Minimum power for 1 or 2 diffraction rings?.

Using Pollux, a 1.0 magnitude star with a f10 4" refractor, what would be the MINIMUM power(or ocular F.L.) to obtain at least 1 diffraction ring for checking the optical path for camera alignment?. Sorry for dumb question...It has been too many years(decades) since I played with telescopes. I have on hand only a 20mm plossl. A 2X barlow will be coming giving me 10mm, will 100X be enough?. -Larry