The Astronomical Telescope

The astronomical telescope makes use of two positive lenses: the objective, which forms the image of a distant object at its focal length, and the eyepiece, which acts as a simple magnifier with which to view the image formed by the objective. Its length is equal to the sum of the focal lengths of the objective and eyepiece, and its angular magnification is -f_o /f_e , giving an inverted image.

The astronomical telescope can be used for terrestrial viewing, but seeing the image upside down is a definite inconvenience. Viewing stars upside down is no problem. Another inconvenience for terrestrial viewing is the length of the astronomical telescope, equal to the sum of the focal lengths of the objective and eyepiece lenses. A shorter telescope with upright viewing is the Galilean telescope.

Galilean Telescope

The Galilean or terrestrial telescope uses a positive objective and a negative eyepiece. It gives erect images and is shorter than the astronomical telescope with the same power. It's angular magnification is -f_o/f_e .

The image below shows parallel rays from two helium-neon lasers passing through a Galilean telescope made from an objective with f=30cm and an eyepiece with f=-10cm.

With the lenses placed 20 cm = f_o+f_e apart, the parallel input rays are rendered parallel again by the eyepiece lens, giving an image at infinity. This shows one of the uses of Galilean telescopes. It is useful as a collimator that takes a large beam of parallel light and reduces the size of the beam, keeping the rays parallel. The angular magnification of this Galilean telescope is 3. The beams of the helium-neon lasers were made visible with a spray can of artificial smoke.