SAOIMAGE package, developed at the Center for Astrophysics. To remove the detector response every raw image was bias subtracted and divided by the flat-field obtained from the twilight sky. Cosmic rays were individually removed from each frame. Bad pixel columns were removed by direct inspection of the frames and replaced with the mean of adjacent columns. The sky background was determined in each frame in concentric object-free annuli about the object and then subtracted from the flat-fielded images. These resulting frames were then used to determine the total counts from the objects of interest (galaxies and stars). Let us consider a frame containing a target galaxy (with total counts $cnts^g$ ) and one or more field stars (with total counts $cnts^*$). Under the assumption that the field stars do not emit $\rm H\alpha$, i.e. that: $flux^*_{on}~/~flux^*_{off}~=~1$ we have: $cnts^*_{on}~/~cnts^*_{off} = T_{on}\times \tau_{on}\times c~/~T_{off}\times \tau_{off}\times c = K$, where: $cnts~=~flux~\times T\times \tau \times c$ (T = effective integration time; $\rm \tau$= integral of the filter transmission; c= conversion factor from $cnts~s^{-1}$ to $erg~cm^{-2}~s^{-1}$).

For a target galaxy the $\rm H\alpha$ equivalent width (E.W.) is then:

1truemm $E.W.~=~{flux^g_{on}~-~flux^g_{off}\over flux^g_{off}}$ 1truemm $=~{cnts^g_{on}~-~K~cnts^g_{off}\over K~cnts^g_{off}}~~~~~[\AA]$

1truemm independent from c, thus derivable also in non photometric 