How To Use a Telescope - Galileo's First Telescope and its History: by John B. Mayall

Galileo Galilee is known as the "father of telescopes" and rightly so. He is the inventor of the telescope and every telescope made after his invention follows the same principle that he used. Galileo's telescope was a primitive prototype of the telescopes that are used widely today. However, the principles he used are the very same ones still being used to this day. Galileo's telescope used two lenses - one concave and one convex - inside a tube-light shaped device. Convex lenses are those lenses whose edges curve inwards and concave lenses are lenses that have outward curves at the edge. The eyepiece in the telescope was constructed with the concave lens. Spy glasses, invented around the same time and used by militants to observe enemy activity in camps, were a major inspiration to Galileo in making his own telescope.
When two lenses are combined together, they are able to collect more light than individual lenses. This is the main principle behind Galileo’s telescope. Most of the telescopes in use today, use the same principle. The human eye also works on a similar principle, but cannot collect too much light. Telescopes are able to gather more light because of the double lenses used in its construction. These lenses gather light and build an image by focusing the light at a point. Refraction is the mechanism in use to form such images. As a result, telescopes are also called refracting telescopes or refractors. The phenomenon by which the collected light bends and forms images is known as refraction.
Images were magnified by a factor of 30 in Galileo's invention. However, the shape of the lenses he used was such that his image became blurred and distorted. But no one had ever invented something so exciting with which to observe the night skies before Galileo's telescope. Galileo used his telescope to view the moon and observe it closely. He was also the one to figure out that the magnification factor of a telescope was provided by the ratio of the power of the concave lens to the power of the convex lens. So he premised that the simplest way to increase this magnification factor was to use a high power concave lens with a weaker convex lens.
In Galileo's time, there were only low strength lenses available. Due to this restriction, Galileo decided to make his own lenses. He was soon able to achieve a magnification of 9x with lenses hat he had ground himself. His telescope was fitted with his own lenses. It was just another feather in his already well-decorated cap.
As time passed, Galileo improvised on his primitive telescope, making several modifications to it. He also demonstrated his invention at the Senate of Venice, and several senators climbed the highest towers of the time to observe the horizon with Galileo's invention. They viewed the distant ships from their perches and decided that the telescope was a very useful military device.
The telescope changed the face of Astronomy and became an indispensable part of the study. Several inventors used the same principle and made telescopes of their own. Gradually over the years, the study of astronomy benefited immensely from the telescope and its uses. The same principle was employed in the construction of much more powerful telescopes that made it possible to understand our plane and its surroundings more comprehensively, all thanks to Galileo's wonderful invention.