To calibrate our CCD magnitudes into the Johnson B, V and Cousins R standard system
(hereafter B, V, R),
we observed the faint standard star sequence NGC 300 from
Graham (1981) and
the bright standard star sequences MARKA, NTPHE, PG0231, SA92 from
Several sequences were observed each night in order to obtain a zero-point for
We use the following linear transformation equations to convert instrumental
magnitudes into standard magnitudes :
where define the standard magnitudes and define the instrumental magnitudes (in adu s) through respectively the three filters B, V, R. The colour term COLOUR is the standard colour provided by Graham or Landolt B-V or V-R. The instrumental CCD magnitude of standard stars is calculated as the "corrected isophotal'' magnitude (see Sect. 4.2.1), thus yielding a good estimation of the total magnitude.
The extinction coefficients for the different filters B, V, R are derived from the La Silla extinction curve provided in the ESO manual (Schwarz & Melnick 1993).
The zero-points are specific to each combination of Telescope/Instrument/ CCD/Filter/atmospheric conditions. These are easily measured using a large number of standard stars observed each night (see Fig. 6 (click here)). The colour coefficients allow the magnitudes from the "observing'' filter (resulting from the Telescope/Instrument/CCD/Filter combination) to be corrected into the B, V, R standard filters. We assume that they remain constant during each observing run, and use the calibrations of an entire run to calculate these coefficients. Two color coefficients for the V band can be calculated depending on whether B-V or V-R colours are used (denoted or respectively).
In practice, we estimate the colour coefficients for each different configuration of Telescope /Instrument / CCD /Filter as shown in Fig. 5 (click here). The resulting colour coefficients are shown in Table 3. The quoted errors are the rms. uncertainties in the linear fit. The measured colour coefficients are then used to determine the accurate zero-point for each night.
Figure 5: Colour term as a function of colour for standard star sequences of an entire observing run (NTT/EMMI-B, CCD TEK#31). The error bars for each star is the quadratic sum of the instrumental error resulting from several measurements and the intrinsic error given by the authors. The solid line represents the weighted least-squares regression whose slope provides the value of . The are chosen for each set of stars of each night to minimize the scatter in the colour term
Figure 6: Resulting zero-point value for one observing night using the color coefficient calculated in Fig. 5 (click here). The error bars for stars are the same as in Fig. 5 (click here), and the zero-point is calculated by weighted least-squares regression by a constant term
Table 3: Measured colour coefficients