Telescope

A refracting (astronomical) telescope has an objective of large focal length and large aperture and an eyepiece of short focal length; it gives angular magnification of distant objects.

In normal adjustment the final image is at infinity, the magnifying power is m = f_o/f_e, and the tube length (separation of the lenses) is f_o + f_e.

The final image of an astronomical telescope is inverted.

A large objective aperture collects more light and improves resolving power, but large lenses are heavy, hard to make free of defects, and can be supported only at their edges.

Reflecting telescopes use a concave mirror as the objective; this avoids chromatic aberration, and a mirror weighs less and can be supported over its whole back surface.

In the Cassegrain design the light from the concave primary mirror is reflected by a small convex secondary mirror through a hole in the primary to the eyepiece or detector.

For a telescope, magnification is about angles, not sizes: the image subtends a larger angle at the eye than the distant object does.