Newtonian telescope

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Image:Newtonian.PNG 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.

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Advantages of the Newtonian Design

Newtonians are usually less expensive for any given aperture than comparable quality 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 style Newtonians can have a 40" primary mirror.

Collimation is relatively easy.

Image:NewtonsTelescopeReplica.jpg

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Disadvantages of the Newtonian Design

Newtonians have 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 at http://www.televue.com.

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.

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