The Deluxe Tele-Extender has two setscrews that are designed to lock the main body of the extension tube to the visual back or onto a projecting eyepiece or lens. One setscrew must be removed from a visual back for the extender to go over the visual back.
Since the tube of the Tele-Extender goes over the eyepiece, the eyepiece or Barlow outside diameter must be smaller than the 45mm inside diameter of the extender. Many eyepieces are too big to fit inside and can’t be used with the extender.
Celestron’s Omni series will fit the Tele-Extender. The older X-Cel series will also fit. The Celestron #93326 Omni and #93506 Ultima Barlows will also fit (once the Barlows setscrews are removed or replaced with smaller setscrews).