The IFW/TCF Telescope Mount Adapter for C-11 and C-14 SCTs by Optec:

Optec Celestron 3" Rear Cell Thread Telescope Mount for IFW, MAX filter 2-inch, and TCF Focuser. Fits the rear cell of the Celestron 11-Inch and 14-Inch SCT.

OPT Product Number: OP-17457