Coronagraphs of SCExAO

This page is dedicated to the various coronagraphs SCExAO is equipped with.

PIAA: designed to apodize a centrally obstructed beam

Laser beam apodization

Red laser beam undergoing apodization.

The Phase Induced Amplitude Apodization (PIAA) uses a set of two tailored optics working as a pair: inserted in a collimated beam, the first changes the distribution of light, while the second recollimates the beam for an on-axis source. This concept has already been the subject of multiple publications. One novelty of the Subaru PIAA Coronagraph is that its design is adapted to the presence of a central obscuration.

The advantages of this approach over what is usually referred to as "classical" apodization is that instead of being absorbed by a mask with a radial transmission profile, the light gets redistributed. This preserves the throughput (which can be as low as 0.1 for a 10-10 contrast), and the angular resolution (indeed, a classical apodizing mask removes the light mostly at the edges of the pupil).

The picture at the top of this page shows a visible collimated laser beam being apodized by the PIAA. The set of lenses leaves the pupil diameter unchanged. As the light travels from left to right in the picture, the beam gets apodized and the central obscuration gradually disappears.

A low dispersion material was chosen (CaF2) to keep chromatic aberrations small. Unlike off-axis reflective systems, this refractive set of optics with circular symmetry are fairly easy to manufacture.

Photo of the
actual SCExAO PIAA lenses

CaF2 PIAA lenses used in SCExAO.