Fig. 1a and b. The emission lines into the order separating filter. a  on the common wavelength scale, the solid gaussian is the filter transmission, the gray boxes are the emission lines of NGC 1084: [NII], and [NII] (see text); as well the lines of the night sky are plotted with their wavelengths and relative intensities values from Osterbrock et al. (1996). Near the wavelength axis the numbers of the interference orders relative of the order are indicated. b  the night sky spectrum on the -order wavelength scale. The thin line is the mean of the sky spectrum from our FPI data. The thick lines are the night sky lines from the neighboring orders. The lines marked with letters are from a , the interference order for each line is given within brackets.