Lyot Coronagraphy on Giant Segmented-Mirror Telescopes

Abstract

We present a study of Lyot style (i.e., classical, band-limited, and Gaussian occulter) coronagraphy on extremely large, highly-segmented telescopes. We show that while increased telescope diameter is always an advantage for high dynamic range science (assuming wavefront errors have been corrected sufficiently well), segmentation itself sets a limit on the performance of Lyot coronagraphs. Diffraction from inter-segment gaps sets a floor to the achievable extinction of on-axis starlight with Lyot coronagraphy. We derive an analytical expression for the manner in which coronagraphic suppression of an on-axis source decreases with increasing gap size when the segments are placed in a spatially periodic array over the telescope aperture, regardless of the details of the arrangement. A simple Lyot stop masking out pupil edges produces good extinction of the central peak in the point-spread function (PSF), but leaves satellite images caused by inter-segment gaps essentially unaffected. Masking out the bright segment gaps in the Lyot plane with a reticulated mask reduces the satellite images'intensity to a contrast of 5x10^{-9} on a 30 m telescope with 10 mm gaps, at the expense of an increase in the brightness of the central peak. The morphology of interesting targets will dictate which Lyot stop geometry is preferable: the reticulated Lyot stop produces a conveniently uni-modal PSF, whereas a simple Lyot stop produces an extended array of satellite spots. A cryogenic reticulate Lyot stop will also benefit both direct and coronagraphic mid-IR imaging.