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.
The Reducer (for focal length reduction) is an optical accessory that allows a more favourable aperture ratio. Required exposure times are hence shorter and the optics 'faster'. Also, the field of view increases, so that even larger astronomical objects can be observed or photographed.