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.