The building of a large telescope at the ORM is a great challenge for the Spanish astronomical community. The project was first promoted by the IAC and the Canarian Government and therefore we shall refer to it as the Gran Telescopio Canario (GTC). This national project has been widened in anticipation of the possible participation of important foreign astronomical institutions (Rodriguez-Espinosa & Alvarez 1996). Some of the authors are in charge of defining the future location of the GTC. A detailed programme is now in progress towards the taking of the decision of where to install the GTC at the ORM.
An imaginary line drawn between the NOT and the Residence divides the ORM into two slopes (see Fig. 1 (click here)). There are more observing installations on the northernmost side of that line. Hereafter this side will be referred to as the WHT slope. On the southernmost side there were no installations until the early 90 s, when the Galileo group chose this area as the site for the TNG. This will be referred to as the TNG slope.
As mentioned in Sect. 1 the WHT slope has been extensively studied, due to the long-standing presence of large facilities and instruments. The NOT location has also been recognized as an excellent site through numerous studies carried out as well as through astronomical results taken with the telescope.
Following the statistics presented in Sect. 3 at site A on the TNG slope, there is no doubt concerning the very good image quality that can be achieved in this area of the ORM, which would be the more suitable one for the installation of new telescopic facilities. However a comparison of the two slopes with standard techniques was imperative.
Two identical seeing monitors were operated simultaneously: the DIMM at site B (Fig. 1 (click here)) near the WHT belongs to the ING (Isaac Newton Group) and was operated by the IAC group during the period corresponding to data shown here. The data acquisition procedure and statistical processing is the same for both sites A and B and the results are presented in Table 3 (click here).
In comparing Tables 2 (click here) and 3 (click here), it is seen that, to within the errors, the data are quite similar at both sites during the winter period. In the summer period, however, the seeing recorded at site A is better. This could be understood as due to local disturbances that might affect the measurements at site B. As mentioned before, the DIMM on site B is located on the WHT roof, which might release more heat during the night in summer than in winter. For what concerns seeing, both sites look very similar, which means that, except the possible disturbance of WHT terrace, the surface layer has a small (Vernin & Muñoz-Tuñón 1994) and uniform contribution whatever the position within the ORM.