In February 1646 he published the book Novae coelestium terrestriumq[ue] rerum observationes, et fortasse hactenus non-vulgatae, where he presented all the observations of the Moon made from 1629 until 1645, the drawings of the bands seen on Jupiter's disc, the strange appearances of Saturn, as well as of the stars of the Milky Way.

This instrument was enthusiastically accepted by Castelli, who used it for planetary observations and urged the Grand Duke of Tuscany Ferdinando II de' Medici in vain to buy it (it was finally purchased by the Prince of Ecchembergh for the original price of 200 scudi).

In fact Fontana had to struggle against the stubborn reluctance of Galileo, now old and blind, to recognise the superiority of the new Neapolitan telescopes (although admitting that their magnification power was greater than those produced in Florence, he believed they could reveal nothing that he himself had not already discovered) and the Medicean court’s fear of losing its unchallenged primacy in optical production.

In Florence, Fontana’s optical devices were carefully examined by the grand-ducal artisans and Galileo himself, as well as Castelli, reflected on the possible techniques adopted by the Neapolitan telescope maker.

In the meantime however, thanks not only to Castelli but also to Giovanni Camillo Glorioso, who from 1613 to 1621 had occupied the prestigious chair at the University of Padua that had once been Galileo’s, the fame of Fontana’s telescopes had spread through much of the peninsula.

Still from Milan, Giovanni Giacomo Cozzolani informed Carlo Antonio Manzini, in Bologna (where the news also reached Bonaventura Cavalieri) of the wondrous new phenomena observed with Fontana’s ‘spyglass’.

Information, albeit confused and fragmentary, on the new 14-palm telescopes, evidently able to show the surface details of planets for the first time, reached Holland, England and France as well (where René Descartes believed that Fontana had managed to fabricate hyperbolic lenses).

As proof of the instrument’s outstanding quality the letter, which has only recently come to light, 12 contains enclosed a drawing of Jupiter made by Fontana himself, which constitutes as of now the earliest representation of the planet’s bands, whose discovery was later to be claimed by (or attributed to) many other authors.