Why was Galileo's telescope important?

What was the importance of Galileo's telescope? That's a good question, considering that Galileo wasn't the person who invented the telescope. But it was Galileo who made the instrument famous. He made the first three-powered spyglass in June or July 1609, presented an eight-powered instrument to the Venetian Senate in August, and turned a twenty-powered instrument to the heavens in October or November.You see, before Galileo's telescope came along, nobody had ever thought to use the device to look at the stars. With this simple telescope he observed the Moon, discovered four satellites of Jupiter, and resolved nebular patches into stars. He published Sidereus Nuncius in March 1610.

Verifying Galileo's discoveries was initially difficult if you did not have Galileo's telescope. In the spring of 1610 no one had telescopes of such quality and power to see the satellites of Jupiter, although many had weaker instruments with which they could see some of the lunar detail Galileo had described in Sidereus Nuncius. Galileo's lead was one of practice, not theory, and it took about six months before others could make or obtain instruments good enough to see Jupiter's moons. So I guess you could call Galileo the father of Astronomy. He was a forward thinker, and it was because of Galileo's telescope that advancements were made. Even so, with the verification of the phases of Venus by others, by the first half of 1611, Galileo's telescope had been copied by many others. The next discovery, sunspots, was made by several observers, including Galileo, independently.

How was it made?

Galileo’s telescope with which Jupiter's moons could be observed was made with a plano-convex objective (the lens toward the object) with a focal length of about 30 to 40 inches, and a plano-concave eyepiece with a focal length of about 2 inches. The eyepiece was in a small tube that could be adjusted for focusing. The objective lens was stopped down to an aperture of 0.5 to 1 inch. , and the field of view was about 15 arc-minutes (about 15 inches in 100 yards). The instrument's magnification was 15-20. The glass was full of little bubbles and had a greenish tinge (caused by the iron content of the glass); the shape of the lenses was reasonable good near their centers but poor near the periphery (hence the restricted aperture); the polish was rather poor. The limiting factor of this type of instrument was its small field of view--about 15 arc-minutes--which meant that only a quarter of the full Moon could be accommodated in the field. Over the next several decades, lens-grinding and polishing techniques improved gradually, as a specialized craft of telescope makers slowly developed. But although Galileo’s telescope, and variations of it, were made with higher magnifications, they were practically useless because of the small field of vision.

Better designs
As mentioned above, the telescopic effect can be achieved with different combinations of lenses and mirrors. As early as 1611, in his Dioptrice, Johannes Kepler had shown that a telescope could also be made by combining a convex objective and a convex ocular. He pointed out that such a combination would produce an inverted image but showed that the addition of yet a third convex lens would make the image erect again. This suggestion was not immediately taken up by astronomers, however, and it was not until Christoph Scheiner published his Rosa Ursina in 1630 that this form of telescope began to spread. In his study of sunspots, Scheiner had experimented with telescopes with convex oculars in order to make the image of the Sun projected through the telescope erect. 5 But when he happened to view an object directly through such an instrument, he found that, although the image was inverted, it was much brighter and the field of view much larger than in Galileo’s telescope. Since for astronomical observations an inverted image is no problem, the advantages of what became known as the astronomical telescope led to its general acceptance in the astronomical community by the middle of the century. Galileo's telescope could be used to view objects on earth as well as the sky. This was not true for the astronomical telescope with its inverted image. Astronomers eschewed the third convex lens (the erector lens) necessary for re-inverting the image because the more lenses the more optical defects multiplied. In the second half of the seventeenth century, therefore, the Galilean telescope was replaced for land viewing purposes by the "terrestrial telescope," which had four convex lenses: objective, ocular, erector lens, and a field lens (which enlarged the field of view even further).

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