Spectroscopy of faint emission line nebulae

Based on results obtained with FORS1 in long-slit mode we demonstrate the power of the VLT for the spectroscopy of faint emission line nebulae such as the gaseous halos of diffuse ionized gas (DIG) in spiral galaxies. It is shown that VLT spectra of DIG allow us to address the ionization and excitation processes for the interstellar medium on galactic scales and that in the future more detailed kinematical studies of DIG could help to constrain the origin of the observed thick H+ layer in galaxies. The currently available instrumentation with regard to this application is compared to other possible designs and efficiencies for integral field spectrographs, in particular Fabry-Perot systems.