  One of the science drivers for the extremely large telescope (ELT) is imaging
and spectroscopy of exo-solar planets located as close as 20mas to their parent
star. The application requires a well thought-out design of the high contrast
imaging instrumentation. Several working coronagraphic concepts have already
been developed for the monolithic telescope with the diameter up to 8 meter.
Nevertheless the conclusions made about the performance of these systems cannot
be applied directly to the telescope of the diameter 30-100m. The existing
schemes are needed to be reconsidered taking into account the specific
characteristics of a segmented surface. We start this work with the classical
system ? Lyot coronagraph. We show that while the increase in telescope
diameter is an advantage for the high contrast range science, the segmentation
sets a limit on the performance of the coronagraph. Diffraction from
intersegment gaps sets a floor to the achievable extinction of the starlight.
Masking out the bright segment gaps in the Lyot plane although helps increasing
the contrast, does not solve completely the problem: the high spatial frequency
component of the diffractive light remains. We suggest using the Lyot stop
which acts on the light within gaps in order to produce the uniform
illumination in the Lyot plane. We show that for the diffraction limit regime
and a perfect phasing this type of coronagraph achieves a sufficient star light
extinction.
