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Period: 3500 BCE to
Creation of a Telescope
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1400 BCE
Concave lenses to correct nearsightedness are developed.
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1300
Convex lenses to correct farsightedness are developed.
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In the Netherlands, Hans Lippershey discovers that holding two lenses up some distance apart bring objects closer.
He applies for a patent on his invention. This is the first documented creation of a telescope. The idea is independently developed by Jacub Metius and Sacharias Janssen. The patent to Lippershey is denied -
Thomas Harriot (1560 – 1621) English astronomer, mathematician, ethnographer, and translator becomes the first person to make a drawing of the Moon through a telescope, on July 26, 1609, over four months before Galileo.
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Galileo, after simply hearing that the device was invented, builds several telescopes of his own and turns them toward the heavens.
He dared to publish his findings and was nearly burned at the stake for it. There are other earlier recorded astronomical uses including viewing stars with Lippershey's own first telescope during its demonstration and Thomas Harriot's views of the moon not long after. -
The term "telescope" is coined by Prince Frederick Sesi at a reception where Galileo is demonstrating his instruments.
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Johannes Kepler switches from a concave eyepiece to a convex eyepiece. This not only allowed a larger field of view, but it allowed for the projection of images (such as the sun) onto a flat white screen.
Although the images are inverted, Kepler demonstrates how a third convex lens turns the images right-side-up again. The use of a third lens also degrades the images, so this form of the telescope is not widely used. For terrestrial applications, particularly military applications, the Galilean form of the telescope is the most widely used. -
Johannes Kepler studies the human eye and notices that the eye's lens is hyperboloidal. He suggests the use of hyperboloidal lenses in the telescope.
Spherical lenses are not very sharp because they smear the rays of light over a very small area, a phenonomenon now known as spherical abberation. -
Marin Mersenne hit upon the idea of using two paraboloidal mirrors instead of lenses, but he never builds this telescope, having been persuaded by Descartes that it could never work.
The main advantage of using mirrors over refracting lenses is that mirrors focus all points of the spectrum at the same point--no chromatic aberration! -
Rene Descartes demonstrated that spherical lenses cannot produce pinpoints of light. He studies elliptical and hyperboloidal lenses and shows that different combinations of thesel lenses will produce a pinpoint of light and sharper image.
He has a Parisian optician produce hyperboloidal lenses for a demonstration, but the lenses are a failure. Although the lenses corrected for spherical abberation, they introduced another probelm--chromatic abberation, which made the problem worse. Chromatic abberation means that different colors are focused at widely differing points, producing smeared images with halos around them. -
James Gregory designed a telescope using a concave primary mirror (slightly hyperboloid) concave ellipsoidal secondary mirror.
The first mirror gathers the light and reflects it onto the secondary. The secondary mirror focuses the light back through a hole in the primary mirror. This is the basis for many telescopes made today, but the opticians of his time were not able to produce mirrors of high enough quality to give good results. -
Robert Hooke demonstrates how to shorten the tube by using three or four perfectly flat mirrors to reflect the image back and forth in a shorter tube.
A 60-foot long telescope can be reduced to 12 feet long, greatly simplifying support and stability. -
Newton produces the first successful reflecting telescope, using a two-inch diameter concave spherical mirror, a flat, angled secondary mirror, and a convex eyepiece lens.
The reflector telescope that Newton designed opened the door to magnifying objects millions of times, far beyond what could ever be obtained with a lens. There were problems with his mirror. It was made of copper and tin, polished to a high degree of reflectivity. It would tarnish quickly and need re-polishing at least twice a year. Newton was the most important thinker of his day, and he believed that only mirrors would eliminate chromatic aberration and that it could never be done with lenses. -
Cassegrain proposed a similar design using a convex secondary mirror that allowed the tube to be shortened even more.
More importantly, it cancelled abberations from the primary mirror and would have resulted in much sharper images, had opticians been able to produce quality mirrors. It is interesting that Gregory, Cassegrain, and later Newton were able to invent designs that were so far ahead of their time that no one could actually make one. -
Johannus Hevelius realized that the longer the telescope was, the closer together the different colored points of light would be at the focal point, yielding a sharper image.
He constructs a telescope 140 feet long which probably gave very sharp images, but it was almost impossible to keep the two lenses aligned because the supporting structure (usually a long tube) could not be made rigid enough. -
Christian Huygens suggests getting rid of the supporting structure and mounting the objective lens on the top of a long pole. These were called "aerial telescopes" because they were open to the air.
They were also much easier to build and use. At the same time, Huygens developed a compound negative eyepiece using two air-spaced convex lenses. This arrangement cancelled out some of the chromatic aberration that occurred in a single lens eyepiece. -
Chester Moor Hall develops an achromatic lens. Two pieces of glass with different indices of refraction can be combined to produce a lens that tends to focus most colors at a very close (though not exact) point.
Red and Green neatly blended at a point, but blue-violet still missed that point by a small amount. The result was a much sharper image with violet halos around brighter objects. Refractors are suddenly popular again. The images still show simple optical distortion around the edges, which mirrors developed around the same time did not. -
The Scottish Instrument maker James Short invents the first parabolic and elliptic, distortionless mirror ideal for reflecting telescopes.
James Short built over 1,360 telescopes. All had speculum mirrors. Short was closely involved with the Transit of Venus observations made throughout the world on 6th June 1761. His instruments travelled on the ship Endeavour with Captain Cook to observe the Transit of Venus on 3rd June 1769. (See The Telescopes of Captain James Cook on this website. -
John Dolland improves upon the achromatic objective lens by placing a concave flint glass lens between two convex crown glass lenses.
This triplet uses the natural differences between the refractive indices of the two types of glass to cancel out chromatic aberration even more. Some historians claim that the triplet was introduced in 1765 by Peter, son of John Dollond. Many excellent telescopes of this kind were made by him. -
Sir William Herschel constructs a forty foot long telescope with a four-foot diameter mirror. \
Reflector telescopes have become popular again because they can be built with enormous mirrors, capable of gathering hundreds or even thousands of times more light than a refractor. Today we call them "light buckets." -
H. Dennis Taylor, optical manager of T. Cooke & Sons of York, makers of astronomical telescopes, designed and patented the revolutionary, and now famous, triplet design (British patent no. 1991).
This lens eliminated the optical distortion at the outer edge of lenses. The Cooke Triplet was a significant improvement of the Dolland triplet of more than a century earlier. The cooke triplet is made of three different types of glass. Schott's baryta light flint glass, Schott's boro-silicate flint glass, and Schott's light silicate crown glass. The lenses are air-spaced.
