Protuberance approach
A protuberance approach is an optical device for observing protuberances at the edge of the sun. The attachment is placed behind a telescope and is often used by amateur astronomers to observe the sun.
With a protuberance approach, an interference filter (H-alpha filter) and a metal cone are arranged one behind the other. The cone shades the sun and thus creates an "artificial" solar eclipse . To do this, the cone must be precisely adapted to the focal length of the telescope used. Since the distance between earth and sun varies by around 2 percent during a year, the sun's diameter also varies - in winter (in the northern hemisphere) the sun is closest to earth and the sun's diameter is the largest. To compensate for this, cones of different diameters are used.
The H-alpha filter only lets through the deep red light of the ionized hydrogen (the H-alpha line ), which the protuberances give off.