Galileo's telescopes

The telescope was unveiled in the Netherlands. In October 1608,  the States General (the national government) in The Hague  discussed the patent applications first of Hans Lipperhey of Middelburg, and then  of Jacob Metius of Alkmaar, on a device for "seeing faraway  things as though nearby." It consisted of a convex and concave  lens in a tube, and the combination magnified three or four  times.

The gentlemen found the device too easy to copy to award the  patent, but it voted a small award to Metius and employed  Lipperhey to make several binocular versions, for which he was  paid handsomely. It appears that another citizen of Middelburg,  Sacharias Janssen had a telescope at about the same time but was  at the Frankfurt Fair where he tried to sell it.



The news of this new invention spread rapidly through Europe,  and the device itself quickly followed. By April 1609  three-powered spyglasses could be bought in spectacle-maker's  shops on the Pont Neuf in Paris, and four months later there  were several in Italy. (Figure 4) We know that  Thomas Harriot observed the Moon with a six-powered instrument  early in August 1609. But it was Galileo who made the instrument  famous. He constructed his 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. With this  instrument 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. In the  spring of 1610 no one had telescopes of sufficient 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. With the verification of the  phases of Venus by others, in the first half of 1611, Galileo's  lead in telescope-making had more or less evaporated. The next  discovery, that of sunspots, was  made by several observers, including Galileo, independently.



A typical Galilean telescope with which Jupiter's moons could be  observed was configured as follows. It had a plano-convex  objective (the lens toward the object) with a focal length of  about 30-40 inches., and a plano-concave ocular with a focal  length of about 2 inches. The ocular was in a little 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 Galilean telescopes of higher  magnifications were certainly made, they were almost useless  because of the concomitant shrinking of the field.