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

Newtonian telescopes are usually less expensive for any given aperture than comparable quality telescopes of other types.
Since light does not pass through the objective (it only bounces off a mirrored surface) exotic glasses are not needed, the material only needs to be able to hold an accurate figure.
Since there is only one surface that needs to be figured (as apposed to four in a refractor) it is easier for amateur telescope makers to fashion their own objective.
A short focal ratio can be more easily obtained, leading to wider field of view.
Long focal length Newtonian telescopes can give excellent planetary views.
There are no lenses to cause chromatic aberration as in a refractor.
The eyepiece at the top end of the telescope combined with short f-ratios allow a shorter and more compact mounting system, reducing cost and adding to portability.

A replica of Newton's first 6 inch reflector

Disadvantages of the Newtonian design

Newtonians suffer from coma, an off-axis aberration which causes imagery to flare outward and away from the optical axis. This flare is zero on-axis, and is linear with increasing field angle and inversely proportional to the square of the mirror focal ratio, equal to the mirror focal length divided by the mirror aperture. The formula for third order tangential coma is 3θ / 16F², where θ is the angle off axis to the image in radians and F is the focal ratio. Newtonians with a focal ratio of f/6 or higher are considered to have insignificant coma for visual or photographic use. Newtonians having a focal ratio of less than f/4 have considerable coma but are the most compact systems, and can still yield beautiful wide-field, low-power imagery. Commercial lenses are also available for Newtonian telescopes that correct for coma from low focal ratio primary mirrors and restore image sharpness over the field. One source is the "ParaCorr" lens from Televue.

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. Visually, these effects can be reduced by using a two or three-legged curved spider. This reduces the diffraction sidelobe intensities by a factor of about four and helps to improve image contrast, with the potential penalty that circular spiders are more prone to wind-induced vibration. Although a four-legged spider causes less diffraction than a three-legged curved spider, the three-legged curved spider often gives a more aestheticly pleasing view.

For portable Newtonians collimation can be a problem. The primary and secondary can get out of alignment from the shocks associated with transportation and handling. This means the telescope may need to be re-aligned (collimated) every time it is set up. Other designs such as refractors and catadioptrics (specifically Maksutov cassegrains) have fixed collimation.

Reference

Smith, Warren J., Modern Optical Engineering, McGraw-Hill Inc., 1966, p. 400.