1 Introduction

Based on several years of meteorological data, Devasthal about 50 km east of Nainital in India (latitude = $29\hbox{$^\circ$}$ 22' N, longitude = $79\hbox{$^\circ$}$ 41' E, altitude = 2420 m above mean sea level) has been found as a potential site for putting up a 3-metre class optical telescope jointly by Tata Institute of Fundamental Research (TIFR), Mumbai and U. P. State Observatory (UPSO), Nainital. For characterizing this site astronomically, an intensive site testing campaign is being carried out using as many modern site testing instruments as possible. For the measurement of seeing, two Differential Image Motion Monitors (DIMMs) mounted on both 52 cm and 38 cm reflector telescopes located at two different sites of Devasthal are already in use, whilst another instrument for detecting the surface layer microthermal fluctuations and for estimating their contribution to the seeing has been developed and recently been installed near the 52 cm telescope site. Broadly, the aim of this study is to evaluate image degradation due to induced atmospheric turbulence within a height of a few metres above the ground. This information can be used to decide the height for locating the telescope in order to achieve better seeing conditions. For this, we started measurements of the turbulence due to surface layer since March 1998.

It is an established fact that atmospheric turbulence with its associated random refractive index inhomogeneities disturbs a passing light beam (Tatarski 1961; Hufnagel & Stanley 1964; Fried 1966) and in consequence deteriorates the optical quality of the star image. Since the earth surface acts as a heat source during day and as a heat sink during night, the magnitude of microthermal fluctuations within turbulent layers are usually maximum near the ground (Erasmus & Thompson 1986). When an electromagnetic wave propagates through a refractively nonuniform medium, fluctuations in the amplitude and phase of the wave are introduced. Consequently, an optical image formed by focusing such a wave exhibits fluctuation in intensity, sharpness and position which are termed as scintillation, image blurring and image motion respectively (Coulman 1985). Seeing effects due to small scale temperature gradients have also been studied by using temperature probes inside the telescope dome and in the immediate vicinity of the telescope site (Lynds 1963; Hall 1967; Coulman 1969, 1974). Lynds (1963) estimated the visual image quality with the 90 cm reflector telescope and compared these observations with the simultaneously taken temperature fluctuation traces. These observations showed that the average value of the seeing was correlated directly with the amplitude and frequency of occurrence of the temperature fluctuations. He also concluded that seeing was never good when the temperature fluctuations were large and frequent. Likewise when the temperature fluctuations were small and infrequent the seeing was generally good.

This paper presents the results of the observations taken on 20 nights during March to June 1998, using microthermal sensor pairs placed at three equally spaced levels on a 18 m high mast. The results have been compared with the seeing data obtained from DIMM instrument mounted on the 52 cm reflector telescope installed at a height of $\sim\! 3$ m above the ground in the vicinity of the mast. The seeing measurements from these two different types of data have been analyzed for an understanding of the seeing quality of the Devasthal site.

The next section describes the details of the principle of seeing measurements in terms of optical turbulence. In Sect. 3, there is a brief account of the instrumental design and data analysis. In Sect. 4, the results and discussions are given followed by the conclusions in Sect. 5.