Abstract
The parameter-search method (PSM) established by Rengaswamy, Ravindra, and Prabhu (Solar Phys. 294, 5, 2019) for measuring the terrestrial atmospheric seeing from long-exposure solar H\(\upalpha\) images is further validated – through simulations – to include the effects of i) intermediate exposure-time, exposures that are neither as small as 1 to 20 ms used in solar speckle imaging in the visible nor as long as 1 second; ii) under-sampling of the data (with respect to the telescope’s diffraction-limit), and iii) non-ideal telescope point spread function. It is then applied to a large dataset to obtain the statistics of Fried’s parameter \(r_{0}\) at Merak: a cold desert in the Himalayas. The bias due to intermediate exposure is well within the error budget. Under-sampling results in an underestimation of \(r_{0}\) only when the seeing-limited point spread function is sampled at a rate worse than two pixels per seeing-limit, implying that \(r_{0}\) estimated in the low-resolution mode is unreliable beyond \(r_{0} = 10\) cm. The non-ideal telescope point spread function tends to reduce the estimated \(r_{0}\)-values significantly only when the RMS wave-front error of the aberrated pupil is worse than \(\lambda/14\). The median seeing at Merak estimated from data spanning over five months is 2.7 arcseconds.
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Acknowledgements
We thank Stanzin Tundup for the performing observations and Angchuk Dorje for his constant support at Merak. We thank the DOT team for making the speckle reconstructed G-band images available for us to use in our simulation. We thank the reviewer for their illuminating comments, which enhanced the content significantly.
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Unni. C, V., Rengaswamy, S., Ravindra, B. et al. Seeing Measurements with the Merak H\(\upalpha\) Telescope. Sol Phys 296, 65 (2021). https://doi.org/10.1007/s11207-021-01807-3
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DOI: https://doi.org/10.1007/s11207-021-01807-3