Tele Vue's 2" Eyepiece Barrel Extender (EBX-2120) increases backfocus distance and provides a mounting location for 2" (48mm) eyepiece filters. It can also help parfocalize eyepiece sets. Extender adds 1.2-inches to the 2" barrel.
Models with dual 2" / 1¼" barrels can use the extender.
12mm Nagler Type-4
The EBX-2120 makes the 12mm approximately parfocal with the 17mm and 22mm Nagler Type-4 eyepieces when used in a Tele Vue 2" diagonal.
Because the field stop is located near the bottom of the chrome eyepiece barrel, you will need to rack your focuser out further, relative to most other eyepieces. If you find that you cannot rack your focuser out enough to achieve focus, we strongly recommend you purchase Televue's EBX-2120. This will make Tele Vue's 55mm Plossl eyepiece approximately parfocal with their 27mm Panoptic eyepiece.
Additional Note: When joining eyepieces with dual 2" / 1¼" barrels to the 2" Barrel Extender (12mm Nagler Type-4 shown in second photo above), remove the lock screw in the 2" barrel section of the eyepiece.