Modern Telescope

The telescope was one of the central instruments of what has been called the Scientific Revolution of the seventeenth century. It revealed hitherto unsuspected phenomena in the heavens and had a profound influence on the controversy between followers of the traditional geocentric astronomy and cosmology and those who favored the heliocentric system of Copernicus. It was the first extension of one of man's senses, and demonstrated that ordinary observers could see things that the great Aristotle had not dreamed of. It therefore helped shift authority in the observation of nature from men to instruments.

In short, it was the prototype of modern scientific instruments. But the telescope was not the invention of scientists; rather, it was the product of craftsmen. For that reason, much of its origin is inaccessible to us since craftsmen were by and large illiterate and therefore historically often invisible.

Galileo, then Professor of Mathematics at the University of Padua (Italy), heard of the new instrument which had been invented in Holland by Hans Lippershey. “Sparing neither labour nor expense” as he wrote, he made a telescope for himself, and applied it to the sky. During the months following the late autumn of 1609 he made a whole series of spectacular discoveries: the mountains and craters of the Moon, the four bright satellites of Jupiter, the phases of Venus, spots on the Sun, and the starry nature of the Milky Way.

The phases of Venus were of particular significance to Galileo, since on the Ptolemaic theory the planet would always appear as a crescent; the fact that it could show up as a half or gibbous (three quarter) disk confirmed Galileo,s view that the Ptolemaic theory must be wrong. Also , the four attendants of Jupiter moved round their parent planet, not round the earth - which proves that at the very least there must be more than one single centre of motion in the Solar System.

Unfortunately, it soon became evident that the refracting telescope had a great disadvantage. The object-glass (that is to say, the main lens responsible for collecting the light) did not bend all wavelengths equally; and this resulted in the red part of the light-beam being brought to a focus at a greater distance from the object-glass. Consequently a brilliant body such as a star seemed to be surrounded by gaudy and confusing rings.

This trouble was only too apparent with Galileo’s telescopes, the most powerful of which magnified a mere 30 times. One remedy was to make telescopes of extremely long focal length, but this made them cumbersome and difficult to use.

Newton’s reflecting telescope (1671)

A second theoretical development came in 1672, when Isaac Newton published his celebrated paper on light and colors. Newton showed that white light is a mixture of colored light of different refrangibility: every color had its own degree of refraction. The result was that any curved lens would decompose white light into the colors of the spectrum, each of which comes to a focus at a different point on the optical axis.

This effect, which became known as chromatic aberration, resulted in a central image of, e.g., a planet, being surrounded by circles of different colors. Newton had developed his theory of light several years before publishing his paper, when he had turned his mind to the improvement of the telescope, and he had despaired of ever ridding the objective of this defect. He therefore decided to try a mirror, but unlike his predecessors he was able to put his idea into practice. He cast a two-inch mirror blank of speculum metal (basically copper with some tin) and ground it into spherical curvature.

He placed it in the bottom of a tube and caught the reflected rays on a 45? secondary mirror which reflected the image into a convex ocular lens outside the tube (see fig. 12). He sent this little instrument to the Royal Society, where it caused a sensation; it was the first working reflecting telescope. But the effort ended there. Others were unable to grind mirrors of regular curvature, and to add to the problem, the mirror tarnished and had to be repolished every few months, with the attending danger of damage to the curvature.

The reflecting telescope therefore remained a curiosity for decades. In second and third decades of the eighteenth century, however, the reflecting telescope became a reality in the hands of first James Hadley and then others.

By the middle of the century, reflecting telescopes with primary mirrors up to six inches in diameter had been made. It was found that for large aperture ratios (the ratio of focal length of the primary to its aperture, as the f-ratio in modern cameras for instance), f/10 or more, the difference between spherical and paraboloidal mirrors was negligible in the performance of the telescope. In the second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope with parabolically ground mirrors came into its own.


Throughout the 18th century telescopes of larger and larger size were built. From 1774, excellent instruments were made by William Herschel, a Hanoverian who settled in England; with one of his home-made telescopes he discovered the planet Uranus (1781), and in 1789 he completed a huge reflector with a 122-cm. mirror. This remained the largest telescope in the world until the installation of Lord Rosse’s 183-cm. in 1845.

Today the largest telescope is the twin Keck Telescopes, the world’s largest optical and and infrared telescopes. Each stands eight stories tall and weighs 300 tons