With certain eyepiece/telescope combinations, it may be necessary to add this Tele Vue Eyepiece Barrel Extension accessory to achieve necessary path length for proper focus or lack of interference.
This eyepiece barrel extension threads on to a 2" eyepiece's filter threads, and can make some eyepieces parfocal with other eyepieces (the Nagler 12mm becomes parfocal with the Nagler 17mm & 22mm eyepieces).