next previous
Up: The ESO-Sculptor faint

6. Calibration

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 Landolt (1992). Several sequences were observed each night in order to obtain a zero-point for each night. We use the following linear transformation equations to convert instrumental magnitudes tex2html_wrap_inline2667 into standard magnitudes tex2html_wrap_inline2669:
where tex2html_wrap_inline2671 define the standard magnitudes and tex2html_wrap_inline2673 define the instrumental magnitudes (in adu stex2html_wrap_inline2675) 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 tex2html_wrap_inline2687 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 tex2html_wrap_inline2695 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 tex2html_wrap_inline2697 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 tex2html_wrap_inline2711 or tex2html_wrap_inline2713 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 tex2html_wrap_inline2715 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 tex2html_wrap_inline2717. The tex2html_wrap_inline2719 are chosen for each set of stars of each night to minimize the scatter in the colour term tex2html_wrap_inline2721

Figure 6: Resulting zero-point value tex2html_wrap_inline2723 for one observing night using the color coefficient tex2html_wrap_inline2725 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

next previous
Up: The ESO-Sculptor faint

Copyright by the European Southern Observatory (ESO)