Whether used for terrestrial or astronomical applications, telescope lenses share some characteristics.
Telescopes bring the far near. Whether on the hunting grounds, on the open seas, or in the astronomical observatory, the same basic optical principles apply to all telescopes. Telescopes using only lenses are called refractive telescopes because the way a lens bends light is called refraction. A refractive telescope has a relatively large diameter objective lens to collect the light, followed by a smaller secondary lens or eyepiece. There are a couple different types of refracting telescopes, distinguished by the type of eyepiece they use.
Objective Lenses
To detect distant objects, a telescope must collect a lot of light. That's where the objective lens comes in.
All objective lenses share a couple basic features. The true power of a telescope lies in its ability to collect light, which is related to the diameter of the objective lens, so they all have a relatively large diameter. Secondly, all objective lenses are converging lenses with a relatively long focal length. The focal length is the distance from the lens to the plane where the lens will form an image of a very distant object, like a star. A shorter focal length would mean the lens would need to curve more, and the more a lens curves, the more errors creep into the system. So objective lenses are large, long-focal-length lenses.
The Eyepiece of a Galilean Telescope
When he observed the moons of Jupiter and the phases of Venus, Galileo used a diverging lens as his secondary lens, or eyepiece. A diverging lens will not focus light from a distant star down to a point. Instead, it spreads out the light. This might not sound very handy, but when the right diverging lens is put behind an objective lens, the diverging lens takes light that is converging --- focusing down to a point --- and spreads it out just enough to straighten it so the observer's eye can see the distant objects. That's how Galileo's telescope worked, so telescopes with this design are called Galilean.
The Eyepiece of a Keplerian Telescope
Johannes Kepler used a different secondary lens; he used, a converging lens. A converging lens will take distant light and focus it down to a point, but it also works backwards. That is, if light is focused down to a point, a converging lens will straighten the light out as if heading towards a very distant point. Kepler figured out that a converging lens could work as an eyepiece if it was set up to look at the point of light focused by the objective lens. So the objective lens focuses light down to a spot one focal length away. Then the secondary lens put its own focal length away from that spot, resulting in light that is straightened out for an observer to see distant objects.
A Specialty Lens
High magnification is particularly useful when looking at planets; a Barlow lens can help with that..
The Galilean and Keplerian telescopes each have advantages, and Peter Barlow figured out mix them together and get the best of both worlds. A Barlow lens is a diverging lens that has features of an objective lens and a converging lens eyepiece. The Barlow lens makes the converging beam from the objective lens spread out just a little, so it doesn't focus quite as quickly. This makes the objective appear as if it has a longer focal length. The Barlow lens is built in a tube, because the converging lens eyepiece needs to be moved a little further away to be aimed at the new focal point. Barlow lenses have the effect of increasing the magnification of the telescope without modifying the objective, the eyepiece or the telescope tube.
Fancy Lenses
Lenses have a problem. Light of different color is bent slightly differently. This ends up blurring the image; when blue light, for example, is focused nicely, red and yellow light will be spread out. To minimize this problem, lenses are made out of two or more pieces of glass put together. Different types of glass bend light differently, so the defocused color problem --- called chromatic aberration --- can be kept small with the fancy, multi-part lenses, called achromats. Any of the lenses in a telescope can be replaced with achromats, which almost always results in a more accurate telescope.
Tags: objective lens, converging lens, focal length, diverging lens, Barlow lens, down point