For years Galileo surpassed all others in the production of telescopes, as clearly shown by two examples. Kepler, having read the Sidereus nuncius, wished to verify Galileo’s discoveries through his own observations. For months he searched for a good telescope; but when finally, in August 1611, he managed to borrow a suitable one, it was the instrument made by Galileo for the Elector of Cologne.

The Sidereus nuncius inspired in Nicolas Claude Fabri de’ Peiresc (1580-1637) instead the idea of determining the orbital periods of Jupiter’s satellites. Sure of success, Peiresc commissioned preparation of the frontispiece of his own book on the subject. Nonetheless, in 1634 he asked Galileo to build him a good telescope, since the instrument he owned, made by Jacob Metius, did not allow him to conduct scientific research of any kind.

Curiously enough, Galileo was able to perfect the first spyglasses from the Netherlands in a few months’ time, while no one managed to perfect his telescope for over twenty years; and even more surprisingly, after the rapid initial development Galileo himself was unable to develop his instrument any further. The facts seem to suggest that in 1610 he had a device that could not be improved upon; and this is basically true.