C, we applied a 2 $\rm \AA$ correction. At the redshift of the target galaxies the $\rm H\alpha$ line lies well within the region of maximum filter transmission.

Table 2 reports the journal of observations as follows:

Col. 1: CGCG galaxy name. Col. 2, 3: central wavelength of the filters used, on-band and off-band respectively. Col. 4: FWHM of the bandpass. Col. 5: telescope used. Col. 6: pixel scale in arcsec. Col. 7: observing date (dd-mm-yy). Col. 8: integration time per filter (minutes). Col. 9: flag indicating if the frame was taken under photometric conditions. Galaxies marked “N” were observed in nearly photometric conditions. Col. 10: normalization factor obtained dividing the flux of several field stars in the off-band frames by the flux in the on-band frames. In photometric conditions this quantity reflects the transmission difference between the on and off band filters and ranges between 0.95 and 1.20 (for the various filters and years). Under non-photometric conditions the quantity includes variations in the sky transparency.

All images were obtained in seeing conditions in the range 1.3-2.5 arcsec.

Image Analysis
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The data reduction of the present frames was based on the IRAF package, developed by NOAO, and on the 