Spiders in Lyot Coronagraphs

In principle, suppression of on-axis stellar light by a coronagraph is easier on an unobscured aperture telescope than on one with an obscured aperture. Recent designs such as the apodized pupil Lyot coronagraph, the ‘band-limited’ Lyot coronagraph, and several variants of phase mask coronagraphs work best on unobscured circular aperture telescopes. These designs are developed to enable the discovery and characterization of nearby jovian or even terrestrial exoplanets. All of today’s major space-based and adaptive optics-equipped ground based telescopes are obscured aperture systems, with a secondary mirror held in place by secondary support ‘spider’ vanes. The presence of a secondary obscuration can be dealt with by ingenious coronagraph designs, but the spider vanes themselves cause diffracted light that can hamper the search for jovian exoplanets around nearby stars. We look at the problem of suppressing spider vane diffraction in Lyot coronagraphs, including apodized pupil and band-limited designs. We show how spider vane diffraction can be reduced drastically, and in fact contained, in the final coronagraphic image, within one resolution element of the geometric image of the focal plane mask’s occulting spot. This makes adaptive optics coronagraphic searches for exojupiters possible on the next generation of adaptive optics systems being developed for 8–10 m class telescopes such as Gemini and the Very Large Telescopes. Subject headings: instrumentation: adaptive optics — instrumentation: high angular resolution — space vehicles: instruments — techniques: high angular resolution — planetary systems