The telescope is one of the scientific instruments that has most revolutionized science and the concept we have of the world. It was an open window to the location of the human being in the Universe and opened the view beyond the Earth and the stars.

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The first known optical instrument was made in ancient Greece and is quoted by Aristophanes in his work "The Clouds" in 424 B.C. It was not a telescope in itself but rather a lens to concentrate the light used to make fire. This first lens consisted of a small water-filled blown glass balloon that concentrated the light. The phenomenon of light refraction was beginning to be used, although without physical foundations. It was not until the year 1200 that the English Franciscan friar Roger Bacon carved the first lenses, in his book "Opus Maius" he describes how the carving of a lens can help the amplification of the written letter, it was the first step for the invention of glasses.

The history of the telescope begins in the sixteenth century when an apparatus is built consisting of a tube and two lenses (one convergent and the other divergent) to amplify images. There are several candidates for inventors of this instrument and it is not clear at present who the real discoverer was. The first of these is Giambattista Della Porta (Italy), who in 1589 in his book "De magiae naturalis" describes an instrument similar to a telescope but which he never built.

In 1590 Zacharias Janssen (Holland) also speaks of an instrument to amplify images that according to him is his invention, but as we will see it was copied from the one designed in 1590 by a Spanish optician, Joan Roget from Girona.   There are several documents on the Roget telescope, including two, the first, dated April 1593, in which Peter of Carolona bequeathed his wife a "ullera de larga adornida de llautó" (long brass magnifying glasses), the second, dated September 5, 1608, belongs to an auction of Jaime Galvany's goods, including an "ullera de llauna per mirar de lluny" (telescope for looking from afar).

Everything suggests that we are looking at the first design of a telescope, but like a thriller, it enters the story of Zacharias Janssen.

An unknown character buys Galvany's telescope in Barcelona and heads to Frankfurt where the books and scientific novelties of the time were presented annually. There he meets Janssen and offers to go half way in exchange for him selling the item to his customers. But Janssen keeps the instrument because he sees that the business may be bigger convinced that it will be able to manufacture something similar, but it is not optical and needs many tools and lenses. He contacts Dutch opticians Lipperhey and Metius to ask them for help without realizing that he is also spreading the usefulness of the instrument. When he manages to build his own telescope and goes to the patent office, he realizes that Lipperhey was ahead of him. Leaving the way to the Catalan optician Joan Roget without the honor of being the official inventor of the Telescope.

This fascinating instrument began as a diversion for the nobles of the time and became so popular that by 1609 it was available in the shops of lens manufacturers in Paris.

That same year the Italian astronomer and mathematician Galileo Galilei (1564-1642) learned of the invention of this instrument and immediately designed his own, this consisted of two simple lenses, a flat convex and a biconcave one placed at the ends of a lead tube achieving a magnification of three times the image. He varied his designs until he got 30 magnifications. Unlike previous Galileo telescope designers, he began to understand the physical mechanism of the instrument and began to do science.

Galileo realized the importance of the quality of the lenses and how they were polished, striving to obtain good lenses for his telescopes. He also noted that reducing the size of the aperture (the lens or end of the telescope through which light enters) increased the definition of the image. But such a small field of view (almost not covering the moon) makes this instrument very difficult to handle because the field of view is very narrow.

 But with it he obtained such spectacular results that they would change the concept of the Solar System that we had at the time. In his 1610 work "Sidereus nuncios" (Messenger or sidereal message) he describes all his astronomical observations in great detail, being this the first treatise on astronomy and also laid the foundations for the end of the geocentric theory that he believed on Earth as the centre of the Universe. Galileo observed the Moon, stars and various planets.