Welcome to the SCExAO Project webpage!

SCExAO project photo during assembly after migration onto the optical table that will be used at the summit (Oct 2009).

Introduction

The SCExAO acronym stands for Subaru Telescope Extreme Adaptive Optics project. It is an active, ongoing effort to equip the Subaru Telescope with a high performance coronagraph and a series of wavefront control solutions that make an optimal use of the angular resolution that an 8-meter telescope has to offer. The ultimate science goal of SCExAO is the direct imaging of extrasolar planets around stars at the limit of diffraction in the near infrared, more specifically in the H-band, at 1.6 microns. Two other large scale projects with comparable science goal are currently being assembled: The Gemini Planet Imager (GPI) that will equip the Gemini South Telescope in 2012, and the Sphe (SPHERE) for the VLT. Instead of competing with these two heavyweight projects, SCExAO is trying to offer complementary capability, by focusing on very small angular separation: 40 - 500 milli-arcseconds.

SCExAO can do this because it implements a high efficiency coronagraph called PIAA (Phase Induced Amplitude Apodization), invented by Olivier Guyon (SCExAO project PI), that exhibits an inner working angle that is as close to the diffraction limit as you can get. The PIAA used in SCExAO was designed to provide a raw-contrast of 10e6 at 1.5 lambda/D.

While reaching such a high level of contrast has already been demonstrated in the well controlled environment of the laboratory, it is unlikely to be achieved at the telescope, as wavefront aberrations induced by the atmosphere and the optics of the telescope quickly degrade image quality. To remedy this, SCExAO has developped a series of appropriate measures, for wavefront sensing and control. SCExAO uses:

• The Subaru Telescope facility AO system called AO188, operating at visible wavelength, to provide the initial wavefront correction, which considerably reduces the strain on the SCExAO components.
• A Coronagraphic Low-Order Wavefront Sensor (CLOWFS) for ultra-fine pointing control, operating in the IR and using the light rejected by the focal plane mask of the coronagraph.
• A phase diversity based calibration of the Optical Path Difference (OPD) responsible for the presence of static speckles in the field.
• A visible pyramid-based High Order Wavefront sensor called CHEOPS (only available in the second phase of the project) to cancel out the dynamic atmosphere induced aberrations responsible for the presence of fast speckles.
• Speckle probing techniques, that use commands sent to a deformable mirror to "probe" the speckles the coherence of the speckles in the field, and eventually erase them.

In September 2011, SCExAO had its first engineering night that wasn't entirely weathered out, and a couple basic capabilities of the system have been demonstrated. The SCExAO bench is currently ongoing an upgrade of the light injection mirror mounts, for better control of the image and pupil location. An external source is also currently being assembled for off-line performance evaluation of wavefront control techniques. The next SCExAO observing run is expected to take place after February 2012.