Simulation of imaging performance for extremely large segmented telescopes

摘要

Two characteristics of next generation extremely large ground-based telescopes are certain: (1) they will be segmented, with active control of segment position, tilt and, possibly, shape, and (2) they will have adaptive correction of atmospheric wavefront errors. Both these aspects place new constrains on wavefront sensing and wavefront correction. For quantitative analysis of the wavefront sensing requirements of these telescopes, it is necessary to analyze the effects of segmentation on image quality. To achieve this we are building a telescope simulator for calculating PSF and related image quality parameters: Strehl ratio, encircled energy, modulation transfer function, etc. A large number of segments (up to 10,000) may be simulated, each affected by piston and tilt errors. Piston and tilt errors are randomly generated to create realistic mirror configurations. Due to the large number of segments, interesting results are observed, both with respect to numerical statistics and diffraction. Strehl ratio as a function of rms piston error is found to depend upon the probability density function of the piston error, and the dominant effect in the case of segment tilt is that of diffraction from a 2D grating rather than the geometrically expected multiple image. Fine analysis of simulated point-spread functions allows a discussion of the pertinence of different image quality parameters for the case of segmentation errors.