The Lumicon 1.25" Drop-in Extension Tube (3" Push Back) provides 1.00-3.25" of extension to the optical path.  The front (where light enters) has a standard 1.25" outer diameter barrel, and the rear accepts 1.25" accessories which are secured with a pair of set screws.  This device is often required to correct the distance of the focal plane of a recording device when the telescope's eyepiece has removed.