Cone aperture
A cone diaphragm is a conical aperture, which in a Koronografen for observing the corona of the sun the solar disk is intended to cover. The diaphragm reflects the incident sunlight sideways into the tube so that only the light from the much weaker corona can pass the diaphragm and thus creates an artificial solar eclipse.
The cone diaphragm sits in the focal point of the telescope and must be precisely adapted to the focal length of the telescope used. Because of the apparently changing diameter of the sun during the course of a year, either differently sized cone diaphragms must be used or the focal length can be tracked in some other way.