With your set up I would just get a bright star in your Finder scope, place the Hartman Mask over the objective and take a short exposure, and view the image in your view finder after the image is taken, you want to merge all of the openings into one point. thats when you know you have achieved perfect focus.
By the Way, A step ring wont work with that tube, the set screw would be covered by the tube walls. I would skip that idea with your current set up.
BUt the Hartman mask will never fail you.
Dennis
