Barrel extension 18mm (for all 1¼” eyepieces). Screw the
extension sleeve into the filter thread of your 1.25″ eyepiece and the
Insertion sleeve is extended by 18mm.
On the eyepiece side: 1.25″ filter thread outside (M28.5mm)
On the opposite side: 1.25″ filter thread inside (M28mm)
Length: 18mm