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Message: <blockquote data-quote="Paul Corfield" data-source="post: 1673478" data-attributes="member: 19132">The cheap ones are 55mm plus the bayonet at each end gives around 60mm. That will take the barlow to around 2.5X magnification. Two sets takes it to to 5X and then the magnification really tapers off. I've tried a third set which only gave me around 6X so the extra 1X  isn't much for the added 60mm of extension or the 180mm overall which is quite a lot. One day I'll get another GSO and because the GSO barlow lens cells are threaded they can be screwed together easily.Paul.</blockquote>