ABSTRACT
A new approach to the use of narrow-band filters in extragalactic research is described. The photometric system vz, bz, yz essentially imitates, at the redshift of the cluster of galaxies in question, the Strömgren v, b, y passbands. This system has some obvious advantages: the uncertainties associated with k-corretions are bypassed, and the passbands avoid the stronger emission lines appearing in the spectra of some normal and most active galaxies. It is necessary to acquire filters for each redshift value and to have accurate spectrophotometry for at least one galaxy at the redshift in order to calibrate the photometry. It is shown that the vz, bz, yz filter system is especially suitable for testing the evolutionary synthesis models. For that purpose 48 CCD frames of three clusters of galaxies (Abell 665, the small cluster around 3C 234, and Cl 1021.1+0427) were obtained. For galaxies, metric magnitudes were derived corresponding to a linear physical standard diameter of 32 kpc (H0 = 60 km s-1Mpc-1, q0 = 0.5). Galaxies showing a strong color gradient along their images (disk galaxies) are distinguished from galaxies with no, or very small, color gradient along their images (elliptical and lenticulars).