Editing Newtonian telescope (section)

==History ==
{{See also|History of the telescope}}

Newton's idea for a reflecting telescope was not new. [[Galileo Galilei]] and [[Giovanni Francesco Sagredo]] had discussed using a mirror as the image forming [[Objective (optics)|objective]] soon after the invention of the refracting telescope,<ref>{{cite book|author=Fred Watson|title=Stargazer: The Life and Times of the Telescope|url=https://books.google.com/books?id=2LZZginzib4C&pg=PA40|year=2007|publisher=Allen & Unwin|isbn=978-1-74176-392-8|page=108}}</ref> and others, such as [[Niccolò Zucchi]], claimed to have experimented with the idea as far back as 1616.<ref>[http://galileo.rice.edu/Catalog/NewFiles/zucchi.html The Galileo Project > Science > Zucchi, Niccolo]</ref> Newton may even have read [[James Gregory (astronomer and mathematician)|James Gregory's]] 1663 book ''[[James_Gregory_(mathematician)#Optica_Promota|Optica Promota]]'' which described reflecting telescope designs using [[Parabola|parabolic mirrors]]<ref>{{cite book|author=Derek Gjertsen|title=The Newton Handbook|url=https://books.google.com/books?id=cqIOAAAAQAAJ&pg=PA562|year=1986|publisher=Routledge & Kegan Paul|isbn=978-0-7102-0279-6|page=562}}</ref> (a telescope Gregory had been trying unsuccessfully to build).<ref>{{cite book|author=Michael White|title=Isaac Newton: The Last Sorcerer|url=https://books.google.com/books?id=l2C3NV38tM0C&pg=PA169|year=1999|publisher=Basic Books|isbn=978-0-7382-0143-6|page=169}}</ref>

Newton built his reflecting telescope because he suspected it could prove his theory that [[Electromagnetic spectrum#Visible radiation .28light.29|white light]] is composed of a [[spectrum]] of colours.<ref>{{cite book|author=Michael White|title=Isaac Newton: The Last Sorcerer|url=https://books.google.com/books?id=l2C3NV38tM0C&pg=PA169|year=1999|publisher=Basic Books|isbn=978-0-7382-0143-6|page=170}}</ref> Colour distortion ([[chromatic aberration]]) was the primary fault of [[refracting telescope]]s of Newton's day, and there were many theories as to what caused it. During the mid-1660s with his work on [[Isaac Newton's early life and achievements#Newton's theory of colour|the theory of colour]], Newton concluded this defect was caused by the lens of the refracting telescope behaving the same as [[Prism (optics)|prisms]] he was experimenting with, breaking white light into a rainbow of colours around bright [[astronomical object]]s.<ref>Newton thought little could be done to correct aberration short of making lenses that were [[F-number#Focal ratio in telescopes|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''”</ref><ref>{{cite book|author=Stephen Parkinson|title=A Treatise on Optics|url=https://archive.org/details/atreatiseonopti03parkgoog|year=1870|publisher=Macmillan|page=[https://archive.org/details/atreatiseonopti03parkgoog/page/n123 112]}}</ref> If this were true, then chromatic aberration could be eliminated by building a telescope which did not use a lens – a reflecting telescope.

[[File:Newton telescope replica 1668.jpg|thumb|A replica of a second reflecting telescope Newton presented to the [[Royal Society]] in 1672 (the first one he made in 1668 was loaned to an instrument maker but there is no further record of what happened to it).<ref>{{cite book|url=https://books.google.com/books?id=KAWwzHlDVksC&q=history+of+the+telescope |title=The History of the Telescope |author=Henry C. King |page=74 |access-date=2013-08-01|isbn=978-0-486-43265-6 |year=1955 }}</ref>]]

In late 1668 Isaac Newton built his [[Newton's reflector|first reflecting telescope]]. He chose an [[alloy]] ([[speculum metal]]) of [[tin]] and [[copper]] as the most suitable material for his [[Objective (optics)|objective]]  mirror. He later devised means for shaping and grinding the mirror and may have been the first to use a [[amateur telescope making#Mirror making|pitch lap]]<ref>{{cite book|author=Raymond N. Wilson|title=Reflecting Telescope Optics I: Basic Design Theory and its Historical Development|url=https://books.google.com/books?id=isH9fTnpc7YC&pg=PA9|year=2007|publisher=Springer Science & Business Media|isbn=978-3-540-40106-3|page=9}}</ref> 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, [[Telescope mount|mount]], and fittings. Newton's first version had a primary mirror diameter of {{convert|1.3|in|mm}} and a [[F-number#Focal ratio in telescopes|focal ratio]] of f/5.<ref>[http://www.telescope-optics.net/reflecting.htm telescope-optics.net Reflecting Telescopes: Newtonian, two- and three-mirror systems]</ref> He found that the telescope worked without colour distortion and that he could see the four [[Galilean moons]] of [[Jupiter]] and the [[Phases of Venus|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 of England|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 [[tarnish]]ed 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.<ref>[http://amazing-space.stsci.edu/resources/explorations//groundup/lesson/scopes/hadley/index.php amazing-space.stsci.edu – Hadley’s Reflector]</ref> Hadley had solved many of the problems of making a [[paraboloid|parabolic]] mirror. His Newtonian with a mirror diameter of {{convert|6|in|mm}} compared favourably with the large [[Aerial telescope|aerial refracting telescopes]] of the day.<ref>[http://labbey.com/Telescopes/Hadley.html The complete Amateur Astronomer – John Hadley's Reflector]</ref>