Coronagraphy

Coronagraphy is a technique in observational astronomy that uses a coronagraph to block or reduce the light from a bright star in order to observe nearby faint objects, such as exoplanets, circumstellar disks, or the solar corona. The term originates from the coronagraph invented by Bernard Lyot in 1939 to study the Sun’s eclipsed corona, and the approach has since been adapted for stellar and solar observations.

Principle and components: A typical coronagraph places an occulting mask in the focal plane to suppress the star’s light. A subsequent pupil stop, or Lyot stop, reduces diffracted light from the telescope aperture. Modern implementations may use apodized masks, phase-mask coronagraphs, or vortex structures to improve contrast and allow smaller inner working angles. Performance is described by achievable contrast (brightness ratio) and inner working angle (minimum angular separation between star and companion).

Applications: Solar coronagraphs, such as LASCO on the SOHO mission, study the solar corona and coronal mass ejections. In stellar astronomy, coronagraphy enables direct imaging of exoplanets and circumstellar disks. Space-based facilities (Hubble, JWST, and planned missions like Roman Space Telescope) and ground-based extreme adaptive optics systems (SPHERE, GPI, SCExAO) employ coronagraphs to reach high-contrast imaging.

Challenges and developments: Residual starlight and speckle noise, optical aberrations, and telescope diffraction limit achievable contrast. Ongoing innovations include adaptive optics, differential imaging techniques (ADI, SDI, RDI), and more sophisticated masks to push inner working angles toward the diffraction limit.