6.5 Galileo's telescope  (Page 4/4)

With the acceptance of the astronomical telescope, the limit on magnification caused by the small field of view of the Galileantelescope was temporarily lifted, and a "telescope race" developed. Because of optical defects, the curvature of lenseshad to be minimized, and therefore (since the magnification of a simple telescope is given roughly by the ratio of the focallengths of the objective and ocular) increased magnification had to be achieved by increasing the focal length of theobjective. Beginning in the 1640s, the length of telescopes began to increase. From the typical Galilean telescope of 5 or 6feet in length, astronomical telescopes rose to lengths of 15 or 20 feet by the middle of the century. A typical astronomicaltelescope is the one made by Christiaan Huygens, in 1656. It was 23 feet long; its objective had an aperture of several inches,it magnified about 100 times, and its field of view was 17 arc-minutes.

Telescopes had now again reached the point where further increases in magnification would restrict the field of view ofthe instrument too much. This time another optical device, the field lens came to the rescue. Adding a third convex lens--of appropriate focal length, and in the right place--increased thefield significantly, thus allowing higher magnifications. The telescope race therefore continued unabated and lengthsincreased exponentially. By the early 1670s, Johannes Hevelius had built a 140-foot telescope.

But such long telescopes were useless for observation: it was almost impossible to keep the lenses aligned and any wind wouldmake the instrument flutter. After about 1675, therefore, astronomers did away with the telescope tube. The objective wasmounted on a building or pole by means of a ball-joint and aimed by means of a string; the image was found by trial and error;and the compound eyepiece (field lens and ocular), on a little stand, was then positioned to receive the image cast by theobjective. Such instruments were called aerial telescopes .

Although some discoveries were made with these very long instruments, this form of telescope had reached its limits. Bythe beginning of the eighteenth century very long telescopes were rarely mounted any more, and further increases of powercame, beginning in the 1730s, from a new form of telescope, the reflecting telescope.

Since it was known that the telescopic effect could be achieved using a variety of combinations of lenses and mirrors, a numberof scientists speculated on combinations involving mirrors. Much of this speculation was fueled by the increasingly refinedtheoretical study of the telescope. In his Dioptrique , appended to his Discourse on Method of 1637, RenèDescartes addressed the problem of spherical aberration, already pointed out byothers. In a thin spherical lens, not all rays from infinity--incident parallel to the optical axis--are united atone point. Those farther from the optical axis come to a focus closer to the back of the lens than those nearer the opticalaxis. Descartes had either learned the sine law of refraction from Willebrord Snell (Snell's Law)

A second theoretical development came in 1672, when Isaac Newton published his celebrated paper on light and colors. Newtonshowed that white light is a mixture of colored light of different refrangibility: every color had its own degree ofrefraction. The result was that any curved lens would decompose white light into the colors of the spectrum, each of which comesto a focus at a different point on the optical axis. This effect, which became known as chromatic aberration, resulted ina central image of, e.g., a planet, being surrounded by circles of different colors. Newton had developed his theory of lightseveral years before publishing his paper, when he had turned his mind to the improvement of the telescope, and he haddespaired of ever ridding the objective of this defect. He therefore decided to try a mirror, but unlike his predecessorshe 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 ). He sent this little instrument to the Royal Society, where it caused asensation; it was the first working reflecting telescope. But the effort ended there. Others were unable to grind mirrors ofregular curvature, and to add to the problem, the mirror tarnished and had to be repolished every few months, with theattending 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 thecentury, reflecting telescopes with primary mirrors up to six inches in diameter had been made. It was found that for largeaperture ratios (the ratio of focal length of the primary to its aperture, as the f-ratio in modern cameras for instance), f/10or more, the difference between spherical and paraboloidal mirrors was negligible in the performance of the telescope. Inthe second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope withparabolically ground mirrors came into its own.