This reducer-flattener from Sky-Watcher is only compatible with the Evostar 150DX focuser. It has a built-in 2" filter cell to mount filters into the light path. With a 34mm fully illuminated field, it is compatible with both APS-C and full frame sensors. It has a 2.22 x 1.48-degree field of view with a full frame camera and 1.45 x 0.97-degree FoV with an APS-C crop sensor. This reducer-flattener will drop your f/ratio down from f/8 (1200 mm) to f/6.2 (930 mm), drastically reducing your exposure time by 66%! It has 89 mm of back focus (55 mm when a back focus spacer and M48 ring are installed). Included with the reducer-flattener are metal thread-on caps, a back focus spacer, and M48 thread adapter.