Occulting disk

A cone diaphragm is a cone-shaped diaphragm that will cover the sun's disk in a Koronograf to observe the solar corona. The aperture thereby reflects the incoming sunlight sideways into the tube so that only the light of the much fainter corona can pass the aperture and thus produces an artificial eclipse.

The cone diaphragm sits at the focal point of the telescope and must be precisely matched to the focal length of the telescope used. Because apparently variable solar diameter during the course of a year either different sized cone aperture must be used, or be the focal length otherwise be tracked.