The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton (1642–1727), using a concave primary mirror and a flat diagonal secondary mirror.Newton’s first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope.
Newton's idea for a reflecting telescope was not a new one.Galileo Galilei and Giovanni Francesco Sagredo had discussed using a mirror as the image forming objective soon after the invention of the refracting telescope, and others, such as Niccolò Zucchi , claimed to have experimented with the idea as far back as 1616. Newton built his reflecting telescope because he suspected it could prove his theory that white light is composed of a spectrum of colors.
* Advantages of the Newtonian Design *
They are free of chromatic aberration found in refracting telescopes.
Newtonian telescopes are usually less expensive for any given objective diameter (or aperture ) than comparable quality telescopes of other types.
The eyepiece is located at the top end of the telescope. Combined with short f- ratios this can allow for a much more compact mounting system, reducing cost and adding to portability.
Disadvantages of the Newtonian design
For portable Newtonians collimation can be a problem. The primary and secondary can get out of alignment from the shocks associated with transport 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.
Newtonians have a central obstruction due to the secondary mirror in the light path. This obstruction and also the diffraction spikes caused by the support structure (called the spider) of the secondary mirror reduce 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.