Sky Watcher Esprit 150ED Reducer/Flattener .77x. This flattener lens module can be installed between the telescope optical elements and the camera objective lens. When installed the 150ED transforms the optical elements of the Sky Watcher 150 ED Telescope into a 2" filter cell converting the focal ratio from f/7 to f/5.4. This feature alone will not only reduce the exposure time but will also yield a brighter and more stable image. The Sky-Watcher 150 ED Flattener module has been designed to work seamlessly with the Sky-Watcher 150 ED Telescope and the Sky-Watcher ALTA 90 ED removing the focal reducer enables the full aperture of the main instrument to be used. The flattener module can also be used with a crop sensor camera. In that case please use the regular 150ED Eyepiece. If you are looking for a reducer for your 150ED Telescope please have a look at this Sky Watcher 150 ED Reducer/Flatter Adapter. Specifications for Sky Watcher Esprit 150ED Reducer/Flattener: Manufacturer: Sky Watcher Additional Features: Three-element Design 55mm Back Focus with M48 Thread Color: Black Weight: 920 g Display Resolution: 5196x5792 pixels Model: 150ED Focal Ratio: f5.4 Fabric/Material: Aluminum Optical Coating: Fully Multi-Coated Features: 36mm fully illuminated field compatible with both APS-C and full frame sensors 90mm back-focus (55mm when back focus spacer and M48 ring installed) Drops f/ratio from native f/7 (1050mm) to f/5.4 (810mm) Reduces exposure time by 68percent compared to native f/7 2.54 x 1.7-degree field of view when used with a full frame camera 1.66 x 1.11-degree field of view when used with APS-C crop sensor Includes reducer metal thread-on caps back focus spacer and M48 thread adapter Built-in 2" filter cell for mounting filters into the light path Package Contents: Sky Watcher Esprit 150ED Reducer/Flattener Sky Watcher S20215: Esprit 150ED Reducer/Flattener .77x