Detection of discontinuous phase steps with a pyramid wavefront sensor

The next generation of large telescopes will use segmented primary mirrors. These segments must be co-aligned to a small fraction of the optical wavelength. Any difference in piston, tip, and tilt will cause a degradation of the final image. In this paper, we investigate some methods to detect, in particular, the discontinuous optical phase steps at the edges between adjoining segments. For this experiment, we use the GHOST bench developed by the ESO Adaptive Optics Systems group. This bench includes a Spatial Light Modulator (SLM) which generates known wavefronts at high resolution and a 4-facet bulk pyramid used as a wavefront sensor. The SLM emulates physical phase plates, and specifically, misaligned segmented mirrors. The measurements of the emulated phase plates are then compared to the results when using physical phase plates. This work is performed in the context of CaNaPy, which will carry out an on-sky test of detecting misalignments of segmented mirrors.