Retrofitting focal reducer spectrographs with removable integral field units

In recent years the interest for integral field spectroscopy has increased consistently among the astronomical community. Larger detectors, efficient fibers, etc. made possible to record simultaneously spatial and spectral information of extended area of sky. The typical spectrograph that processes the Integral field data has a focal reducer configuration, i.e. a refractive collimator re-imaging the pupil on a dispersing element and a camera to re-image the focal plane with suitable pixel sampling. The focal reducer configuration is very common in the panorama of astronomical instrumentation. Nevertheless few of them offer an integral field mode. Via the example of d.o.lo.res, the Low Resolution Spectrograph of the Galileo National Telescope (TNG), we analyze the possibility to retrofit some of them with an Integral field unit designed and manufactured a posteriori.