5 Photometric calibration

We have measured instrumental magnitudes of standard stars by constructing for each star its growth curve through circular concentric apertures. The magnitude has been taken at the convergence of the curve. From the spectral energy distributions of our observed standard stars (Massey & Strobel 1988), we have derived their apparent magnitudes at the effective wavelengths $\lambda_{\rm eff}$ of our filters, through the usual relation

$$m_{vF}=-2.5 \cdot \log_{10}f_{\lambda_{\rm eff}}(m_{vF})+2.5 \log_{10}f_{\lambda_{\rm eff}}(0)$$ (2)

where F is a generic filter, $f_{\lambda_{\rm eff}}(m_{vF})$ and $f_{\lambda_{\rm eff}}(0)$ are the spectral irradiances in erg cm^-2 s^-1 Å^-1 within the F filter having the effective wavelength $\lambda_{\rm eff}(F)$ of a star of magnitude m_vF and of a star of m_vF=0 respectively. From each star we have derived the zero point $Z_{\rm p}$ of the photometric calibration for the different filters and nights.

The standard deviation of the zero point values thus obtained is within 0.05 mag during all but one night. During this night the scatter is much larger than a factor of four. Thus with the aim at maintaining the uncertainty on the galaxy photometry within few hundreds percent, we have considered only those galaxies observed during photometric nights, i.e. those nights for which $\sigma_{Z_{\rm p}}\le0.05$ mag.