How did Galileo Galilei improve early telescopes?

After learning of the invention of the telescope in the Netherlands in 1609, Galileo Galilei quickly deduced the secret of the device through trial and error. While others had also constructed their own versions, Galileo distinguished himself by mastering the art of lens grinding, which allowed him to produce increasingly powerful instruments.1

By refining his manufacturing techniques, Galileo improved the telescope's magnification significantly. While his initial model was a simple three-powered spyglass, he soon presented an eight-powered instrument to the Venetian Senate and eventually developed telescopes capable of magnifying up to 20 times.1

The resulting Galilean telescope utilized a convergent lens as an objective and a divergent lens as an eyepiece, a design that produced an upright image. These technical improvements enabled him to conduct systematic astronomical observations that were previously impossible, such as discovering the phases of Venus, the craters and mountains on the Moon, and the four largest moons of Jupiter.231