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richardm
16th November 2005, 09:37 AM
I've got a telescope that has two objective lenses. One of them is labelled K10, and is higher magnification than the one labelled K20. I've figured out (or think I have!) that the K stands for Kellner, but can't figure out the way the numbers are derived. Any ideas?

SpaceFluffer
16th November 2005, 09:44 AM
The number refers to the focal length (in mm, I would guess). The smaller the focal length, the greater the magnification.

richardm
16th November 2005, 10:06 AM
The smaller the focal length, the greater the magnification.

Isn't it normally the other way round? On a camera I'd expect a lens with a 50mm focal length to have less magnification than one with 400mm. Or is this some fundamental misunderstanding on my part?

roger
16th November 2005, 10:19 AM
Richard, think of how a camera works. The lens collects the light, and the film plane "captures" it. Translate this to your telescope. The telescope is the lens, and the eyepiece is the (now moveable) film plane. What happens as you move closer to the film plane? You see less of the film plane, hence you are seeing a smaller piece of the world blown up to fit your eye. Thus, as you move closer, you are actually magnifying.

Now, if you made the focal length of the telescope longer, then the situation would be analogeous with the camera.

Apologies to the real telescope people: I recognize my paragraph was a bit more hand waving then exact optics, but analogies often get the point across better than equations.

kookbreaker
16th November 2005, 10:22 AM
Isn't it normally the other way round? On a camera I'd expect a lens with a 50mm focal length to have less magnification than one with 400mm. Or is this some fundamental misunderstanding on my part?

Nope. Telescope magnification is the focal length of the objective lens divided by the focal length of the eyepiece. So lower focal lengths mean higher magnifications. If you must think of a crude model, imagine you are mangifying the image produced by a magnifier.

Shorter focal lengths in all positive lenses mean greater magnification as well.