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 directly screwed to the thread back on the focuser's tube of the AIRY BLACK telescope: in this way you avoid the unwanted flexures! In the camera side, it has a thread for connection to DSLRs (it requires ring) or cooled CCD cameras.