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Subject:
From:
Nathan Hearn <[log in to unmask]>
Reply To:
Informal Science Education Network <[log in to unmask]>
Date:
Thu, 10 Sep 2009 12:19:16 -0600
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ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related institutions.
*****************************************************************************

Hi Marc,

    You actually raise a very fundamental question about why
telescopes work.  Part of the problem is that these diagrams do not
illustrate the magnification properties of the telescope, as you have
discovered.  For astronomical telescopes (which do not make an object
look closer, only bigger), the import consideration is what the rays
that are not parallel to the axis of the telescope do.  Here, rays
that come in at some angle to the axis end up emerging from the
eyepiece at some larger angle, and end up looking bigger.  Terrestrial
telescopes are similar, but the diagrams are a bit more complicated,
since the focusing is not done "at infinity."

    Most college-level optics textbooks should cover this in detail
(with more enlightening diagrams), but for lack of a good reference
right now, here is one I threw together:

http://flash.uchicago.edu/~nhearn/astc/astro_telescope.pdf


- Nathan


-
Nathan C. Hearn
[log in to unmask]

Project Scientist
Computational & Information Systems Laboratory
National Center for Atmospheric Research


On Thu, Sep 10, 2009 at 09:19, Marc Taylor <[log in to unmask]> wrote:
> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
> *****************************************************************************
>
> 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
> [log in to unmask]
>
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>
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Check out the latest case studies and reviews on ExhibitFiles at www.exhibitfiles.org.

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