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History Newtonian Telescope Journal Article Reflecting Sir Isaac Newton
   
Newtonian telescopes are usually less expensive for any given aperture than comparable quality telescopes of other types.
The earliest telescopes, such as those used by Galileo, consisted of glass lenses mounted in a tube. Newton discovered that when light (from a star, for example) passed through a lens the different colours were refracted by differing amounts. This meant that the components of white light were brought to a focus at different places and the image of a star would then appear to be surrounded by a spectrum of colours. This effect is called chromatic aberration, and was not easily rectified using the technology available in Newton's time.
Newtonians suffer from coma, an off-axis aberration which causes imagery to flare inward and towards 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.
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