Microthermal measurements of surface layer seeing at Devasthal site

Abstract

In order to detect the microthermal fluctuations introduced by the atmospheric turbulence very near to the ground at Devasthal site, a PC based instrumentation has been developed. The optical image degradation due to such turbulence has been quantified. The results of the optical seeing due to the surface layer at Devasthal site are presented and compared with the seeing results obtained from the Differential Image Motion Monitor. Microthermal measurements were taken on 20 nights between March and June 1998, using sensors placed at three equally spaced levels on a 18 m high mast. We found a significant decrease in the optical turbulence over the height of the mast with a mean value of 0.32" for the 12 to 18 m slab and 1.28" for the 6 to 12 m slab. For Devasthal site, a seeing of ~ 0.6" can be achieved, if the telescope is located at a height of ~13 m above the ground.