Newbie Eyepieces 101

On Sat, 19 Jul 2003 12:25:05 -0400, "Stephen Paul"
wrote:

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.

That is essentially how an eyepiece functions.

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.

Any given eyepiece will magnify the (real) image at any telescope's
focal plane by the same amount; but the size of the image at the focal
plane will be different for telescopes of different focal lengths.
Thus the same eyepiece, even though it magnifies different real images
by the same amount, will result in different magnifications when used
with telescopes of different focal lengths.

IOW, the focal length of the telescope is just as important as the
focal length of the eyepiece when it comes to computing the
magnification of the system.

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.

The image at the focal plane is a "real" image. The image in the
eyepiece is a "virtual" image. The relationship between the two is
very much like that between a postage stamp (a real object) and the
enlarged virtual image of the postage stamp that a magnifying glass
provides.

Bill Greer