Telescope is a simple device which makes far away things appear closer. It gives the observers a new perspective about the universe by enabling them to look deep into the skies and unveil the secrets of the cosmos. Through this device, Galileo argued that Ptolemy’s geocentric model for the universe was wrong and that Copernicus was right about heliocentrism. Over the centuries telescope has went under enormous transformations and has helped in the development of advanced sciences.
Before the telescope, a device known as ‘spyglass’ was used to magnify distant objects on Earth. The telescope became possible with the sophistication of glass making and lens grinding technique. The earliest known version of the telescope appeared in the year 1608 when a Dutch eyeglass maker, named Hans Lippershey, applied for a patent on a device called Kijker (looker), which was capable of magnifying an image up to three times. It consisted of two parallel pieces of the lens (a concave eyepiece and a convex objective lens) aligned along the same horizontal line .
Although Galileo was not the first to build the telescope, he improved it by using a systematic technique of changing one factor at a time and recording the data. The device that he built is known as a crude refracting telescope .
The earliest known sketch of a telescope.
Optical path of Galileo’s telescope.
The initial version of Galileo’s telescope could only magnify to 8x power. Soon he was able to refine it to 20x magnification. He experimented with the device and built hundreds of telescopes. He figured out that greater the distance between the lenses, the closer the distant object will appear. But, reduce the distance, and objects will appear clearer. Also, as Galileo’s telescopes magnified an object, they narrowed their concentration on smaller and smaller sections and suffered from chromatic and spherical aberration. To solve this problem, he put a large lens in the telescope and covered it with thick paper so that light could only pass through the central part of the curved lens. Soon he was able to refine the magnification about 30 times .
Galileo’s telescope with 1330 mm focal length and a 26 mm aperture. It’s magnification power is 14x.
Galileo used his telescope to study the skies systematically by observing a small part of the sky for a longer period. He recorded his observations in his book Sidereus Nuncius. He mapped the surface of the moon, and the diffused arch of light across the sky – the Milky Way. He discovered sunspots, four of Jupiter’s moon, and Saturn’s rings .
One of Galileo’s drawings of the moon showing craters, mountains, and terminator for lunar day and night.
After Galileo, Johannes Kepler did a detailed study of telescopic optics and designed an apparatus which consisted of two convex lenses – Keplerian telescope. Kepler’s telescope allowed a much wider vision field than Gallio’s telescopes and had better magnification power but produced an inverted image and needed a high focal length .
Keplerian astronomical refracting telescope built by Johannes Hevelius. Its focal length is 46 m (150 ft).
By the end of the 18th century and with the refinement of glass making techniques the quality of the device was improved. With the invention of the achromatic refracting lens, scientists were able to minimize chromatic and spherical aberration, and overcome the need for long focal lengths .
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