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You need about 1.5X per mm of aperture to clearly see the diffraction rings. With an aperture of 7-8" that means 250-300X. On a short focal ratio Newtonian this may be difficult (impossibly short eyepieces), but on an f/15 Mak it is easily achieved as Brendan indicated.
If your refractor was a little 60mm one, it takes just 90X to see diffraction rings. More than that is a waste...
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