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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 of our filters, through the
usual relation

| |
(2) |

where *F* is a generic filter, and
are the spectral irradiances in
erg cm^{-2} s^{-1} Å^{-1} within the *F* filter having the effective
wavelength 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 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
mag.

** Up:** An H catalogue of

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