Coronagraph

A coronagraph is an instrument that blocks out the light from a bright object, enabling the viewer to detect the light from a fainter object nearby. It's the equivalent of a visor that blocks the Sun's glare when you're driving, making it possible for you to see taillights of the car in front of you.

The usual coronagraph is simply a dark spot that blots out the light from a bright object. Most of JWST's instruments have traditional coronographs that, in addition to obscuring the source of the bright light, also block out some of the region around the bright object.

Only MIRI has a phase-plate coronograph, consisting of a piece of glass divided into four squares that intersect in the center. Two of the diagonal squares are just slightly thicker than the other two diagonal squares. Light is made of waves with peaks and troughs, or high points and low points. The glass manipulates the wavelengths of light so they overlap in just such a way as to cancel each other out. In theory, when an object is in the center of the coronagraph, the manipulation makes all its light vanish.