Re: Astrophotography (was Re: *istD EOL...)

[Bob Blakely]

Think of the lenses as simple, one element lenses. Think of the center of 
the lenses as fulcrums (pivot points) of a "light lever" too. If the 
eyepiece has a short focal length, the distance from the image in the 
telescope to the lens "fulcrum" is short. The distance from the lens 
"fulcrum" to your retina is longer. Small image in the tube, big image on 
your retina. Shorter focal length on the lens even larger image on your 
retina.

Yes, I know there is another lens involved, your eye's lens, but this is 
sufficient to demonstrate the principle.

Regards,
Bob...
From: "Tom C" <[EMAIL PROTECTED]>

Bob wrote:
Think of it like a lever. The objective is focusing the image in the air 
inside the telescope's tube. The longer the focal length, the larger this 
image (like a camera lens). The eyepiece is used like a magnifying glass 
to view this image "in the ether". The shorter the focal length of the 
magnifying glass, the larger the image to your eye.

There's something fundamental I'm missing, maybe you can help.  I've been 
wrestling with the idea for a while... why, on let's say a camera lens or 
optical tube, does longer focal length = larger image, and on an eyepiece 
longer focal length = smaller image (less magnification).  In my mind, it 
seems that an eyepiece is a lens with an optical tube and therefore it 
should work reverse of what you've stated regardless of whether it's 
focusing on he object itself or an image of the object "in the ether".

I realize your statement is quite correct.  What am I not getting?  I'm 
sure I need to dig out a basic optics book.