Adaptive optics (AO) is widely used in optical/near-infrared telescopes to remove the effects of atmospheric distortion, and laser guide stars (LGSs) are commonly used to ease the requirement for a bright, natural reference source close to the scientific target in an AO system. However, focus anisoplanatism renders single LGS AO useless for the next generation of extremely large telescopes. Here, we describe proof-of-concept experimental demonstrations of a LGS alternative configuration, which is free of focus anisoplanatism, with the corresponding wavefront sensing and reconstruction method, termed projected pupil plane pattern (PPPP). This laboratory experiment is a critical milestone between the simulation and on-sky experiment, for demonstrating the feasibility of PPPP technique and understanding technical details, such as extracting the signal and calibrating the system. Three major processes of PPPP are included in this laboratory experiment: the upward propagation, return path, and reconstruction process. From the experimental results, it has been confirmed that the PPPP signal is generated during the upward propagation and the return path is a reimaging process whose effect can be neglected (if the images of the backscattered patterns are binned to a certain size). Two calibration methods are used: the theoretical calibration is used for the wavefront measurement, and the measured calibration is used for closed-loop control. From both the wavefront measurement and closed-loop results, we show that PPPP achieves equivalent performance to a Shack–Hartmann wavefront sensor.
PPPP, Pupil Plane Projected Pattern, is a LGS alternative (described more fully in the paper by Yang, this meeting #10703-26) which is inherently free of focal anisoplanatism. It has other practical and scientific advantages, but it is the disadvantages that this paper concentrates on since they are foremost when considering a real implementation. An on-sky test of the technique is funded and here we describe progress in solving the fundamental questions for any new technique: how to actually do it at a real telescope? Our targeted platform is a Nasmyth platform of the 4.2m WHT on La Palma. We discuss the difficulties of projecting an afocal beam from the primary mirror without causing excessive back-reflections/-scatter, which drowns the beam-profile, and instead suggest two alternative experiments. By splitting the validation of PPPP on-sky into two parts, each experiment can address a separate aspect of the validation without the disadvantage of trying to “do it all” within one experiment.
For the next generation of extremely large telescopes with the primary mirrors over 30 m in diameter, focal anisoplanatism renders single laser guide star AO useless. The laser tomography AO (LTAO) technique demonstrates an effective approach to reduce focal anisoplanatism, although it requires multiple LGSs & WFSs, and complex tomographic reconstruction. Here we propose a novel LGS alternative configuration with the corresponding wavefront sensing and reconstruction method, termed Projected Pupil Plane Pattern (PPPP). A key advantage of this method is that a single collimated beam is launched from the telescope primary mirror, and the wavefront sensed on the uplink path, which will not suffer from the effects of focal anisoplanatism. In addition, the power density of the laser beam is significantly reduced compared to a focused LGS, which decreases aircraft and satellite safety hazards. A laboratory experiment for PPPP has been setup to anchor the PPPP concept and compare against a Shack-Hartmann WFS.
Speckle imaging techniques are effective post-processing methods to eliminate atmospheric perturbations on the imaging
of space objects, in which speckle interferometry and bispectrum methods are usually used to estimate the magnitude and
phase spectrum of the objects separately. The spectral ratio technique used in this paper is convenient and efficient to
evaluate r0, which is crucial for calibrating the speckle transfer function in the magnitude estimation. It is shown that
power spectrum, the second moment of the magnitude spectrum, needs bias removal whereas bispectrum processing does
not. Reconstructed images from the observed data of binary stars and Jupiter are presented.