In October 1608, the Dutch craftsman Hans Lipperhey built the first rudimentary telescope. Galileo saw a few copies of it in Venice in 1609 and immediately tried to increase its magnification power. The Galilean telescope consists of a tube carrying two lenses: a plane-convex objective and a plane-concave eyepiece. The best examples built by Galileo exceeded twenty magnifications. They were plagued, however, by severe resolution problems due to heavy spherical and chromatic aberration; their field of view was also very limited. For example, Galileo's telescope covered only a portion of the Moon's surface. Galileo also used the telescope as a projection instrument for drawing the surface features of the Moon and sunspots, and as a device for measuring the angular distance between heavenly bodies by means of a micrometer.

In about 1630, the "astronomical" or "Keplerian" telescope was introduced. The concave eyepiece was replaced by a convex lens that substantially widened the field of vision, but gave an upside-down image. After Galileo, increasingly sophisticated instruments were built by Francesco Fontana, Evangelista Torricelli, Eustachio Divini, and Giuseppe Campani. The body tubes grew steadily longer, while the image was turned right side up by adding two lenses to the convex eyepiece.

James Gregory, in 1663, and Isaac Newton, in 1668, developed the so-called reflection telescope, in which the objective was replaced by a mirror: this eliminated chromatic aberration.