Sky Watcher Evoguide 50ED Flattener Compatible with Evoguide 50ED. This Flattener allows the Field Flattener to be retracted in towards the camera which brings the image to a nice round number of degrees (within the limiting sphere of most astroimaging softwares) while ensuring the main image is not truncated. Field Flatteners work best with telephoto lenses that have a similar focal length as they allow the Flattener to be tripped at the same time. For a simple and quick install choose the Sky-Watcher Field Flattener for an unblocked 28mm imaging circle. A 17.5mm back focus is required when using this Flattener with a DSLR camera with a crop sensor. A must have accessory for anyone doing astrophotography with a Telescope Specifications for Sky Watcher Evoguide 50ED Flattener: Manufacturer: Sky Watcher Color: Black Display Resolution: 8688x5792 pixels Model: Evoguide 50ED Features: Corrects for field curvature allowing the Evoguide 50ED and 50DX to be used for astrophotography Thread on metal caps T-thread connection or 1.25" opening for camera mounting 28mm imaging circle 17.5mm back focus 5.33 x 3.57-degree field of view when used with APS-C crop sensor Package Contents: Sky Watcher Evoguide 50ED Flattener Sky Watcher S20214: Evoguide 50ED Flattener Compatible with Evoguide 50ED