# Silesia

A telescope is a optical Instrument which makes it possible to increase the apparent size and the Luminosité of the objects of the sky at the time of their observation.

Equipped with a rectifying of image (vehicle or prism) it behaves then in Lunette of approach: it is named terrestrial telescope or marine telescope.

## History

Its invention is anonymous and would come from Italy (towards 1590) or from the north of the Europe (Netherlands, about 1608). Descartes speaks about this invention at the beginning of its Dioptrique :

But, with the shame of our sciences, this invention, if useful and so admirable, were firstly found only by the experiment and fortune. There is approximately thirty years, that one named Jacques Metius, of the town of Alcmar in Holland, man which had never studied, although it had a father and a brother who made profession of mathematics, but which particularly took pleasure to make extreme mirrors and glasses, by composing even the winter with ice, as well as the experiment showed that one can about it make, having on this occasion several glasses of various forms, was happily warned to look through two, of which one was a little thicker in the medium than at the ends, and the other on the contrary much thicker at the ends than in the medium, and he applied so fortunately to the two ends of a pipe, that the first of the glasses of which we speak, of it was made up.

The invention of the telescope is difficult to allot, because several people sought to obtain the patent from it: Hans Lippershey, which made a demonstration of a telescope of enlargement three at the end of September 1608, Jacques Metius and Sacharias Jansen.

As soon as the telescope was known and started to be spread, several people, whose Thomas Harriot, turned it towards the sky at the beginning of 1609 to observe the celestial objects. But it is Galileo which, as from August 1609 establishes truly the telescope like instrument of astronomical observation by the whole of its celestial observations and especially by the new glance that it carried on the sky and the objects that it observed: it was astonished by the phenomena that it saw and it studied them. It built its own glasses and initially an enlargement of six instead of three gave them, to gradually change it to 20 then to 30.

## Composition

Glasses are made up of an objective and a Oculaire laid out on both sides of a closed tube. The tube can be fixed or telescopic like in the case of the telescopes of marine. The eyepiece is located, as its name indicates it, on the side of the eye, and it is low-size. The objective is other side, and is generally of greater dimension than the eyepiece.

These first glasses of approach , terrestrial or astronomical, had a convex objective and a concave eyepiece (see description of Rene Descartes higher) due to the principle of chance of their invention by opticians. Most recent (see lower description) have objective and ocular convex. In spite of the History, the two systems preserve each one their advantages:

• ocular concave: right image allowing the use out of terrestrial telescope and shortening the length of the tube compared to the focal distance of the objective. The assembly of two small of these glasses creates the apparatus known as twin of Galileo (use with the theater considering the weak performances).
• ocular convex: reversal of the image (high and low) and lengthening compared to the length of the focal distance of the objective. The use out of telescope is obstructed by these consequences (neither high nor low in the sky, mechanical mounting to support the system). On the other hand, the marine or terrestrial use imposed a telescopic tube and an optical system of recovery of the image, said vehicle composed of a Doublet or an even number of prisms (which fold, shorten the obstruction) in the case of prism the glasses or the binoculars known as of navy.

One can make a single-tube telescope with two Loupe S. large, with rather remote hearth being used as objective, and small, with brought closer hearth being used as eyepiece. Indeed, the objective and the eyepiece are two convergent optical systems, i.e. they concentrate (focuses) the luminous rays, with the manner of a magnifying glass. These two convergent systems have like principal characteristics the diameter and the focal Distance. The focal Distance is the distance between the center of the convergent optical system (for example the center of the lens of a magnifying glass) and the hearth (the point where luminous rays coming from infinite converge).

The modern glasses have all of the objectives and the eyepieces made up of several lenses. Indeed, a simple lens has an acceptable quality only under certain conditions. One can correct or decrease certain defects by pairing several lenses having glasses of different index, one thus created doublets (Achromatique S) or triplets (Apochromatique S) which are free from defects on larger beaches.

The enlargement of the glasses is given by: $G= \ frac \left\{\ alpha\text{'}\right\} \left\{\ alpha\right\}$ where $\ alpha\text{'}$ is the angle under which one sees the final image through the glasses and $\ alpha$ is the angle under which one sees the object with the naked eye. Since we observe celestial objects, one can consider that the angles are small and thus, as one has (for small angles):

$\ tan \ alpha \ simeq \ sin \ alpha \ simeq \ alpha \left(rad\right)$

Then, the enlargement of the glasses is calculated by dividing the focal distance from the objective by that of the eyepiece.

## Telescope afocale

A telescope is known as afocale when the hearth image of the objective is with the same position as the hearth object of the eyepiece. The object observed being ad infinitum its image is in the focal plan image of the objective. However the focal plan image of the objective is also the focal plan object of the eyepiece, the image provided by this one is ad infinitum. The human eye being made to observe an object located ad infinitum, it does not adapt when it observes an image through a telescope afocale.

## See too

• List of the largest glasses

 Random links: Yao (ethnos group) | Ginnastica Triestina | Saint-Peter (Barbados) | Theater of Insane Caramel | Brown (river of Aisne)