AO4ELT5 Proceedings

The use of an axicon beam-shaping element in non-modulated pyramid wavefront sensors

Allain, Guillaume (Université Laval)

Coronagraphs designed for high contrast imaging need high performance adaptive optics correction in order to reach typical contrast ratios requirements of about $10^{-9}$. The use of non-modulated pyramid wavefront sensors allows adaptive optics systems to measure wavefront errors with a high sensitivity, allowing such high contrast ratios. The lack of modulation, however, makes them more sensitive to the pyramid defects. It is notably difficult to find a pyramid on which the apex is manufactured to a high enough quality to allow its use in a non-modulated system. We propose an innovative way to counter this problem, in the situation where the telescope has a central obscuration, by adding known non-common path aberrations to the wavefront by means of a beam shaping optical component. This component (e.g. axicon) is placed at a position conjugate to the telescope pupil plane and the resulting image plane is incident onto a standard pyramid. This element shapes the point spread function (PSF) from a spatially compact point to a shape that has minimal light at the center. This insures that next to no light is incident on the pyramid apex where most of the defects are present. The images of the sub-pupils are then used to retrieve the wavefront aberration using a similar way to what is done with a typical pyramid wavefront sensor.

DOI: 10.26698/AO4ELT5.0114- Proceeding PDF

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