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7. Standard magnitudes

7.1. Transformation equations

The calibration of the five different CCDs used in the imaging survey allows us to derive a homogeneous set of standard magnitudes for all detected objects. Instrumental magnitudes are related to standard magnitudes by the transformation Eq. obtained from Eq. (1):
where tex2html_wrap_inline2761
We assume that no correction for galactic absorption is necessary because our fields lie at high galactic latitude tex2html_wrap_inline2763 and the widest extent of the full survey is only 1.53 deg.
Because we have a priori no knowledge of the colours of the detected objects in the Johnson-Cousins system, we invert the set of Eq. (2) so that the colour correction appears in terms of observed colour. We give the equations for the V band where two colour coefficients are measured:

Most objects detected in one filter are identified in the other 2 filters (see next subsection). For the galaxies detected in only one filter, we derive the magnitudes in the other filters using the standard colours B-V = 1.0 and V-R = 0.5. These mean values are derived from the mean galaxy colours obtained for the survey and reported in Sect. 9.2.2. To check the reliability of our colour coefficients listed in Table 3 and the resulting standard magnitudes, we estimate V in two possible ways using Eqs. (3) and (4). The mean differences in the 2 corresponding magnitudes for 7000 objects observed at the NTT and 5000 objects at the 3.60 m are tex2html_wrap_inline2773 mag. This test demonstrates the reliability of our measured colour coefficients. Because the R and V frames cover almost the same area of the sky, the final V magnitudes are calculated in terms of V-R colour (Eq. 4).

7.2. Colour completeness

Although R and V frames cover almost the same area of the sky, the B frames cover only about 80% of area observed in R and V, partly because of the smaller CCD size in the EMMI blue channel. Within the common area of the B, V and R bands, we define a colour completeness rate for objects selected in the B band as the fraction of B detected objects with associated V or R or both detections (in bins of 1 mag). We calculate the analogous curves for the V and R bands inside the same area. These various curves are shown in Fig. 7 (click here). Typically, the completeness drops below 90% 1 mag brighter than the limit of the catalogue in each band. At the limit of the spectroscopic catalogue tex2html_wrap_inline2811, the completeness in B and V is greater than 95%.

Figure 7: Colour completeness of B, V and R detected objects in B-V and V-R colours. The completeness is estimated in bin sizes of 1 mag. The coding of the curves is specified inside the graph

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