This 2-Inch T-thread nosepiece screws into the female T-threads on ST cameras. All ST cameras include at least one nosepiece. Each nosepiece is threaded to accept filters and notched for extra security in the event the retaining screw comes loose slightly during the night. For a more solid connection to Schmidt-Cassegrain scopes, consider the SCT to T-thread Visual Back.
Image shows both the 1.25-inch and 2-inch Nosepieces.