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This may seem like an odd question, but I've been trying not to assume I know how this works...

We all know (if we didn't before this year) that a very basic telescope consists of a converging lens at the objective and a diverging lens at the eyepiece. You may have, like me, drawn diagrams and made animations showing the path of light in reflecting and refracting telescopes. You may have, like me, handed lenses to kids and let them make tubeless telescopes for themselves.

But what I'm trying to understand is exactly how this works. Every light-path drawing I make seems to show that the image made by the telescope should be **smaller** than the original. I can understand how a Galilean telescope would make an image **brighter,** but not larger. Look at the light path diagram here to see what I mean.
http://www.lhup.edu/~dsimanek/scenario/lenses.htm

Expermenting with a pair of lenses I have, I've noticed that even a single converging lens will make distant objects bigger, but the image is out of focus. Put the diverging lens in the light path and the distant, magnified vista snaps into focus without changing apparent size. Try it.

Great -- but what exactly is going on here? Perhaps the way to think about it is that the converging lens makes a large image, then the diverging lens snags a small part of that image and allows you to focus on it. But that would seem to imply that the converging lens is closer to the objective than the objective's focal point...

I'm starting to wonder if this is one of those diagrams like the tongue "taste regions," or the "we only use 10% of our brains" business, frequently stated but wrong in some subtle, fundamental way.

Maybe what I'm really looking for is what it means to "focus" light. I would like to make a concise and accurate display for an exhibit component, and I would like to get this right.

Marc Taylor
Coordinator, Andrus Planetarium
Hudson River Museum
511 Warburton Avenue
Yonkers, NY 10701
914 963 4550 x223
Fax 963 8558
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