Development of the Telescope
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The first telescopes were built in Holland at the start of the seventeenth century. Galileo improved the telescope designed by Lippershey and used it to study the Moon the planets and hitherto unseen stars. Galileo's instrument was feeble by modern standards but it allowed him to view for the first time the four maj or moons of the planet Jupiter. It was also his ruin for his unwise observations of the Sun (revealing sun-spots and the Sun's 27-day period of rotation) scarred the retinas of his eyes and finally blinded him.
All refractors have at least two lenses: the objective, which gathers light and brings it to a focus, and the ocular, which straightens the refracted light rays for seeing or photography. Galileo's first telescope used a divergent lens as ocular. The astronomer Kepler used a convergent lens, which is superior but creates an inverted image; for example the Moon appears in the eyepiece upside down.
To compensate for inversion, telescopes which were designed for use on land or sea had yet another lens added between the objective and the ocular so that the image appeared right way up. Since the instrument was rather long and difficult to store, the tube containing the lenses was made of several pieces which could slide or 'telescope' inside one another when it was not in use.
It was soon found that simple lens arrangements of this kind distorted the image. White light is made up of various wavelengths, each associated with a different colour. When a white beam is bent by a convergent lens, the violet component is bent most and the red least. The colours focus at different points, producing a blurry image. This chromatic aberration can be corrected by adding a plano-concave lens in front of the main objective, which cancels out the original error. The combined assembly (often of more than two lenses) is known as an achromatic lens.
An ordinary amateur astronomer's telescope today can magnify the stars about 300 times. The maximum size for a refracting telescope is about 100 cm; there is one of this size at Yerkes in the United States. Because of the problems of getting glass of sufficient purity in a large lens, any instrument larger than this must use a mirror as its objective.