The telescope. One of Galileo's most famous devices, the first one was constructed in 1609. His model could magnify objects twenty times. Beating the European records of three. With his telescope Galileo was able to see the moon, discovering craters on it's surface.

With the discovery of his telescope he was able to see Jupiter more clearly and gather new information on it. 

Along with Jupiter, the largest planet is our galaxy, he discovered it's four moons. Ganymede, Callisto, Europa, and Io.

He more clearly observed Venus' phases,which provided  even more support for his heliocentric model and even more support for the Copernican theory.

Also with his telescope he viewed sun spots, black circles on the sun which showed it was flawed and was not a perfect body. Galileo was able to view mountains and craters on the moon and identify it's texture.