This flattener is made for our f-7 refractors. It threads to the rear of the Feather Touch 3” focuser. This flattener uses an over-sized 48 mm camera attachment thread. Using one of the over-sized 48 mm T rings below, a DSLR camera may be used with this field flattener. This provides 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 48mm extension tubes.