An Acre of Glass - The History of the Telescope

The history of the telescope makes for interesting reading. During the development of the telescope, practical experiments with reflectors had already begun in 1639, but it was not until 1663 that they gained any prominence.

The Gregorian Telescope

In that year a Scottish mathematician, James Gregory, at the age of 24, published a treatise entitled Optica Promota. In this he gave a description of a compound reflecting telescope employing two concave specula (metal mirrors). The larger one was to be perforated, and to have a paraboloidal surface; the smaller was to be ellipsoidal. High magnification could be had with this instrument, the second reflection amplifying the focal length of the primary in the ratio of fs to Fs.

Construction of the telescope was undertaken, but whatever chance it may have had of performing creditably was lost by polishing the speculum on a cloth lap - putty (tin oxide) being used as the polishing agent. The unyielding lap was an insurmountable barrier to parabolizing, interest apparently ebbed, and about 60 years were to elapse before a workable model was finally produced.

The Cassegrainian Telescope

Sieur Cassegrain, a Frenchman, in 1672 designed a second compound reflector, differing from Gregory's in that it employed a convex secondary, to be of hyperboloidal figure, placed inside of the focus of the paraboloidal primary .

While the Gregorian is seen to be capable of higher magnification, all that is necessary can be had from the Cassegrainian, and it has the advantage of being a much more compact instrument. Although little was heard of this telescope for the next two centuries, it is worth observing that it survived the Gregorian, and is still widely used in observatories.

The Newtonian Telescope

The history of the telescope takes an interesting turn at this point. In the same year, Newton designed and constructed two small reflectors, of the type so popular with amateur astronomers today and which still bears his name. They were not large, as we know telescopes today, the effective apertures of the concave specula being about 1 1/3". Their focal length was 6", making the focal ratio f/4.5.6

Newton, according to his Opticks (1704), polished his specula on pitch, using putty as the polishing agent. His methods were ingeniously calculated to yield a spherical surface, and it is quite probable that a close approach to that figure was attained. But the performance of even a spherical mirror of the proportions of Newton's could hardly be satisfactory because of the great amount of spherical aberration present.

Although Newton thought that his mirror might fail of good definition, he "despaired of doing the work" (parabolizing the speculum), yet he "doubted not but that the thing might in some measure be accomplished by mechanical devices."

It might be concluded that if the center of the mirror were properly deepened, that is, given a shorter radius, or if the radii of the outer zones were progressively lengthened, or if a little of each were done, all the reflected rays could be brought to a common focus. That is a practical solution, and the resulting surface in each instance is a paraboloid.

The standard practice is to deepen the spherical mirror so that, for a 6-inch f/8 mirror, the glass removed in the operation is but half a wave length of light in thickness at the center. Incredible though it seems, this represents the difference between poor and good definition.

The single-lens eyepiece of Kepler's had already been improved, with the addition of another element, by Christian Huygens, a Dutch astronomer and mathematician, about the year 1650. The field lens, like Galileo's concave lens, is placed before the focal plane of the objective. As it is convex, however, it further converges the rays to form' a slightly smaller image in a new focal plane, which is then magnified by the eye lens. Thus, a much wider field of view is encompassed by the eyepiece.

These were important developments in the history of the telescope on the way to our present-day powerful telescopes

You'll Soon Be Gazing At The Stars Through Your Very Own Telescopes

Visit: http://www.maketelescopes.net

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The History of Early Telescopes

An investigation into early telescopes makes for interesting reading. In 1722, John Hadley, an English mathematician, completed a form of reflector for the telescope in the style of Newton in which the mirror evidently was suitably figured. This instrument attracted considerable attention, and presently other makers were turning out Newtonian reflectors, following Hadley's technique, which consisted of removing the spherical aberration as it was revealed by the extra-focal diffraction rings of a star image.

Hadley then turned his attention to a design by James Gregory and in 1726 he produced an instrument slightly over 2" in diameter and 12" in focal length. This proved so successful that construction was undertaken by others.

Notable among these was James Short, who made both Newtonians and Gregorians in great numbers, from about 1732 to the time of his death in 1768. Observatories purchased his larger instruments, a tribute to his skill, and the smaller ones were marketed chiefly among the aristocracy and amateur astronomers.

The principal attraction of the Gregorian design was the erect image it gave, which made it suitable for terrestrial use. This circumstance influenced its preference over the Newtonian, notwithstanding the fact that its images must have been pretty dull. Well into the 19th century, however, the Gregorian rode a wave of popularity that no type of telescope has known, until overwhelmed in comparatively recent years by the flood of amateurs who have flocked to Newton's design.

