Sidereus Nuncius
The first telescopes appeared in the Netherlands in 1608 when Middelburg spectacle-maker Hans Lippershey tried to obtain a patent on one. By 1609 Galileo had heard about it and built his own improved version. He probably was not the first person to aim the new invention at the night sky but his was the first systematic (and published) study of celestial bodies using one. One of Galileo's first telescopes had 8x to 10x linear magnification and was made out of lenses that he had ground himself. This was increased to 20x linear magnification in the improved telescope he used to make the observations in "Sidereus Nuncius".
Juan Roget
A 1959 research paper by Simón de Guilleuma claimed that evidence he had uncovered pointed to the French born spectacle maker, Juan Roget, as the inventor of the telescope before Hans Lipperhey's patent application for the same device in October 1608 and that Dutch spectacle makers had copied Roget's device. Guilleuma referenced a book published in 1618, ""Telescopium: sive ars perficiendi novum illud Galilaei visorium instrumentum ad sydera in tres partes divisa"" ("Telescope, or a performance of the art and means to Galileos's new vision of the stars, in three volumes") by Italian author Hieronymi Sirturi Mediolanensis (aka Girolamo Sirtori of Milan) in which the author describes a 1609 meeting with a "withered old" spectacle maker in Girona named "Roget" who claimed to have invented the telescope. Simón de Guilleuma researched the register of deaths of Rodez Cathedral in Aveyron and found that there was a "Roget" family of spectacle makers, leading him to conclude that the spectacle maker in the Sirtori story was Juan Roget.