The flattener is a lens that evens out the field, as this is slightly curved by the primary optics. This curvature causes stars that are imaged at the edge of the field of view to be less sharp. The flattener, also called a field flattener, corrects this effect. Astrophotographers can then enjoy images where the stars remain sharp right out to the edge of the exposure. The flattener is installed between the telescope and the camera.
This flattener is ideal for small refractors with focal lengths from 400 to 600 millimetres and an aperture ratio of f/5 to f/7.5 and photography with a DSLR camera. The back focus (the required distance from the flattener to the camera sensor) is 55 millimetres. This is also ideal for DSLR cameras with a connected T2 ring.
