The astronomical telescope
The function of a telescope is to increase the angle which a distant object appears to subtend at the eye, and therefore produces the same effect as if the object were either larger or else closer to the eye. Fig. 24.23 shows how this is done by an astronomical telescope. For a very distant object, e.g., a star which is effectively at infinity, rays coming from any point on it are sensibly parallel on reaching the telescope. A real image I is therefore formed in the focal plane of the objective. The focal plane of the objective coincides with the focal plane of the eye lens. Consequently, the image I acts as an object for the eye lens
and, in normal adjustment, a final virtual, greatly magnified image is formed at infinity. Clearly the telescope owes its magnifying power to the fact that the angle subtended at the eye by the final image at infinity is very much greater than that subtended by the distant object.
For astronomical purposes it does not matter that the final image is inverted. For high magnifying power the objective should have a long focal length j, , and the eye lens a short one Ie. In this respect, a telescope differs from a compound microscope.