How Does a Telescope Work?

This is the basic principle of a telescope: it lets you see distant objects clearly, by making the image bigger. How is this done? We can find the beginnings of our answer in the humble magnifying glass; notice that when you look at an object through a magnifying glass, it appears bigger. That’s because a magnifying glass uses a convex lens (a lens whose middle is thicker than its sides) to focus the light from the object into your eye, and thus it makes the image appear larger.

Now there’s a small problem with this method; beyond a certain distance (the focal length, or f) the virtual image created by the lens will appear smaller and upside down. The main remedy for this is to use another lens to turn the image right side up again, and magnify it. In this two-lens arrangement, the main, big lens is called the objective lens, and the lens that turns the image right side up is called the eye lens. This arrangement is common in smaller telescopes, where one can adjust the focus of the telescope simply by changing the distance between the two lenses. This is where the verb form of “telescope” derives from, incidentally; the objective and eye lens are usually mounted in a series of tubes that fit inside one another. A user can then can extend or retract to change the distance between the two lenses and thus change the focus of his telescope to look at objects more clearly.

More complex telescopes have multiples lenses instead of just the objective and the eye lenses; this is done to prevent any imperfections in a big single lens from marring the quality of a picture. Mind you, all lenses are machined to a high degree of precision; it’s just that for seriously top-notch stuff, only perfection (or coming as close to it as humanly possible) will suffice. They may alter the path of the light rays so you can view objects at an angle (this is done so one can sit down and be comfortable while using a telescope; serious astronomers can watch the skies all night, every night). Some of them are linked to computers so you can record everything you observe through the telescopes. The biggest telescopes tend to be far larger than anything a human could operate by hand; instead large, robotic telescope mounts pan them, zoom them and focus them, while all the operator has to do is open his eyes and look. 

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