After previous unsuccesful attempts with a Celestron Ultima barlow, I managed now to make a small telescope from a Canon 70-300 f/4-5.6L . The key is the Televue Powermate which I got second hand from a
Dutch astronomy trade shop for $120 in excellent condition.
I already made earlier an Canon lens mount to 1.25" barrel adapter by glueing together a rear lens cap and a 32mm (1.25") PVC plumbing socket with PVC glue.
Now I first zoomed in the lens to 300mm and focused the lens to infinity. This moves the rear lens as far inside as possible. Then I inserted (carefully not to touch the rear lens !!) the Powemate barrel into the PVC tube after mounting the latter to the lens, and a star diagonal with an eyepiece (Nagler 13T6) in it behind it.
Then I pointed to a distant tree and saw it was NOT in focus ... AArgh
But fortunately I could (carefully) focus the lens and saw it got focus !!! Yes !!!
The image was remakrable free of color errors at 58x (300 * 2.5 / 13), but one catch: the edges had some coma. I don't know why because the Nagler is excellent and the focal ratio is f/(5.6*2.5) = f/14, so not short anyway.
The lens is also not the issue: mounted on my eos 7 it makes razor sharp images even to the edge at full aperture.
But this trick can be used when you have no scope available or take it on a holiday trip where you are short on baggage space.
The diameter is only 52mm but the same trick can be done with a 70-200 f/2.8L which is 70mm diameter.
http://pix.northsouthsky.com/cutelittleapo.jpg
Neato.
