Systematic Difference

The difference between the Northern and Southern mean calibrated magnitudes of the field stars is examined to search for any systematic difference in the solutions. Figure 3 shows the correlation of the mean Northern and Southern magnitudes of the field stars. The straight line fit to the relation yields

$$<m>_{North} = (0.998\pm0.001) <m>_{South} + (0.006\pm0.013),$$

with the slope consistent with unity at $2\sigma$ level. A closer look at the residuals (i.e. the difference between Northern and Southern magnitudes) in Figure 4 shows that the residuals appear more extended toward the negative amplitudes, suggesting there might be a systematic difference between North and South. The linear fit gives the slope of the relation of $(-1.80\pm 1.10)\times 10^{-3}$. Figure 5 shows the distribution of the residuals as the function of the R.A. coordinate. Each point represents a field star, and the 10 concentrations of points in the plot correspond to 10 calibration fields. The mean reasidual in every field is also plotted. No clear correlation pattern is readily seen. However, the distribution of the residuals with the R.A. offset from the central scan position (Figure 6) shows a clear dependence at the left edge of calibration scans, a known effect of non-optimal PSF photometry in the Southern data (Skrutskie 1998).

This effect is reduced by considering only field sources from the central 1/3 of the scans. The residuals as the function of magnitude for the central third of the scans are shown in Figure 7, with much tighter distribution as compared to Figure 4. The linear fit slope is $(0.95\pm 1.63) \times 10^{-3}$. The residuals as the function of the R.A. coordinate for the central third of the scans are plotted in Figure 8.