TPF-O (Terrestrial Planet Finder - Occulter, Fig. 8.22) concept came from the realization than an internal coronagraph (as in the TPF-C design) requires both the wavefront correction and control to be of very high quality to avoid leakage of the stellar light. The consequence is a complex design with adaptive optics to correct wavefront residuals. By contrast, the use of an external occulter permits the elimination of the major part of the light before it enters the telescope.
The main difficulty of this 'pure' separate-occulter version is the requirement for the manufacture of a perfect apodizing and occulting device, so that the stellar residual is reduced to a level that allows off-axis observation. The most intensively studied design is a mix of an occulting device and a coronagraphic device.
Formation flying of the two spacecraft is also a key issue in obtaining optimal pointing of the instrument.
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