Olivier Guyon/University of Arizona

Phase-Induced Amplitude Apodization (PIAA) is a high performance coronagraphic technique well suited for direct imaging and spectroscopic characterization of potentially habitable Earth-like planets from space. It delivers high contrast images by performing a lossless geometric remapping of the telescope beam with two aspheric mirrors, and simultaneously offers nearly full throughput, full angular resolution of the telescope, small inner working angle, high contrast and low chromatism. While the early PIAA systems (which are currently undergoing testing at several facilities) were designed to deliver high contrast at 2 λ/D, newer hybrid designs can theoretically deliver on a point source a 10^-10 contrast down to 0.64 λ/D inner working angle with full throughput, approaching limits imposed by fundamental principles. PIAA coronagraph designs offer nearly optimal coronagraphic solutions for a wide range of telescope size and astrophysical goals, from debris disks and giant planet imaging with a small size (<1-m) telescope to high efficiency spectroscopic characterization of Earth-like planets with a 4 to 8-m diameter optical telescope, for which PIAA reaches the fundamental inner working angle limit imposed by the stellar angular size.

Our team is currently funded by the NASA technology demonstration for exoplanet missions (TDEM) program to demonstrate high contrast performance of the PIAA concept in monochromatic light. We have achieved at 2 λ/D 3 x 10⁸ and 4 x 10^-8 contrast respectively at our NASA JPL and NASA Ames testbeds, and are expecting demonstration of 10^-9 contrast at 2 λ/D in monochromatic light by the end of the current funded effort. In parallel, on-sky operation of a PIAA system has started on the Subaru Telescope, and active sensing and control of pointing has been achieved in the laboratory to the 1-3 λ/D level required for 10^-10 coronagraphic detection with PIAA (corresponding to 0.1 mas on a 1.4-m visible telescope).

We are requesting NASA support to continue technology development of PIAA, with a focus on demonstrating high contrast in polychromatic light. Our primary goal, which will be our milestone #1, is to demonstrate 10^-9 raw coronagraphic contrast at 2 λ/D in a 10% wide band with a high throughput PIAA system with a flight-like configuration. This work is a direct continuation of current efforts, and will use existing facilities and hardware. NASA Ames will provide a high quality set of PIAA optics, recently manufactured to broadband light operation requirements, for this experiment, which will be conducted in the MAM chamber at JPL. In parallel to this effort, we will develop and test in air a new hybrid PIAA architecture with a significantly smaller inner working angle (1.3 λ/D for a point source free of wavefront aberrations) and apodizing mirrors easier to manufacture. Thanks to new low-cost PIAA optics manufacturing techniques and the use of a flexible easily reconfigurable testbed at NASA Ames, we will be able to rapidly develop this new concept. Our milestone #2 will be to demonstrate simultaneously in monochromatic light 10^-8 contrast and 50% transmission at 1.5 λ/D at the NASA Ames testbed in air.

A key challenge associated with achieving <2 λ/D IWA with our new PIAA designs is the management of chromatic issues: the two efforts will therefore be closely linked. The PIAA system using the high quality PIAA optics in the MAM chamber at JPL can be modified at little cost (change of an apodizing mask and a Lyot stop) to implement our new smaller IWA architecture. Following completion of our milestones #1 and #2, we will, time permitting, reduce the IWA of the PIAA coronagraph system in the MAM chamber below 2 λ/D while preserving the 10^-9 contrast in a 10% spectral bandwidth.