Idiot Newbie question

:S
Hello all. Nice to be here and please excuse my general ignorance. I'm on unfamiliar ground.
I haven't really done any serious sky-gazing since high-school, and even then I had trouble knowing my "ass." from my "dec". So maybe I'm a bit more clued-in now and can find Orion on a good night. Recently my wife has expressed an interest in buying a telescope. A decent one is expensive here in the down-under (about 35% more than you woud pay in the US).
However. I do have a honking-great, Canon 500mm f4 IS apochromatic camera lens.
Being a bit of a tightwad, I love to have things do double-duty. Given that the focusing & magnification on telescopes occurs at the back-end (DO correct me if I'm wrong - I told you I am idiot!), is there any reason I can't have some clever boffin build me an eyepiece / focussing assembly that I can mount on this lens and use it as a telescope as well?
All comments welcome and many thanks.

Whatwasthat?

Peter:

I believe people do this, there are adapters available. However various problems arise, there may not be enough focusing range to use a diagonal for example.

Camera lenses are not optimized for getting the sharpest possible image at the center, rather they want a wide flat field of view. The resolution of film or a CCD is low in comparison to an eyepiece and the human eye at high magnifications... Camera lenses are generally not operated at full aperture and are not their sharpest at full aperture.

A 500mm F4 lens would be considered a 125mm F4 telescope, at high magnifications you will probably need to stop it down to increase the sharpness. Also at F4 you will be looking at problems with astigmatism in the eyepieces, the steep light cone causes problems...

So, I think the bottom line is that a camera lens will not provide the same sort of views that a telescope will and the focal plane range may preclude using a diagonal.

But, since you have it, you might as well buy an adapter and see how it goes... Just keep in mind that you will probably want to buy a telescope at sometime in the future.

jon

Yes, you can do what you describe. I don't know where you can find a commercial one, (I used to have one for my Olympus)....But you can make one fairly easily by taking a rear lens cap and gluing it to a 1.25 to 2 inch eyepiece adapter. (Cut a hole in the rear lens cap!!!) Then, when you put the cap on the back of the lens, and stick an eyepiece into the 1.25 adapter, you have an eyepiece holder attached to your lens. (Or you can use a piece of pvc or copper pipe with a 1.25 ID.)

You won't get as good a view as you might expect because Camera lenses and telescope lenses have different requirements (as covered elsewhere).

Focus your contraption by sliding the eyepeice in and out, and moving the regular focusing ring on the camera lens. And you may have to make sure you have an adapter such that you can position the field stop exactly the right distance behind the end of the lens.....but with that warning, you can do what you describe.

Alex

Google: copyscope