Measurement of Astronomical Seeing Using Long Exposure Solar Images

Abstract

We extend our modified parameter search method (Sridharan, Dashora, and Venkatakrishnan, Solar Phys. 222, 35, 2004) of estimating the Fried parameter ( $r_{0}$ ) from long exposure images to the images obtained from the H $\alpha $ -telescope installed at Merak, a cold desert on the Himalayas, with a modified criterion – the contrast ratio of the observed and the deconvolved images – to identify the true $r_{0}$ . We validated our new method by applying it to the solar granulation data and found that it is as accurate as our modified parameter search method. The median seeing at H $\alpha $ – estimated from the data spanning over 5 months – is ≈ 2 arc-sec and it corresponds to a median $r_{0}$ of 6 cm. About 30% of the estimated values are above $r_{0}=7~\mbox{cm}$ . The diurnal variation of the seeing is consistent with that expected for a lake-shore site. The significance of our method – extracting the underlying seeing from a single long exposure solar image – lies in the fact that it can be easily adopted to monitor seeing with small H $\alpha $ -telescopes that are coveted and eagerly possessed by many solar observatories for patrolling solar flares and filament eruptions.