Site Evaluation Study for the Indian National Large Solar Telescope Using Microthermal Measurements

Abstract

A microthermal seeing measurement device has been developed in-house to measure the temperature structure function DT(r, h) and the air temperature Tair(h). A pressure sensor, located adjacent to it, measures the average barometric pressure P(h). From the data measured, the temperature structure coefficient C2T(r,h) and the refractive index structure constant C2N(h) are computed for the five equidistant microthermal seeing layers in the 3–15 m range in the surface layers. A statistical analysis is performed on the local coherence length ro(loc)(h1, h2). Corresponding values of the atmospheric seeing ϵ(loc)(h1, h2) for all 10 microthermal seeing slabs is also computed and plotted, and the data are logged in real time. Because the characterization of the three sites is under way and the best site for the National Large Solar Telescope facility is yet to be determined, in this paper I discuss the preliminary results obtained from the Hanle site. A summary of the first results is as follows: ϵ(loc) (3 m, 6 m) = 0.663 arcsec, ϵ(loc) (6 m, 9 m) = 0.465 arcsec, ϵ(loc) (9 m, 12 m) = 0.363 arcsec and ϵ(loc) (12 m, 15 m) = 0.315 arcsec.