PROCEEDINGS ARTICLE | September 21, 2012

Proc. SPIE. 8442, Space Telescopes and Instrumentation 2012: Optical, Infrared, and Millimeter Wave

KEYWORDS: Actuators, Point spread functions, Telescopes, Mirrors, Wavefronts, Adaptive optics, Coronagraphy, Space telescopes, Planets, James Webb Space Telescope

The main components of the SPICA coronagraphic instrument have initially been bar-code apodizing masks,
i.e. shaped pupils optimized in one dimension. Their free-standing designs make them manufacturable without
a glass substrate, which implies an absolute achromaticity and no additional wavefront errors. However, shaped
pupils can now be optimized in two dimensions and can thus take full advantage of the geometry of any arbitrary
aperture, in particular obstructed apertures such as SPICA's. Hence, 2D shaped pupils often have higher
throughputs while offering the same angular resolutions and contrast. Alternatively, better resolutions or contrast
can be obtained for the same throughput. Although some of these new masks are free-standing, this property
cannot be constrained if the optimization problem has to remain convex linear. We propose to address this
issue in different ways, and we present here examples of freestanding masks for a variety of contrasts, and
inner working angles. Moreover, in all other coronagraphic instruments, contrast smaller than 10<sup>-5</sup> can only
be obtained if a dedicated adaptive optics system uses one or several deformable mirrors to compensate for
wavefront aberrations. The finite number of actuators sets the size of the angular area in which quasi-static
speckles can be corrected. This puts a natural limit on the outer working angle for which the shaped pupils
are designed. The limited number of actuators is also responsible for an additional diffracted energy, or quilting
orders, that can prevent faint companions to be detected. This effect can and must be taken into account in
the optimization process. Finally, shaped pupils can be computed for a given nominal phase aberration pattern
in the pupil plane, although the solutions depend in this case on the observation wavelength. We illustrate this
possibility by optimizing an apodizer for the James Webb space telescope, and by testing its chromaticity and
its robustness to phase changes.