The original design Galileo Galilei came up with in 1609 is commonly called a Galilean telescope. It uses a convergent (plano-convex or bi-convex) objective lens and a divergent (plano-concave or bi-concave) eyepiece lens. Galilean telescopes produce upright images.
Galileo’s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon and four moons orbiting Jupiter.
Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F' L1 / y’). However, the (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more, but traveling at a larger angle (α2 > α1) to the optical axis. This leads to an increase in the apparent angular size.
The final image (y’’) is a virtual image, located at infinity and is the same way up as the object.
(Wikipedia)
Subject: THE REFRACTING TELESCOPE 
Sun Jul 24, 2011 2:37 pm
