2 Data reduction

Treatment of the data is accomplished in three stages. First, the primary data is acquired and saved to image files, usually of quite large sizes. Second, the images are located and identified, and the centering is done to produce (x, y) files. Finally, the set of files covering the aimed object are reduced.

The plate overlap concept (Eichhorn & Williams 1963) is adopted for the least squares adjustment of a three constants model to both coordinates observed values. The model is simply a polynomial of first degree in x and y. The solution is obtained by an iterative procedure (Teixeira et al. 1992). The iteration step consists in the conventional reduction of separated tours, followed by the averaging of the residuals for the objects present in more than one tour and subtraction from the observed values.

The iteration proceeds at a fast pace and, as a rule, attains a satisfactory point after three to five steps. Except for the origin, scale and alignment, the solution is, in principle, independent from the adopted reference catalogue, as shown in Fig. 1.

For the present sample of observations, the internal standard error for a single tour amounted to 0$\hbox{$.\!\!^{\prime\prime}$}$04 for both coordinates and for objects within the optimal magnitude range interval, say V = 9 to 13. It increases steadily for fainter magnitudes, attaining 0$\hbox{$.\!\!^{\prime\prime}$}$15 for V = 15.5.

  

Figure 1: Right ascension and declination frequency distribution for standard error. The reference stars with more than two used observations contribute to the plot; that is 409 ACT, 447 TAC and 397 Tycho stars. ACT solution is displayed by the dashed line, Tycho solution by the solid line and TAC solution by the dotted line