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 blurring 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 structure (called the spider) of the secondary mirror reduces contrast.