A coronagraph is an optical device that blocks the light from the Sun’s disk, making it possible to observe the corona, the very thin and hot upper level of the solar atmosphere, at the edge of the disk.
A lens focuses an image of the Sun onto a masking or occulting disk that prevents the light from proceeding farther into the telescope.
A coronagraph requires very high quality optics assembled in a dust-free atmosphere. Near sea level, a coronagraph would be virtually useless, because the scattered light from the Earth’s atmosphere would overwhelm light from the corona. Scientists put corona-graphs high in the mountains or launch them into space.
But any telescope can be turned into a coronagraph, simply by taking it to observe a total solar eclipse. (Just remember, for safety, never to look at the eclipse directly; project the image out through the eyepiece onto a piece of paper.)
Because of the happy coincidence of the Moon’s size and distance, when the Moon passes directly between the Sun and the Earth, it is usually in just the right position to make a very effective occulting disk. And during an eclipse, the Earth’s atmosphere is also in the Moon’s dark shadow, so light scattering is not a problem.
In fact, scientists still find a total solar eclipse to be an unexcelled opportunity to study the Sun’s corona.