High Dynamic Range and the Search for Planets
  General arguments for optimized coronagraphy in the search for planets are presented. First, off-axis telescopes provide the best telescopic platforms for use with coronagraphy, and telescope fabrication technology now allows the fabrication of such telescopes with diameters of up to 6.5 m. We show that in certain circumstances a smaller telescope with an off-axis primary has a signal-to-noise advantage compared with larger Cassegrain telescopes. Second, to fully exploit the advantages of the coronagraph for suppressing stray light, it is necessary to use a high Strehl ratio adaptive optics system. This can be best achieved initially with modest aperture telescopes of 3--4 m in diameter. Third, application of simultaneous differential imaging and simultaneous polarimetric techniques are required to reach the photon-limit of coronagraphic imaging. These three developments, if pursued together, will yield significant improvements in the search for planets. 
A. T. Tokunaga, C. Ftaclas, J. R. Kuhn, and P. Baudoz