coronagraphs

Coronagraphs are optical instruments designed to suppress the light of a bright source in order to reveal nearby faint features. They were introduced to solar physics by Bernard Lyot in 1939 to study the solar corona, and they remain essential for observing faint solar structures beyond the disk. In astronomy, coronagraphs enable direct imaging of objects such as exoplanets and circumstellar disks that would otherwise be lost in glare.

Principle: In most optical coronagraphs, a mask or occulting disk sits in the telescope's focal plane to block the star's image. A subsequent pupil-plane stop, or Lyot stop, reduces diffraction from the telescope aperture. Variants use apodized pupils or phase masks to control diffracted light. Coronagraphs come in internal forms, placed inside the telescope, and external forms, such as a starshade, which blocks starlight before it enters the telescope.

Advances in wavefront control, including deformable mirrors and adaptive optics, have greatly improved contrast. Data-processing techniques like angular differential imaging help remove residual starlight in images. Coronagraphy is used on ground-based and space-based platforms to image exoplanets, circumstellar disks, and other faint surroundings, contributing to studies of planetary systems and solar wind.