Lyot Coronagraphy on Giant Segmented-Mirror Telescopes

We present a study of Lyot style (i.e., classical, band-limited, and Gaussianocculter) coronagraphy on extremely large, highly-segmented telescopes. We showthat while increased telescope diameter is always an advantage for high dynamicrange science (assuming wavefront errors have been corrected sufficientlywell), segmentation itself sets a limit on the performance of Lyotcoronagraphs. Diffraction from inter-segment gaps sets a floor to theachievable extinction of on-axis starlight with Lyot coronagraphy. We derive ananalytical expression for the manner in which coronagraphic suppression of anon-axis source decreases with increasing gap size when the segments are placedin a spatially periodic array over the telescope aperture, regardless of thedetails of the arrangement. A simple Lyot stop masking out pupil edges producesgood extinction of the central peak in the point-spread function (PSF), butleaves satellite images caused by inter-segment gaps essentially unaffected.Masking out the bright segment gaps in the Lyot plane with a reticulated maskreduces the satellite images'intensity to a contrast of 5x10^{-9} on a 30 mtelescope with 10 mm gaps, at the expense of an increase in the brightness ofthe central peak. The morphology of interesting targets will dictate which Lyotstop geometry is preferable: the reticulated Lyot stop produces a convenientlyuni-modal PSF, whereas a simple Lyot stop produces an extended array ofsatellite spots. A cryogenic reticulate Lyot stop will also benefit both directand coronagraphic mid-IR imaging.Comment: 4 pages, 2 figure