The direct imaging of extrasolar planets is a prominent goal for modern astrophysics. Direct imaging is a technique which can detect some planets that are inherently not compatible with the transit or radial velocity techniques. Direct imaging can also make it possible to measure the spectrum of planet light, which can help to characterize the planet atmosphere. A coronagraph is a telescope instrument with the purpose of enabling direct detection of extrasolar planets and performing exoplanet science. A coronagraph operates by nulling the light from a single on-axis star so that the faint light of a nearby exoplanet can be observed. A typical coronagraph creates a focus of the starlight in the focal plane, and has an element known as a focal plane mask to block or manipulate the starlight. Another element in the pupil plane, known as the Lyot mask, is used to control diffraction from the blocked starlight.