"Bill Greer" wrote in message
...
The image of an extended celestial object at the focal
plane of a 400mm focal length telescope is one third the linear size
of the image of the same object at the focal plane of a 1200mm focal
length telescope.

That description is very useful. I'd not read that anywhere else. Thank you.

I have to admit though, I am a little more confused now about the term
magnification as it concerns the image at the focal plane. I picture the
eyepiece being a fixed microscope under which the image at the focal plane
is presented by the telescope objective. Although just now thinking about
it, I can see that this couldn't really be an accurate way to define
magnification, since doing so means that the eyepiece always magnifies the
image at the focal plane by the same amount, and cleary (I think) this isn't
true, since different focal length objectives dictate the magnification
provided by the eyepiece.

To say that the angular size of an object in the eyepiece is 50x larger than
the angluar size of the unaided eye is also very useful, but I'm now
wondering just what is the correct relationship between the image at the
focal plane and the image "in" the eyepiece.

Maybe I need to re-read Brian's response a couple more times for it to sink
in. I skimmed it over, and I'm not sure if he addressed that.

-Stephen