Extreme adaptive optics imaging with a clear and well-corrected off-axis telescope subaperture

Rather than using an adaptive optics (AO) system to correct a telescope's entire pupil, it can instead be used to more finely correct a smaller subaperture. Indeed, existing AO systems can be used to correct a subaperture 1/3 to 1/2 the size of a 5-10 m telescope to extreme adaptive optics (ExAO) levels. We discuss the potential performance of a clear off-axis well-corrected subaperture (WCS), and describe our initial imaging results with a 1.5 m diameter WCS on the Palomar Observatory's Hale Telescope. These include measured Strehl ratios of 0.92-0.94 in the infrared (2.17 mu m) and approximate to 0.12 in the B band, the latter allowing a binary of separation 0.34" to be easily resolved in the blue. Such performance levels enable a variety of novel observational modes, such as infrared ExAO, visible-wavelength AO, and high-contrast coronagraphy. One specific application suggested by the high Strehl ratio stability obtained (1%) is the measurement of planetary transits and eclipses. Also described is a simple "dark hole'' experiment carried out on a binary star, in which a comatic phase term was applied directly to the deformable mirror, in order to shift the diffraction rings to one side of the point-spread function.