1.2 arcsec. The SPM Cassegrain focus (f/7.5) was coupled with a 1024x1024 pixel Thx31156 CCD in 1993 and 1994, with 0.25 arcsec/pixel and a gain of 2.12 $\rm e^-$/ADU. Since 1995 the system was upgraded with a 1024x1024 pixel TEK1024AB CCD with nearly double quantum efficiency, 0.30 arcsec/pixel and a gain of 4.88 $\rm e^-$/ADU.

Each galaxy was observed using two narrow band interferometric filters: one that includes the redshifted wavelength of the $\rm H\alpha$ line (on), the other, of similar bandwidth, centered at least 100 $\rm \AA$ off the line (off) to secure the continuum measurement. The flux from the \[NII\] emission lines ($\rm \lambda$ 6548 $\rm \AA$ and 6584 $\rm \AA$) is included in the on-band observations. Fig. 2 shows the transmission profiles of the filters used (as provided by the manufacturer for $\rm T=20^o$ C). Fig. 2a refers to the 1990-1995 data. The transmission curves of the SPM filters were remeasured in 1996 and are presented in Fig. 2b. These show a slightly lower transmission than previously measured. We allowed for a transmission shift toward the blue of 1 $\rm \AA$ per 5 deg of temperature decrease. Since we observed with a dome temperature around $\rm 10^o$ 