Use a telephoto lens as short refractor ?

[skysurfer]

Twenty years ago I had a Tokina AT-X 80-200 f/2.8 for my Canon T90 and I got the idea to use a rear lans cap and a 32mm PVC plumbing socket to attach an eyepiece to it. It worked ! I got a 70mm spotting scope, but quality was so-so, as the Tokina is not premium quality.

Last week I got a Canon 70-300 f/4-5.6L (which is a really great lens !) and tried the same. Only 53mm in diameter but quality is superb just like photos taken with it on my 7d body. Using my Nagler 13mm results in a high quality 23x50 with 3.5º FOV spotting scope. But the image is upside down (which is not a big issue), but using on a tripod, like any refractor without diagonal is very uncomfortable, so it can only be used handheld. And in that case the Image Stabilizer of the lens obviously does not work as power is required which is only available when mounted on a camera.
I found a website of an Oman amateur astronomer who managed to use a Canon L telephoto lens as a small high quality telescope. He used an EF mount extension ring where I use a 1 1/4" barrel extension tube and furthermore the same setup (Celestron 2.2x barlow before the diagonal).
But for me NO eyepiece gets focus. Tried everything, removing the barlow, putting the barlow behind the diagonal, zooming the lens in and out, NO sharp focus appears.

Does anybody have experience with this ? This would be great for deep sky viewing when I have no telescope or binos with me.

[leon]

Now that is a niffty idea, might be worth a try, just for fun.

Leon

[tlgerdes (Trevor)]

We played around with using a 70-200mm lens as a guide scope with an autoguider, the problem was in-focus, we couldnt get the CCD chip close enough to get a good focus. I think you will find the same with an eyepiece.

Think about where the CCD plane is in comparison to the lens when fitted to a camera, this is the distance your eyepiece lens need to be as well.

[astroboy]

Hi
I made a converter myself a while back to use on a 300mm F4 L and worked great , I now use it on a 100 - 400mm L as a spotting scope and works well too.
I's made from the lens cell bit from a Televue 2.5 x barlow ( 1.8x should be the same optics just with a longer tube ) fitted into a rear cap and a diagonal glued to that , it seems to make about a 70mm F11 scope and with a 24mm EP gives 10x - 40x ( I think ) .
The key is to have the barlow optics fit up inside the camera lens slightly , which the 300 L and 100 -400 L is fine , I don't know about the 70 - 200 .

Good luck
Zane