Solar Multi-Conjugate Adaptive Optics

Abstract

Ground-based telescopes are severely limited in their performance by the turbulence of the Earth’s atmosphere. The wavefronts emitted by the celestial bodies are perturbed and we lose high spatial frequency information. One solution to overcome the deleterious effects is to use adaptive optics (AO) systems to compensate for the distortions in the wavefront in real-time. They use a wavefront sensor to measure the perturbations of the wavefront and an adaptive optical element like a deformable mirror (DM) to compensate for the perturbations. The simplest AO systems are Single Conjugate AO (SCAO) systems; they offer an AO-corrected field of view of about 10 - 15 arc-seconds. This is due to the lack of correlation of the turbulence experienced by wavefronts arriving from different angles on the sky. The angular region over which the wavefronts arriving from different directions have a significant correlation is called the isoplanatic angle. When studying extended objects like the Sun, it would be beneficial if the AO-corrected field of view is larger than that offered by SCAO systems. This can be done by implementing a Multi-Conjugate AO (MCAO) system. MCAO systems use two or more DMs that are conjugated to distinct layers of the Earth’s atmosphere and they can offer AO correction over 1 arc-minute fields. With plans for larger ground-based telescopes like the 2 m National Large Solar Telescope (NLST), it is essential that we develop the expertise in solar AO and related technology by developing and testing them on existing smaller telescopes. One of the crucial parameters in the design of an MCAO system is the height of the strong layer of turbulence above the site to which the second DM will be conjugated. We need to study the vertical distribution of turbulence strength (C 2N (h)) at the site to identify this layer. The first part of this thesis deals with C2N (h) measurements in the daytime. We have used an optical method called S-DIMM+ (Solar-Differential Image Motion Monitor+) that uses the motion of images obtained with a 2-D array of lenslets to estimate the C2N profile. We have performed extensive simulations to study the performance of this technique with our system parameters. Following this, we also carried out the experiment at the Kodaikanal Observatory to estimate the C 2N(h) up to ∼6 km. We identified the presence of a strong turbulence layer at about 3 km above the site. Near-simultaneously, we have used balloon-borne temperature sensors to measure the near-Earth turbulence up to a maximum of 350 m. The results agreed with earlier seeing-measurements at the observatory. These two experiments are the first daytime profiling campaigns at the Kodaiakanal Observatory’s 125 year history. The second part of this thesis deals with the measurement of the isoplanatic angle during the daytime. We used an iterative deconvolution method to estimate the isoplanatic angle from the long-exposure seeing-limited images taken with the 20 cm H-α telescope at Merak to be about 15′′ − 20′′. The next part of the thesis details the first designs of SCAO and MCAO systems for KTT. Some preliminary work on the code for wavefront sensor has been completed and this is also discussed. Finally, in the last part of this thesis, we have developed simulations to quantify the performance of solar telescopes and associated AO systems. Traditional image quality metrics like Strehl ratio and encircled energy require knowledge of the point spread function (PSF). During the daytime, due to the extended nature of the Sun, we do not have access to the PSF. Therefore, we have performed extensive simulations in Python to quantify the performance of solar AO systems using rms granulation contrast as the metric. We obtain semi-logarithmic plots indicating the correspondence between the Strehl ratio and the rms granulation contrast for most practical values of the telescope diameters (D) and the atmospheric coherence diameters (r0), for various levels of adaptive optics compensation. We estimate the efficiency of a few working solar adaptive optics systems by comparing the results of our simulations with the Strehl ratio and rms granulation contrast published by these systems. Our results can be used in conjunction with a plausible 50% system efficiency to predict the lower bound on the rms granulation contrast expected from ground-based solar telescopes.