Abstract
Context. The pyramid was proven to be a highly sensitive and versatile wave-front sensor (WFS) and has been selected to be installed on the single conjugate adaptive optics (AO) systems of the extremely large telescope (ELT). The pupil of the ELT is fragmented by the secondary support spider arms, which are larger than the spatial coherence length of the atmospheric turbulence. This causes a rupture of the incoming wavefront continuity, which means that we need to be able to measure the differential pistons across the spider arms to achieve full wavefront reconstruction. Aims. We investigate the reaction of the modulated pyramid WFS to discontinuous aberrations in presence of phase residuals after AO compensation for a range of expected observing conditions at the location of the ELT. We then explore some parameters of the sensor in order to improve its sensitivity to the wavefront discontinuities, including optical gain compensation, specific sensor modulation paths, and the number of faces of the pyramidal prism. Methods. We derived sensitivity loss and modal cross-talk strength coefficients around static post-AO residual phases using the COMPASS end-to-end AO simulation software. Moreover, extensive closed-loop AO simulations let us derive two wavefront error criteria that are appropriate for a fragmented pupil geometry. We used these to assess the wavefront reconstruction performance. Results. We show that on the ELT, the pyramid experiences a drastic loss in sensitivity and also non-linear modal cross-talks. Added to the limited capture range, this makes it poorly suited for phase discontinuity measurements at visible wavelengths. The strategies we studied to increase the sensitivity and reduce the modal cross-talk provide an improvement of the reconstruction for low D/r0(λWFS) values, that is, for a K-band pyramid. In presence of a large residual wavefront variance, however, a similar sensor in visible light fails to provide the necessary trade-off to measure both the continuous modes and the wavefront discontinuities. Conclusions. The ELT instruments, designed with only visible-band pyramid WFSs, will not be able to perform a direct measurement of the wavefront discontinuities. They will have to rely on Kolmogorov statistics to restore the continuity of the atmospheric corrugated wavefront. If any other source of discontinuities arises on the ELT, instruments will need an additional, dedicated WFS.
Highlights
The ability of adaptive optics (AO) to compensate for atmospheric distorted wavefronts and to enable a diffraction-limited beam is demonstrated on every large groundbased optical telescope
To investigate the properties of the P-wave-front sensor (WFS) response to petal modes, we first need to introduce an appropriate modal basis Bpetal that explicitly expresses petal modes to address and characterise their properties. This basis is orthogonal in the deformable mirror (DM) command space to ensure that the confusion between the modes we study reflects a confusion in the pyramid wavefront sensor (P-WFS) measurements and not from the command space
The pyramid wavefront sensor was selected for its great sensitivity and adaptive linearity range, but it exhibits a highly variable sensitivity to the phase steps across the telescope spider arms depending on the observing conditions
Summary
The ability of adaptive optics (AO) to compensate for atmospheric distorted wavefronts and to enable a diffraction-limited beam (or close to it) is demonstrated on every large groundbased optical telescope This technology has matured rapidly over the past three decades, and the AO community is currently paving the way for the class of extremely large telescopes, ranging from 20 m diameter and larger. While the arms provide the necessary structural stiffness, their width is larger than the spatial coherence length r0 of the perturbed wavefront, which is typically 10–15 cm at 500 nm at the ELT site of Cerro Armazones It results in a significant fragmentation of the optical pupil and in a break of the spatial continuity of the incoming wavefront phase
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
