The first reflecting telescope built by Sir Isaac Newton in 1668[2] is a landmark in the history of telescopes, being the first known successful reflecting telescope.[3][4] It was the prototype for a design that later came to be called a newtonian telescope.
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Isaac Newton built his reflecting telescope as a proof for his theory that white light is composed of a spectrum of colors.[5] He had concluded that the lens of any refracting telescope would suffer from the dispersion of light into colors (chromatic aberration). The telescope he constructed used mirrors as the objective which bypass that problem. To create the primary mirror Newton used a custom composition of metal consisting of six parts copper to two parts tin, an early composition of speculum metal.[3] He devised means for shaping and grinding the mirror and may have been the first to use a pitch lap[6] to polish the optical surface. He chose a spherical shape for his mirror instead of a parabola to simplify construction: he had satisfied himself that the chromatic, and not the spherical aberration, formed the chief faults of refracting telescopes. He added to his reflector what is the hallmark of the design of a "Newtonian telescope", a secondary "diagonal" mirror near the primary mirror's focus to reflect the image at 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 all the tube, mount, and fittings.
Newton described his invention as:
The diameter of the sphere to which the Metal was ground concave was about 24 English Inches, and by consequence the length of the Instrument about six Inches and a quarter. The Eye-glass was Plano-convex, and the diameter of the Sphere to which the convex side was ground was about 1/5 of an Inch, or a little less, and by consequence it magnified between 30 and 40 times. By another way of measuring I found it magnified 35 times. The concave Metal bore an Aperture of an Inch and a third part, but the Aperture was limited not by an Opake Circle, covering the limb of the Metal round about, but be an opake Circle, placed between the Eyeglass and the Eye, and perforated in the middle with a little round hole for the Rays to pass through to the Eye. For this Circle being placed here, stopp'd much of the erroneous Light, which other wise would have disturbed the Vision. By comparing it with a pretty good Perspective of four Feet in length, made with a concave Eye-glass, I could read at a greater distance with my own Instrument than with the Glass. Yet Objects appeared much darker in it than in the Glass, and that partly because more Light was lost by Reflexion in the Metal, than by Refraction in the Glass, and partly because my Instrument was overcharged. Had it magnified but 30 or 25 times, it would have made the Object appear more brisk and pleasant.
Newton describes a telescope with an objective concave primary mirror diameter of about 1.3 inches (33 mm) ground to fit a sphere that was 24 inches in diameter giving it a radius of 12 inches and a focal length of 6 inches (152 mm). It had a flat diagonal secondary mirror bouncing the light at a 90° to a Plano-convex eyepiece.[7] Newton completed this reflecting telescope in late 1668 and first wrote about it in a February 23, 1669 letter to Henry Oldenburg (Secretary of the Royal Society).".[8]
Newton found that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus with his new little telescope.[7] Newton's friend Isaac Barrow showed the telescope to small group from the Royal Society of London at the end of 1671. They were so impressed with it they demonstrated it for Charles II in January 1672. The last record Newton made of having it was in 1704, but the final fate of Newton's first reflecting telescope is unknown.[1] It was recorded as having been seen for a while at an instrument makers shop (Mr. Heath's) in Strand, London before 1723 but nothing further is known about it.[1]
The practical potential of Newton's first telescope was made more clear by the end of the 18th century, when the largest reflector had grown to nearly 50 inches aperture (126 cm)[9] while the largest achromatic lens objective was not more than about 5 inches (13 cm).[10]
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