Welcome to the SCExAO project webpage!
Image of various components in SCExAO: (Left) 2000 actuator deformable mirror (middle) IR/visible dichroic splitter, fixed pupil
mask and delivery fiber for internal calibration source (foreground) (Right) Starbowl wheel.
Applying to use SCExAO? See science ready capabilities
under the "current capabilities" link for details
The SCExAO acronym stands for Subaru Coronagraphic Extreme Adaptive Optics. 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 a separation corresponding to the diffraction limit of the telescope in the near infrared, more specifically in the y-K-bands, at 0.95-2.4 μm. Two other large scale projects with comparable science goals are currently being assembled: The Gemini Planet Imager (GPI) that is currently being commissioned at the Gemini South Telescope and the SPHERE for the VLT. Instead of competing with these two heavyweight projects, SCExAO is trying to offer complementary capabilities, by focusing on very small angular separation: 40 - 500 milli-arcseconds.
SCExAO can do this because it implements several a high efficiency, low inner working angle coronagraphs. The workhorse of these units is the 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 (1 λ/D). The PIAA used in SCExAO was designed to provide a raw-contrast of 106 at 1.5 λ/D. In addition SCExAO offers other low inner working angle coronagraphs such as the Vortex, Four-Quadrant Phase Mask and 8-Octant Phase Mask versions. In addition it also offers a shaped pupil coronagraph for high contrast work where the inner working angle can be relaxed.
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 is/has developed a series of appropriate measures, for wavefront sensing and control. SCExAO uses:
- The Subaru Telescope facility AO system called AO188, operating at visible wavelengths, 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 to fine tune the tip/tilt.
- A visible pyramid-based High Order Wavefront sensor (only available in the second phase of the project) to cancel out the dynamic atmospherically 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. This also offers the ability for precise astrometry.
SCExAO has been undergoing commissioning of various modules since 2011. Most runs have been hampered by weather but the few that haven't, have been very successful. The modules/features that have been commissioned and can now be offered for open use are listed in the following section. Beneath this is a list of the modules we are currently engineering and hope to offer in the not to distant future.
Image of SCExAO mounted at the Nasmyth IR platform. AO188 can be seen to the left (large black box), SCExAO is the
dual level instrument in the middle (black and white panels) and HiCIAO is to the right of the image (Blue instrument).