The design method and the performance of two types of concave gratings for operation as multiplexers/demultiplexers, with 10 nm/mm reciprocal linear dispersion, required to multiplex/demultiplex channels separated by 0.5 nm in wavelength, are presented. The first type of grating is designed in such a way that the principal ray of the central wavelength in the spectrum is diffracted at normal angles to the image field. The second type of grating is a Rowland circle mount. In each case, two recording configurations are obtained. The performances of the mounts are almost identical. In the first configuration, the coupling loss is less than 22.7 dB and the optical isolation is better than 237 dB. In the second configuration, the losses are less than 21.8 dB and the optical isolation is better than 243 dB. Finally, a Rowland circle grating with 4 nm/mm reciprocal linear dispersion, which can multiplex/ demultiplex at least 175 channels with loss less than 22.6 dB and crosstalk better than 228 dB, is presented. These spectrographs can be used for wavelength routing applications in wavelength division multiplexing (WDM) networks.