Limits on achievable intensity reduction with an optical occulter: comment

Abstract

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. A 28, 1668 (2011)] 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.

Limits on achievable intensity reduction with an optical occulter: reply to comment

“…This note is the response to the comments in [1] (hereinafter referred to as CL2011) where the authors analyzed the petal function introduced in [2] (hereinafter referred to as WS2011).…”

“…The main focus of WS2011 is twofold: (1) to bring attention to the limitation associated with the petal-style occulters due to implicit requirements for the sharpness of the radii at the petal tips and (2) to highlight the observation that, if the improvement in the intensity reduction is a result of using petal tips with increasing sharpness, care must be taken not to violate the general restriction imposed by the methodology of physical optics (PO) where the smallest structure in the geometry must be much larger than the wavelength. Thus, if the petal ends are modeled by smooth arcs, an intensity reduction of 10 orders appears possible only if the radii of the curvature at the petal tips are reduced to orders smaller than the wavelength.…”