The CPC 1100 SCT usually requires knobs with metric threads. The CPC 1100 flat secondary can be identified by a factory-installed plastic cap over the collimation screws. This cover, which fits into the two slots in the above photograph, must be removed and set aside when Bob's Knobs are installed. The cover is cosmetic only and will not affect operation of the scope.
The CPC 1100 SCT may also have a Trifid Fastar secondary, characterized by a rotary cover and black Phillips collimation screws. This secondary usually requires these Bob's Knobs with metric threads.
FITS most with black Phillips collimation screws (CPC 1100, EdgeHD and metric Fastar).
A few post-2005 scopes have black Phillips collimation screws with standard threads, and the only way to determine whether the threads are standard or metric is to remove one collimation screw and measure it. Alternatively, you can order both standard and metric knobs and return the unneeded set for a full refund. Check that our screw threads match the factory screw threads before installing.
Celestron 11 inch SCTs manufactured prior to 2006 require knobs for the C11 with standard collimation screw threads. Do not use metric knobs on those telescopes.