Keplerian Refractor:

Johannes Kepler, whose listeners was friendlier than Galileo's whenever it came to his theories regarding the universe, developed Galileo's telescope design. Kepler's refracting telescope engaged a convex-lens objective with a long focal length and a smaller convex-lens eyepiece with a small focal length. Dissimilar the Galilean telescope, the keplerian refractor (as shown in figure below) generates an inverted image; it is upside-down and backwards. The distance among the objective and the eyepiece should be precisely equivalent to the sum of the focal lengths of the two lenses in order for the image to be lucid. The magnification factor based on the ratio of the focal length of the objective to the focal length of the eyepiece.

Figure: The keplerian refractor consists of a convex objective and a convex eyepiece.

The keplerian telescope is favored over the Galilean type primarily since Kepler's design gives a larger apparent field of view. The Galilean telescopes in general have visible fields of view so narrow that looking via them is not a comfortable experience. The magnification factor of a keplerian telescope can be changed by using eyepieces with longer or shorter focal lengths. The short the focal length of the eyepiece, the bigger is the magnification factor, casually termed as power, supposing that the focal length of the objective lens stays constant.

The biggest refracting telescope in the world is situated at the Yerkes Observatory in Wisconsin. Its objective lens consists of a diameter of 40 in (a little more than 1 m). The Keplerian refractors are used by thousands of amateur astronomers globally.