First on-sky high-contrast imaging with an apodizing phase plate

We present the first astronomical observations obtained with an apodizing phase plate (APP). The plate is designed to suppress the stellar diffraction pattern by 5 mag from 2 - 9λ/D over a 180° region. Stellar images were obtained in the M′ band (λ_c = 4.85 μm) at the MMTO 6.5 m telescope, with adaptive wave-front correction made with a deformable secondary mirror designed for low thermal background observations. The measured point-spread function (PSF) shows a halo intensity of 0.1% of the stellar peak at 2λ/D (0.36″), tapering off as r^-5/3 out to radius 9λ/D. Such a profile is consistent with residual errors predicted for servo lag in the AO system. We project a 5 σ contrast limit, set by residual atmospheric fluctuations, of 10.2 mag at 0.36″ separation for a 1 hr exposure. This can be realized if static and quasi-static aberrations are removed by differential imaging, and is close to the sensitivity level set by thermal background photon noise for target stars with M′ > 3. The advantage of using the phase plate is the removal of speckle noise caused by the residuals in the diffraction pattern that remain after PSF subtraction. The APP gives higher sensitivity over the range (2-5) λ/D than direct imaging techniques.