2" Locking Ring (for Celestron SC's 8"/9.25"/11"/14"), with thread female 2" (5mm optical length). The locking ring allows the direct connection of zenith mirrors with the SC thread connection, without the use of a 2" sleeve. This saves about 50mm in length. Whenever there is little space at the back of the device and you still want to use 2" accessories. Fork-mounted Schmidt-Cassegrains are a good example. With normal adaptation of zenith mirrors with SC thread or 2" plug, you could no longer swivel the device.