First On‐Sky High‐Contrast Imaging with an Apodizing Phase Plate

We present the first astronomical observations obtained with an ApodizingPhase Plate (APP). The plate is designed to suppress the stellar diffractionpattern by 5 magnitudes from 2-9 lambda/D over a 180 degree region. Stellarimages were obtained in the M' band (4.85 microns) at the MMTO 6.5m telescope,with adaptive wavefront correction made with a deformable secondary mirrordesigned for low thermal background observations. The measured PSF shows a halointensity of 0.1% of the stellar peak at 2 lambda/D (0.36 arcsec), tapering offas r^{-5/3} out to radius 9 lambda/D. Such a profile is consistent withresidual errors predicted for servo lag in the AO system. We project a 5 sigma contrast limit, set by residual atmosphericfluctuations, of 10.2 magnitudes at 0.36 arcsec separation for a one hourexposure. This can be realised if static and quasi-static aberrations areremoved by differential imaging, and is close to the sensitivity level set bythermal background photon noise for target stars with M'>3. The advantage ofusing the phase plate is the removal of speckle noise caused by the residualsin the diffraction pattern that remain after PSF subtraction. The APP giveshigher sensitivity over the range 2-5 lambda/D compared to direct imagingtechniques.Comment: 22 pages, 5 figures, 1 table, ApJ accepte