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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

Originally Posted by SpaceFluffer

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

Originally Posted by richardm

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.

16th November 2005, 09:37 AM	#1
richardm Philosopher Join Date: Aug 2001 Posts: 9,270	Question about telescopes 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?
richardm Philosopher Join Date: Aug 2001 Posts: 9,270	__________________ Rimmer: Look at her! Magnificent woman! Very prim, very proper, almost austere. Some people took her for cold, thought she was aloof. Not a bit of it. She just despised fools. Quite tragic, really, because otherwise I think we'd have got on famously. Last edited by richardm; 16th November 2005 at 09:39 AM.

16th November 2005, 09:44 AM	#2
SpaceFluffer Muse Join Date: May 2004 Posts: 595	The number refers to the focal length (in mm, I would guess). The smaller the focal length, the greater the magnification.
SpaceFluffer Muse Join Date: May 2004 Posts: 595

16th November 2005, 10:06 AM	#3
richardm Philosopher Join Date: Aug 2001 Posts: 9,270	Originally Posted by SpaceFluffer 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?
richardm Philosopher Join Date: Aug 2001 Posts: 9,270	__________________ Rimmer: Look at her! Magnificent woman! Very prim, very proper, almost austere. Some people took her for cold, thought she was aloof. Not a bit of it. She just despised fools. Quite tragic, really, because otherwise I think we'd have got on famously.

16th November 2005, 10:22 AM	#5
kookbreaker Evil Fokker Join Date: Aug 2001 Posts: 9,257	Originally Posted by richardm 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.
kookbreaker Evil Fokker Join Date: Aug 2001 Posts: 9,257	__________________ Thanks for helping me win Best Children's Gifts and Best Toys in Philly Voter in 2011 & 2012! Spectrum Scientifics - My store - Google it people!

16th November 2005, 10:19 AM	#4
roger Penultimate Amazing Join Date: May 2002 Location: Mountain View, CA Posts: 11,062	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.