This flattener is made for our f-7 refractors. It threads to the rear of the 2.5" Stellarvue or Feather Touch focusers. This flattener uses an over-sized 48 mm camera attachment thread. Using one of the over-sized 48 mm T rings, a DSLR camera may be used with this field flattener. This provides a completely threaded mounting solution for larger chip cameras when used a 2.5” 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.