Help me understand how a telescope work?

[NeilFawcett]

The trick is to examine ray diagram. From that you can see how the angles have changed to get a larger image. It's true that in the demo I linked to they're squashed and it's hard to see them. I'll look for a better image...

EDIT: Ok! This is a good start: http://brunelleschi..../eesplora2.html

The angle does indeed support/show how the closer the eye piece is the, the greater it's angle. But if we take the (bottom) image from that site:-

Imagine we moved the eyepiece further back. The magnification lessen and the FOV increase. But that does quite seem to sit right...

Look at for example the rays travelling (left to right) from the top to of the first lens to the bottom of the second. Imagine now that second lens moving back further... Most of those rays would now miss the second lens.

So is it that infact a different path is used for an eyepiece further back? Look at this concocted example with the eye piece furtherback. We can see the apparent size of the object is now smaller (shallower angle), NOT NOTICE how the rays travel through the first lens differently for an eyepiece furtherback:-

That's the only way I can seem to make sense of it, as the rays from the first diagram will not work with the eye piece lens shown inthe second position!

This is madening

[NeilFawcett]

^^

"NOT NOTICE" = "NOW NOTICE"

[dph1nm]

Most of those rays would now miss the second lens.

You cannot just take a lens and move it to the right. It will no longer focus. Each lens has only one position where it will focus. Also, the vertical height of the lenses in these diagrams is entirely arbitrary, so which rays pass the lens is of no signifcance (I could draw the lens taller then more rays would pass through!). In fact the FOV really depends on how the manufacturer constructs the lens - you could buy a 12mm lens with the same FOV as 24mm one.

Nigel

[NeilFawcett]

You cannot just take a lens and move it to the right. It will no longer focus. Each lens has only one position where it will focus. Also, the vertical height of the lenses in these diagrams is entirely arbitrary, so which rays pass the lens is of no signifcance (I could draw the lens taller then more rays would pass through!). In fact the FOV really depends on how the manufacturer constructs the lens - you could buy a 12mm lens with the same FOV as 24mm one.

Nigel

There was no suggestion the same lens was in both places - I'm just trying to get my head around the notion an eye piece at say 12mm gives twice the magnification (& FOV all other things equal) as a 24mm lens. The closest I've come thus far is my noddy attempt of a diagram above...

[dph1nm]

Your second diagram is misleading, as you have changed the distance from the objective lens (on the left) to the image in the focal plane (the small upside-down arrow). This distance must remain fixed, even when you change the eyepiece from a 12mm version to a to 24mm one.

NigelM

[NeilFawcett]

Your second diagram is misleading, as you have changed the distance from the objective lens (on the left) to the image in the focal plane (the small upside-down arrow). This distance must remain fixed, even when you change the eyepiece from a 12mm version to a to 24mm one.

NigelM

Agreed! My mistake! So if I moved the focal plane back to the original position we're headed in the right direction? NOTE the different path of the rays though!!!?

[NeilFawcett]

Agreed! My mistake! So if I moved the focal plane back to the original position we're headed in the right direction? NOTE the different path of the rays though!!!?

Infact I can't see how that works

I'm so very very confused how such a simple question can't be answered by a pair of simple diagrams