a focal length 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.