When purchasing the QuadTCC the proper length Backfocus Spacer is included for the scope specified. However, when ordering the QuadTCC for a TEC scope you will also need to order a Drawtube End Cap (S35RFAA) for the focuser. This only applies to the TEC focusers. This part is integral to the Astro-Physics 3.5" focusers and is not needed as a separate purchase. The S35RFAA screws onto the drawtube's 4.100" x 24 thread (same as Starlight Instrument's 3.5" focuser end cap thread).