When a coronagraph is used to observe the Sun, a round metal screen blocks the overwhelming brightness of the Sun's central disk, or photosphere. Other screens and diaphragms block reflections of the photospheric image. The coronagraph must be used at high altitudes and on very clear days, when the atmospheric diffusion of light is at a minimum; imperfect lenses or dust inside the instrument will scatter enough photospheric light to obscure the dim solar corona.