Phase Induced Amplitude Apodization (PIAA) coronagraphy

PIAA is a lossless beam apodization technique. Beam apodization is very useful in coronagraphy: an apodized pupil produces a high contrast image free of Airy ring. The conventional method to apodize the pupil is to introduce in the beam a mask which is fully transmissive in the center and becomes opaque at the edge of the pupil. With PIAA, the same apodized pupil is created by geometric redistribution of the light rather than selective absorption. This is achieved by aspheric optics (mirrors or lenses), as shown in the figure on the left.

Advantages of PIAA for coronagraphy

The Airy rings produced with a conventional imaging telescope are due to diffraction originating from the sharp edges of the pupil. Pupil apodization, by creating a soft-edged pupil, can therefore greatly reduce these rings, and can be used either by itself or in combination with other coronagraph techniques (for example, the apodized pupil Lyot coronagraph combined pupil apodization with Lyot coronagraphy). The conventional technique used to apodize the telescope beam is with an amplitude mask (continuous or binary) with variable transmission from the center to the edge of the pupil. This leads to a difficult compromise between reaching high contrast (which requires a strong apodization) and achieving high coronagraph throughput & good angular resolution (which favor a weak apodization). In a conventional apodized pupil coronagraph, for example, reaching 10^-10 contrast requires a strong apodization with a 10% throughput which degrades the telescope's angular resolution to approximately 2.5 λ/D (instead of 1 λ/D) and does not allow high contrast imaging within approximately 4 λ/D.

With PIAA optics, strong apodizations can be achieved with no loss in throughput or angular resolution, enabling high contrast imaging at small angular separation from the optical axis (from 0.64 λ/D for an aggressive PIAACMC design to 2 λ/D for a more conventional PIAA design - design choice depends on the goal contrast, ability to mitigate chromatic issues and angular size of the central source) with almost no loss in efficiency. PIAA does not absorb light, and it therefore preserves the sensitivity and angular resolution of the telescope. When using mirrors, PIAA can be made to operate at high contrast over a wide spectral band.

The performance gain offered by PIAA for detection and characterization of exoplanets over other coronagraph is quantified in Guyon et al. 2006. Compared to the more conventional coronagraphs which were considered for TPF-C, adopting the PIAA is equivalent to a 2x to 3x gain in telescope diameter. Recent improvements on the PIAA concept may allow even higher performance, with high contrast detection of exoplanets closer in than 2 λ/D, as shown in the PIAACMC description.

More information

For more information, you may visit the following pages:

PIAA system architecture. This link describes how to build a PIAA-based coronagraph system, and how to integrate wavefront control within such a system.
The PIAA complex mask coronagraph (PIAACMC) is a high performance coronagraph using PIAA optics. This study explores how PIAA can be hybridized with Lyot coronagraphs for maximum performance.
PIAA aspheric optics design: how to design PIAA optics.

More information on the PIAA coronagraph concept can also be found in PIAA-related publications webpage.

Applications

PIAA coronagraph for ground-based Extreme-AO systems

The PIAA technique is attractive for direct imaging of exoplanets with ground-based telescopes, as it can provide (together with an Extreme-AO system), high contrast at small angular separation without loss in efficiency. The PIAA coronagraph is part of the Subaru Coronagraphic Extreme-AO (SCExAO) system which is soon (end 2010) to be deployed on the Subaru Telescope.

Proposed space missions using PIAA

The Pupil mapping Exoplanet Coronagraphic Observer (PECO) mission concept uses a coronagraphic 1.4-m space-based telescope to both image and characterize extra-solar planetary systems at optical wavelengths
EXCEDE (PI: G. Schneider, University of Arizona) uses a small (<1m) optical telescope to image debris disks and massive planets around nearby stars.

Current status of technology development efforts: PIAA laboratory testbeds

PIAA technologies have been developed through laboratory demonstrations and modeling since late 2002.

The Subaru Telescope PIAA system laboratory prototype (2003 - Feb 2009)

A PIAA coronagraph testbed effort was initiated at Subaru Telescope / Research Corporation of the University of Hawaii in 2003, and was operated with funding from NASA JPL and Subaru Telescope/NAOJ. This monochromatic testbed in air included reflective PIAA optics, a 32x32 MEMS deformable mirror for wavefront control, and a coronagraphic low order wavefront sensor (CLOWFS) for accurate pointing/focus measurement. This testbed demonstrated 2e-7 raw contrast in monochromatic light at 1.65 λ/D separation in air and a 1e-3 λ/D closed loop pointing control. Raw contrast performance was limited by incoherent ghosts in the system, which used several non AR-coated lenses. The testbed however demonstrated control of coherent light to the 3.5e-9 contrast level in long time-averaged integrations. The Subaru testbed effort was discontinued in early 2009 and its final results have been compiled in Guyon et al. 2009 .

For more info on this system, see The Subaru PIAA system laboratory prototype webpage.

The NASA Ames Coronagraph Experiment (2008 - present)

A new testbed effort dedicated to PIAA technology development was initiated at NASA Ames in 2008. The testbed, optimized to provide flexibility and rapid testing of new configurations/components, operates in air and has recently (early 2010) reached raw contrasts of 1e-7 at 2 λ/D with a lensbased PIAA system (Belikov et al. 2009).
See this link for an introduction to the testbed.

The High Contrast Imaging Testbed (HCIT) PIAA table at the Jet Propulsion Laboratory (March 2009 - present)

The HCIT is a vacuum facility to demonstrate high contrast coronagraphy. The PIAA is currently (2010) one of the techniques tested at HCIT.

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