From the time of the invention of the telescope, and the startling discoveries of Jupiter's moons and the rings of Saturn, interest in astronomy had become something infectious. Each new discovery was accorded the widest publicity, stimulating a desire among those of learning to gain at first hand a glimpse of these celestial wonders. It was not practicable as yet for the average individual to make his own speculum, but many contrived to fit spectacle lenses into tubes, much as Galileo had done some 150 years earlier. This in fact was one of the first of the early telescopes.

Those whose means permitted bought telescopes, and envied was the gentleman who possessed one of three or four inches aperture, by an "exclusive" artist. But, judged by present-day standards, many of those reflectors were tiny. There is one (maker unknown) in the Fugger Collection at Augsburg, barely 1" in diameter and 6" in focal length, that was concealed in a walking stick! Eyepiece lenses of 1/6" or less in focal length were quite common.

The metal used in those early mirrors was an alloy of copper and tin, the usual proportion about 75 to 25, which could be given a beautiful polish. But the metal was extremely hard to work, and a prodigious amount of labor was involved in grinding and polishing the curve. To facilitate the work, the comparatively thin disks were cast to the approximate curve, the backs also being curved to give uniform thickness and equalization of temperature effects. Grinding was done on convex iron tools of similar radius, using emery, and sometimes sand. Polishing was done on a pitch lap, with rouge.

Manufacturers usually devised their own machines to do the work of grinding and polishing. Except where the utmost perfection was imperative, figuring seems to have consisted for the most part of a final brief variation of the stroke, in an unguided attempt to concentrate the polishing at the center. Critical testing, undoubtedly seldom indulged in on account of its laboriousness, could as yet only be performed on a star. In reflective ability, speculum was only about 60 per cent efficient, and the surface tarnished rapidly, effecting a further serious light loss. This meant frequent repolishing, and repolishing meant refiguring.

It is interesting to inquire into the prices that were asked for telescopes in that period, the latter half of the 18th century. Listed below are prices and sizes of a few of the Gregorians made by Short, selected from his catalogue. Newtonians in similar sizes were priced only slightly lower.

Diameter (inches)	Focal Length (inches)	Magnification	Price (guineas)*

1.1			3			18		3

1.9			7			40		6

4.5			24			90-300		35

6.3			36			100-400	        75

18			144			300-1,200	800

*An English gold coin, issued until 1813, equivalent to 21 shillings, or about five dollars.

The early telescopes were certainly gaining popularity by this time.

You'll Soon Be Gazing At The Stars Through Your Very Own Telescopes
Visit: http://www.maketelescopes.net

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History Trivia Questions & Answers - Famous Last Words

Whose last words were "I am still alive?"

Answer: Gaius Caligula.

Caligula has been dubbed, Rome's most tyrannical emperor. His reign was noted not only for cruel and treacherous acts but also of truly insane ones. His more insane acts included attempting to have his horse, Incitatus, inducted as a senator and after marching his troops to the coast of France, he ordered them to gather seashells on the beach. He also began publicly appearing dressed as a god and insisted that he be treated as one. His peers also accused him of both incest and murder. It is hardly surprising that his own guard eventually assassinated him.

"I'm bored with it all" were the last words of which famous person?

Answer:Winston Churchill

Winston Churchill was born on November 30th 1874 and was the Prime Minister of Britain during WWII. Winston served in the military in both India and Cuba before beginning his life in politics. Churchill is one of only two people to have received the distinction of being made an Honorary Citizen of the USA. Also, among his many distinctions was a Nobel Prize for literature. Churchill died nine days after having a major stroke on the 24th of January 1965.

Who said on their deathbed, "All my possessions for a moment of time"?

Answer:Elizabeth the I

Elizabeth was born on September 7th 1533. She ascended to the throne in 1559 after the death of her sister Mary. She was the only child of Henry the 8th and Anne Boleyn. Her mother Anne was executed by her father Henry when she was only two years of age. Elizabeth never married despite many proposals and petitions. She died in 1558 on the 17th of November.

"Go on, get out - last words are for fools who haven't said enough" are said to be the last words of which famous person?

Answer:Karl Marx.

It is reported that he said these words to his housekeeper who wanted to write down his dying words. Karl Marx was born in Germany in 1818. He was a noted philosopher, historian, communist and revolutionary. He died in March, 1883. He was buried in Highgate Cemetery in London. His tombstone reads "Workers of all Lands Unite".

"Go away. I'm all right" was the last thing said by which famous person?

Answer:H. G. Wells

Novelist H. G. Wells was born in Bromley, England on September 21st 1866. He is probably best known for his works in science fiction, which include such classics as the Time Machine, the Invisible Man and The War of the Worlds. Wells died at his home in Regent's Park in 1946. He had wanted his epitaph to read, "I told you so. You damned fools."

If you are someone who enjoys history trivia, you can find many different history trivia questions, games and other related resources at Triviachamp.com

If you would like to print out some free printable quiz sheets, you might also enjoy TriviaChamp's sister site at: -- Printable Trivia Questions

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