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newtonian telescope : ウィキペディア英語版
newtonian telescope

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.〔(Isaac Newton: adventurer in thought, by Alfred Rupert Hall, page 67 )〕 The Newtonian telescope's simple design makes it very popular with amateur telescope makers.〔(Telescope Basics - Mark T. VandeWettering, 2001 )〕
== History ==

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,〔(Stargazer - By Fred Watson, Inc NetLibrary, Page 108 )〕 and others, such as Niccolò Zucchi, claimed to have experimented with the idea as far back as 1616.〔(The Galileo Project > Science > Zucchi, Niccolo )〕 Newton may even have read James Gregory's 1663 book ''Optica Promota'' which described reflecting telescope designs using parabolic mirrors〔(Derek Gjertsen, The Newton handbook, page 562 )〕 (a telescope Gregory had been trying unsuccessfully to build).〔(Isaac Newton By Michael White Page 169 )〕
Newton built his reflecting telescope because he suspected it could prove his theory that white light is composed of a spectrum of colours.〔(Isaac Newton By Michael White Page 170 )〕 Colour distortion (chromatic aberration) was the primary fault of refracting telescopes of Newton's day, and there were many theories as to what caused it. During the mid-1660s with his work on the theory of colour, Newton concluded this defect was caused by the lens of the refracting telescope behaving the same as prisms he was experimenting with, breaking white light into a rainbow of colours around bright astronomical objects.〔Newton thought little could be done to correct aberration short of making lenses that were f/50 or more."''the object-glass of any telescope cannot collect all the rays which come from one point of an object, so as to make them convene at its focus in less room than in a circular space, whose diameter is the 50th part of the diameter of its aperture''”〕〔''Treatise on Optics'', p. 112〕 If this were true, then chromatic aberration could be eliminated by building a telescope which did not use a lens – a reflecting telescope.
In late 1668 Isaac Newton built his first reflecting telescope. He chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror. He later devised means for shaping and grinding the mirror and may have been the first to use a pitch lap〔(Reflecting Telescope Optics: Basic Design Theory and Its Historical Development ) By Ray N. Wilson Published by Springer, 2004 ISBN 3-540-40106-7, ISBN 978-3-540-40106-3.〕 to polish the optical surface. He chose a spherical shape for his mirror instead of a parabola to simplify construction; even though it would introduce spherical aberration, it would still correct chromatic aberration. He added to his reflector what is the hallmark of the design of a Newtonian telescope, a secondary diagonally mounted mirror near the primary mirror's focus to reflect the image at a 90° angle to an eyepiece mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made the tube, mount, and fittings. Newton's first version had a primary mirror diameter of and a focal ratio of f/5.〔(telescope-optics.net Reflecting Telescopes: Newtonian, two- and three-mirror systems )〕 He found that the telescope worked without colour distortion and that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus with it. Newton's friend Isaac Barrow showed a second telescope to a small group from the Royal Society of London at the end of 1671. They were so impressed with it that they demonstrated it to Charles II in January 1672. Newton was admitted as a fellow of the society in the same year.
Like Gregory before him, Newton found it hard to construct an effective reflector. It was difficult to grind the speculum metal to a regular curvature. The surface also tarnished rapidly; the consequent low reflectivity of the mirror and also its small size meant that the view through the telescope was very dim compared to contemporary refractors. Because of these difficulties in construction, the Newtonian reflecting telescope was initially not widely adopted. In 1721 John Hadley showed a much-improved model to the Royal Society.〔(amazing-space.stsci.edu - Hadley’s Reflector )〕 Hadley had solved many of the problems of making a parabolic mirror. His Newtonian with a mirror diameter of compared favourably with the large aerial refracting telescopes of the day.〔(The complete Amateur Astronomer - John Hadley's Reflector )〕 The size of reflecting telescopes subsequently grew rapidly, with designs doubling in primary mirror diameter about every 50 years.

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