The segmentation of the telescope pupil (by spiders & the segmented M4)
create areas of phase isolated by the width of the spiders on the wavefront
sensor (WFS), breaking the spatial continuity of the wavefront. The poor
sensitivity of the Pyramid WFS (PWFS) to differential piston leads to badly
seen and therefore uncontrollable differential pistons. In close loop
operation, differential pistons between segments will settle around integer
values of the average sensing wavelength. The differential pistons typically
range from one to ten times the sensing wavelength and vary rapidly over time,
leading to extremely poor performance. In addition, aberrations created by
atmospheric turbulence will contain large amounts of differential piston
between the segments. Removing piston contribution over each of the DM segments
leads to poor performance. In an attempt to reduce the impact of unwanted
differential pistons that are injected by the AO correction, we compare three
different approaches. We first limit ourselves to only use the information
measured by the PWFS, in particular by reducing the modulation. We show that
using this information sensibly is important but will not be sufficient. We
discuss possible ways of improvement by using prior information. A second
approach is based on phase closure of the DM commands and assumes the
continuity of the correction wavefront over the entire unsegmented pupil. The
last approach is based on the pair-wise slaving of edge actuators and shows the
best results. We compare the performance of these methods using realistic
end-to-end simulations. We find that pair-wise slaving leads to a small
increase of the total wavefront error, only adding between 20-45 nm RMS in
quadrature for seeing conditions between 0.45-0.85 arcsec. Finally, we discuss
the possibility of combining the different proposed solutions to increase
robustness.Comment: 12 pages, 15 figures, AO4ELT5 Proceedings, Adaptive Optics for
Extremely Large Telescopes 5, Conference Proceeding, Tenerife, Canary
Islands, Spain, June 25-30, 201