Two-mirror apodization for high-contrast imaging

Direct detection of extrasolar planets will require imaging systems capable of unprecedented contrast. Apodized pupils provide an attractive way to achieve such contrast, but they are difficult, perhaps impossible, to manufacture to the required tolerance, and they absorb about 90% of the light in order to create the apodization, which of course lengthens the exposure times needed for planet detection. A recently proposed alternative is to use two mirrors to accomplish the apodization. With such a system, no light is lost. In this paper, we provide a careful mathematical analysis, using one-dimensional mirrors, of the on-axis and off-axis performance of such a two-mirror apodization system. There appear to be advantages and disadvantages to this approach. In addition to not losing any light, we show that the nonuniformity of the apodization implies an extra magnification of off-axis sources and thereby makes it possible to build a real system with about half the aperture that one would otherwise require or, equivalently, resolve planets at about half the angular separation as one can achieve with standard apodization. More specifically, ignoring pointing error and stellar disk size, a planet at 1.7λ/D ought to be at the edge of detectability. However, we show that the non-zero size of a stellar disk pushes the threshold for high contrast so that a planet must be at least 2.5λ/D from its star to be detectable. The off-axis analysis of two-dimensional mirrors is left for future study.