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 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.
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