This eyepiece sockets / visual back fits the thread of the Schmidt Cassegrain telescopes and enables the use of 1.25-inch accessories- for example for straight insight with 1.25 "anocular, for ZenitSpiegel and the like. This adapter belongs to the Standard delivery scope of all Celestron 5 ", 6", 8 ", 9.25" and 11 "Schmidt-Cassegrains.