got a better collimation and this resulted in better focus.
The way I achieved better collimation is awkward: tried with star test, but image of diffraction was so small that was difficult to make corrections, so I gave up and plug the Canon to the primary focus, switched to live view and of course it was out of focus but ... what?!? diffraction pattern was clearly not concentrical and I was able to make corrections it in live view, crazy but it worked
Michele