Antares 2" 2X Barlow for Newtonians, with Twist-lock.

2UB2N can be used in a refractor without diagonal but not with one.  The reason is that the barrel holding the lens has to get down Newtonian (eg.SW dob) extension tubes that are less than 2" diameter and so can't be clamped in a diagonal.  This is why this product is primarily for Newtonians.