A New Coronagraph for NAOS-CONICA -- the Apodising Phase Plate

The Messenger 141 – September 2010 use light diffracted from the Airy core of the central star to cancel out the coherent light in the outer diffraction rings. Small sinusoidal ripples of phase added to the incoming wavefront act like a simple dif­ fraction grating, creating a pair of “speck­ les” that can be adjusted to cancel out diffraction on one side of the star, but reinforcing it on the opposite side. Mathe­ matically adding many of these virtual gratings together forms the resultant APP pattern, seen in Figure 2. Its effect on the Very Large Telescope (VLT) point spread function (PSF) is seen in the com­ missioning data in Figure 3. The APP optical element itself is shown in Figure 4 prior to installation.