Sebastian
Eyepieces come usually with either a 1.25" barrel or a 2" barrel. Many of the longer focal length EPs will have the 2". The focuaser is designed to take a 2" EP but the adapter enable the 1.25" EP's to fit.
If you have got the Skywatcher 150mm Dob, it usually comes with a 25mm and a 10mm focal length epepice, both should be 1.25" barrels, so just leave the adaptor in place.
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