1 Introduction

According to diffraction theory, the image of a point source produced by a telescope of diameter D at a wavelength $\lambda$ is an Airy's disc of size $\varepsilon_D \sim \lambda/D$. Due to degradation by the Earth's atmosphere the stellar image formed at the focus of the ground based telescope is much bigger than $\varepsilon_D$. The size of the image also depends on the thermal effects of the dome, building and immediate surroundings as well as local atmospheric turbulence which depends on the location of the site. We refer here to seeing (FWHM of a star image) as the overall quality of the optical image. For many purposes the power of a telescope is therefore proportional to the primary collecting area divided by the solid angle formed by the image and thus a 2.5 m telescope with 0 $.\!\!^{\prime\prime}$5 seeing is equivalent in performance to a 5 m telescope with 1 $.\!\!^{\prime\prime}$0 seeing (Woolf 1982). Hence, smaller telescopes situated at sites with good seeing can perform better than larger telescopes located at sites with poorer seeing. Thus, it is of primary importance to evaluate a site before putting up a large telescope and the same has been carried out here for locating a modern 3 m class optical telescope jointly by Uttar Pradesh State Observatory (UPSO), Nainital and Tata Institute of Fundamental Research (TIFR), Mumbai.

The principal requirements of a site for optical astronomical observations are clear, dark and transparent skies with good seeing, modest atmospheric extinction, low precipitable water vapour, small changes in night time air temperature etc. In addition, the site should be far away from human activities so that for at least a few decades the deterioration due to light and atmospheric pollution is minimal. At the same time, one has to take into account the logistics of access and availability of water and power to the site so that infrastructural development and telescope operation do not become too expensive. The cost involved in providing easy accessibility to an otherwise excellent site often comes in the way of its choice. A compromise between the factors mentioned above is always made. Consequently, there are only a few excellent astronomical sites on the Earth. Most of them are located either on island (such as Hawaii and La Palma) or on coastal areas (such as Chilean and Midwest American). Good sites are generally located in the subtropical zone (25 to 35 positive or negative latitudes) and are mostly located at heights $\ge$ 2 km above the mean sea level.

Figure 1: The upper and lower panels of the diagram show the location and contour maps of Devasthal region. Site 1 and Site 2 are marked as 1 and 2 respectively