This large photographic field flattener is made for our f-7 refractors. It threads to the rear of the Feather Touch 3” focuser. This flattener uses the standard 42 mm t-thread for camera attachment. Your DSLR and a standard t-ring will attach to this flattener, providing a completely threaded mounting solution for larger chip cameras when used with the 3” Feather Touch focuser. This field flattener requires 55 +/- 0.5mm (standard DSLR) back focus. When using a CCD camera, the remaining back focus required can be made up with 42mm extension tubes.