Tip-tilt error in Lyot coronagraphs

The direct detection of extrasolar planets by imaging means is limited by the large flux of light from the host star being scattered into the region of interest by a variety of processes, including diffraction. Coronagraphs are devices that suppress the undesirable scattering of light caused by diffraction. In a coronagraph the sensitivity limit for high dynamic range is limited by the propagation of errors introduced by the imperfect optical system to the final image. In this paper we develop theory and simulations to understand how such errors propagate in a coronagraph. We describe the response of classical and band-limited Lyot coronagraphs to small and large errors in the placement of the central star, and identify ways of making such coronagraphs more robust to small guiding errors. We also uncover features of the decentered point-spread function that can lead to the spurious detection of companions, especially with aggressive, high dynamic range coronagraphs dedicated to companion searches aimed at finding extrasolar terrestrial or Jovian planets.