A new technique has been developed for improving the spectral resolution of spectrographs mounting discrete array detectors. The basic concept is to acquire various spectroscopic images, each one being shifted on the detector by a fraction of the pixel (i.e., the detector resolution element), and to apply suitable numerical procedures in order to extract a spectral profile with subpixel resolution. This technique has been applied to a vacuum spectrograph, adopting the Johnson‐Onaka configuration. The dispersion element is a concave toroidal grating that can rotate around a pivot axis displaced from its vertex, in order to maintain a good spectral focus on the detector. The latter is a multianode microchannel‐plate array (MAMA) and operates in the photon‐counting mode. Several stigmatic images of the H I Ly‐α line at 1216 A˚ emitted by a D2 lamp have been acquired for various rotation of the grating. The results of the application of this new technique and the numerical algorithm are presented and discussed in terms of potentialities and limitations due to the signal‐to‐noise ratio and intrinsic spectral broadening of the signals. © 1996 Society of Photo−Optical Instrumentation Engineers.