Building a Galilean Telescope — The Instrument That Changed Our View of the Universe

In 1609, Galileo Galilei built a telescope based on reports of a Dutch invention and turned it toward the sky — and nothing in astronomy was ever the same. Within months he discovered the craters and mountains of the Moon, the four largest moons of Jupiter, the phases of Venus, and the stars of the Milky Way. A Galilean telescope uses two lenses: a large convex (converging) objective lens at the front that gathers light and forms an image, and a smaller concave (diverging) eyepiece lens that intercepts the light before it comes to focus and produces an upright, magnified image. The magnification equals the focal length of the objective divided by the focal length of the eyepiece. Galileo's first telescope magnified about 8 times; his best achieved roughly 20 times. This blueprint builds a functional Galilean refracting telescope using two glass lenses mounted in a cardboard or wooden tube — simple enough to build in an afternoon, powerful enough to see Jupiter's moons and the craters of our Moon, exactly as Galileo did over four hundred years ago.