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.
The EdgeHD visual backs are longer than the standard #93653-A visual backs. This is to take advantage of the EdgeHD’s optimum image size which is farther back than the same position for a standard Schmidt-Cassegrain telescope.
The additional length of the EdgeHD visual back is a tapered, faceted cone surface that won’t allow the Deluxe Tele-Extender’s setscrews to achieve a mechanical grip on the visual back. So the extender can’t be used in the conventional way attached to the visual back.
It can be used if the eyepiece is gripped with the Tele-Extender's setscrews and not the EdgeHD visual back. This configuration isn’t as solid as when the unit is fixed to the visual back itself, so it's best with a small imager.
NOTE: The EdgeHD optimum image distance is of most importance for the outer parts of the FOV only. If you are imaging planets or small center of field of view objects, it makes no difference whether or not you are at the optimum distance. So a regular visual back #93635-A can be used with the Tele-Extender secured to it.