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Image Quality Specification for Solar Telescopes


Modern large ground-based solar telescopes are invariably equipped with adaptive optics systems to enhance the high angular resolution imaging and spectroscopic capabilities in the presence of Earth’s atmospheric turbulence. The quality of the images obtained from these telescopes cannot be quantified with the Strehl ratio or other metrics that are used for nighttime astronomical telescopes directly. In this paper, we propose to use the root mean square (rms) granulation contrast as a metric to quantify the image quality of ground-based solar telescopes. 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 (\(r_{0}\)), 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.

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Data Availability

The authors received the simulated solar granulation images from another source (Prof. S. P. Rajaguru of the Indian Institute of Astrophysics) and therefore cannot share the same. The method of generation of phase screens has been described in detail in Unni. C et al. (2021) and can be used to reproduce the phase screens.


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We would like to thank the referee for their helpful comments. We would also like to thank Prof. S. P. Rajaguru of the Indian Institute of Astrophysics for providing the solar granulation images which were used for the simulations. This research has made use of the High Performance Computing (HPC) resources ( and made available by the Computer Center of the Indian Institute of Astrophysics, Bangalore.

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First and corresponding author did the simulation in Python with verbal guidance from second author. First author also generated all the plots. Both authors wrote and reviewed the manuscript together with a few iterations.

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Correspondence to Saraswathi Kalyani Subramanian.

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The authors declare no competing interests.

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Subramanian, S.K., Rengaswamy, S. Image Quality Specification for Solar Telescopes. Sol Phys 298, 15 (2023).

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