Seeing and microthermal measurements near Devasthal top

Abstract

Results of the seeing measurements carried out near Devasthal top on 43 nights during March to April 1999 are presented. Open air seeing measurements were carried out with a differential image motion monitor (DIMM) using a 38 cm telescope with the mirror about 2 m above the ground. This, in combination with our earlier reported measurements carried out during October to November 1998 on 37 nights show a median seeing of 1.?1 and 35% of the time seeing is better than 1.?0. A trend in the seeing evolution is noticed with better seeing towards the later part of the night. By analysing the temporal evolution of seeing for seeing fluctuations it is found that the mean ratio of two seeing values separated by a time interval of ?t grows with a time constant of about 17 minutes. To quantify the optical image degradation caused by atmospheric turbulence very near to the ground, microthermal measurements were also simultaneously performed with DIMM observations. Microthermal measurements show that most of the contribution to seeing comes from the 6 - 12 m slab of the atmosphere with a mean value of 0.?86. A significant decrease in turbulence over the height of the mast is noticed with a mean value of 0.?22 for the 12 - 18 m slab. A seeing of ~ 0.?6 can be achieved by locating the telescope at a height of ~ 13 m above the ground.