Last year I posted a topic on a small APO made from a Canon 'L' lens, (70-300mm f4-5.6L) but at that time I could it not focus to infinity. I use it only fully zoomed in.
I was inspired by an Omanese guy who dit the same successfully.
http://www.samirkharusi.net/lensdiagonal.html
Now I did it by using a Televue Powermate 2.5 instead of a Celestron 2.2x barlow to extend the (too short) backfocus. The reason is that the Powermate protrudes a few mm deeper into the rear end on the lens.
That is also the catch: the lens should NOT be zoomed out and NOT be focused too close otherwise the rear element will hit the Powermate barrel.
So far so good, it provides excellent images and pinpoint stars it is actually a 53mm f/14 refractor (14 = 2.5 * 5.6), so rather small aperture.
But what happens: the edges of the image contain lots of coma and other errors. Strange, my Nagler 13T6 normally provides pinpoint sharp star images at the edges even at a f/5 Genesis or f/4.7 Dobson.
Yes, of course, a camera lens is not designed for visual telescopic use, but due to the Powermate its f/ratio is extended from f/5.6 to f/14.
What am I doing wrong ?
