2 Selection of the fields

Table 1 summarizes the list of low-extinction windows in which we have defined a relatively dense set of standards for further use. Due to specific needs of our project, but also in view of more general applications, we have proceeded to select standard stars up to $m_{V\rm al} \, = \, 13$ (we will use "$m_{V\rm al}$" to designate the magnitudes obtained with the Valinhos system ($V_{\rm V}$ filter) and "V" for magnitudes in the standard Jonhson system ($V_{\rm J}$ filter)). We should emphasize that, since the primary objective was to construct an astrometric and photometric reference set, the selected stars do not constitute a complete sample up to the stated magnitude limit. It is also clear that, due to instrumental limitations and sky quality of the site, the extension of quality observations to fainter magnitudes is not devoid of problems. However, according to the performed tests and preliminary results (Dominici 1998), we are be able to monitor variable stars up to $m_{V\rm al} \, =\, 14.5$; and construct complete stellar catalogues up to $m_{V\rm al} \, \sim \, 15.5$, the latter limit being slightly dependent on the field declination (see below). This relatively shallow sampling naturally means that our standards do not reside in the bulge but are just foreground stars.

  

Table 1: Coordinates of the 12 low-extinction windows for wich secondary catalogs based on Tycho data are presented (Blanco & Terndrup 1989 and Blanco 1988)