Limits on achievable intensity reduction with an optical occulter: comment.

An external occulter is used as a means to suppress starlight and enable the observation of faint, Earth-like planets. A recent paper in this journal [J. Opt. Soc. Am. A28, 1668 (2011)JOAOD60740-323210.1364/JOSAA.28.001668] suggested there were unrecognized limits on starlight intensity suppression with an external occulter. It claimed that, at the level of intensity reduction needed, tips of petal-shaped occulters are too small to be manufactured. We present analysis of that publication’s apodization function and show that it is mathematically distinct from the external occulter apodization functions being discussed in the literature. Their broad conclusion that the tips cannot be built is therefore invalid. They suggest that transmitting screens might be superior but do not explain why. We show their transmission function is substantially similar to those in use for the last five years and therefore may not represent an advance in the field.