Optical performance and phase error budgeting of future ELTs

摘要

Next generation giant telescopes are under study around the world, with great variety in size and pupil segmentation scheme. We present performance calculations for different pupil segmentation geometries, in the presence of segment surface errors and residual atmospheric phase errors after Adaptive Optics correction. Optical performance is evaluated through point-spread function (PSF) calculation. Pupil segmentation parameters include segment size and shape, gap diffraction, and segment surface errors. We consider large, 8m-class, circular or polygonal segments, and small, 2m-class, hexagonal segments. These options represent the choices of the different telescopes design groups. Our segmentation scheme consists of eight polygonal petals forming a filled octagonal pupil. All segment and pupil edges are along four unique directions, minimizing the number of diffraction spikes in the PSF, creating large areas of low levels of scattered light close to the core. This is important for high-dynamic range imaging. Comparison between polygonal and circular petals shows that, in addition to the presence of low-scatter areas, the encircled energy is higher. One of the most challenging goal for these future ground-based telescopes is the exo-planet detection and observation. The integration in a single model of the telescope pupil shape, aberrations on the mirrors, AO residual wavefront errors provides an evaluation of the capacity of future extremely large telescopes in the key domain of high dynamical range imaging.