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Published online by Cambridge University Press: 26 May 2016
We describe a procedure for the efficient measurement of large quantities of PN radial velocities (RVs) in early-type galaxies, using the Focal Reducer and Spectrograph (FORS) at the Cassegrain foci of the ESO Very Large Telescope. The PNs are detected using the classic on-band/off-band filter technique. The radial velocity is obtained by measuring and calibrating the displacement of a PN image on the CCD produced by the insertion of a grism in the light path. Since no slits are used, it is possible to measure RVs for all the detected PNs, irrespective of their number and distribution in the field. All the images needed to find the PNs and measure their brightness and RVs can be collected in just one observing run, without the complicated preparation procedures typical of multi-object slit spectroscopy. The RVs have errors of about 40 km/s.