First Study of Optical Turbulence Over an Astronomical Site in the Central Himalayas Using ST Radar Observations

Abstract

Optical turbulence in the atmosphere is a key factor which governs the sharpness of images produced from ground-based optical telescopes. The intensity of optical turbulence can be conveniently assessed by the estimation of the refractive index structure constant (C 2 n). Here, observations from a stratosphere tropo sphere radar (206.5 MHz) that is co-located with the optical telescope facilities in the central Himalayas, are used for the estimation of C 2 n. The integrated pro¯le of C 2 n and winds over about 2–20 km have been used to estimate seasonal and diurnal variation of \seeing", wavefront coherence time, isoplanatic angle and scintillation rate. The mean C 2 n over the site varies from 10 15:7 to 10 19 m 2=3 with the largest values in the monsoon and post-monsoon season. The best \seeing" conditions are observed in the winter and post monsoon season with median \seeing" varying from 0.3900 to 0.8100. The largest contribution to the diurnal variation of \seeing" comes from turbulences at 2–5 km height region. A weak correlation of diurnal variation of \seeing" has been observed with surface temperature and relative humidity (RH). Among astroclimatic parameters, the largest isoplanatic angle is associated with winter season with median value of 2.5700 and wavefront coherence time of 4.2 ms. The least scintillation in intensity is observed in the winter season with a rate of 10% and most in monsoon season with 45% rate. This study is expected to have a positive impact on the optimization of the operation of adaptive optical techniques, observing time and scheduling of scienti¯c programmes for optical telescope facilities in this region of the central Himalayas