A question to Experts

Hi, I just got a Canon FD300mm F2.8 Lens. The optics of this lens is shown here: The first element is a protective glass, the 2nd element is a fluorite and the 3rd is a UD glass.

My question is:
can I just take the first 4 elements(with the lens cell) out and make a telescope from it? Will the SA and CA very high without the other elements? What will be the focal length of it?(longer or shorter than 300mm?)

Thanks,
GY

Attached Image:

That lens is designed as a complete optical system, removing any of the elements will cause a huge decrease in performance. If you need a telescope that is well suited for photography and viewing, I would consider the Borg 100 mm ED. Depending on the condition of your lens, you could sell it and easily afford a used Borg optical tube assembly.

Joplin

I built an adaptor out of a rear lens cap for a telephoto lens (75-205 f3.5 zoom) just so I could try that as a small telescope. It worked better than I thought it would. However, it only works as straight through, (my adapter had no focus room for a diagonal) so it's not good for terrestrial observation. And not all of my eyepieces focus in the right place. Also, it was much heavier than most scopes, even cheap ones, that would work better.

Sounds like you have a nice lens that you might not want to take apart. My guess is that at f2.8 you might need those extra elements anyway. I would also guess that you could modify a diagonal somehow to adapt directly onto the back. Good luck.

Charlie

I doubt it'll work properly if you take the lens cell out, as those other lenses may well do more than just reduce the focal length.

University Optics used to sell a right angled, erect image, 3X Barlowed gadget that one attached a camera lens and eyepiece to - and you had a telescope. Mine worked as advertised, except that it showed how poor the quality of my lenses were.

Various big photography stores used to sell gadgets that fit on the back of your lens. But those gadgets didn't enable you to change eyepieces. But for 50 or 60 dollars, you'd have a nice low power telescope.

What you may wish to try, is to get a spare rear plastic cap of your lens and some 1.25" internal diameter tube, and see if you can make an eyepiece holder that fits into the plastic cap. You would then have a working telescope.
Renato