A coronagraph is a device for studying the solar corona at any time of the day by creating an artificial eclipse, invented by Bernard Lyot in 1930. It consists of a high-quality refractor in whose focal plane a small disk occults the image of the Sun. The diameter of the disk exactly equals the diameter of the solar image, so that only the faint light from the surrounding corona reaches the end of the instrument where a camera is mounted. For good results, the objective lens must be superbly polished and be entirely free of internal defects such as striations or bubbles. Great care is taken to reduce scattered light by mounting a series of diaphragm stops inside the coronagraph tube. Observations need to be carried out under the most favorable atmospheric conditions – clean air being absolutely essential – so that coronagraphs are normally installed at high-altitude stations.