Seeing measurements with the Merak Hα Telescope

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 Hα 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 r0 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 r0 only when the seeing-limited point spread function is sampled at a rate worse than two pixels per seeing-limit, implying that r0 estimated in the low-resolution mode is unreliable beyond r0=10 cm. The non-ideal telescope point spread function tends to reduce the estimated r0-values significantly only when the RMS wave-front error of the aberrated pupil is worse than λ /14 . The median seeing at Merak estimated from data spanning over five months is 2.7 arcseconds.