The James Webb Space Telescope (JWST) will be equipped with a Near Infrared Multi-Object Spectrograph (NIRSpec), in order to record simultaneously several hundred spectra in a single observation run. The selection of the objects in the field of view will be done by a micro-elecro-mechanical system (MEMS): a micro-shutter array. This
array is easily reconfigurable and can generate any slit mask geometry. At the Laboratoire d’Astrophysique de Marseille, we have developed since several years different tools for the modeling and the characterization of these MOEMS slit masks. Our model, based on Fourier theory, addresses two key parameters for the MOS performance: contrast and spectral photometric variation (SPV). The integration in a single model of the JWST telescope, the micro-shutter slit mask, the
spectrograph and the detector is proposed. The JWST telescope simulator assess pupil shape with low, mid and high frequencies aberrations on the whole telescope as well as on each segment. Additional wavefront aberrations generated by the fore-optics and the spectrograph are also taken into account. NIRSpec performance is then calculated as function of multiple parameters such as telescope pupil shear, object position in the field, plate scale, object location within the slit, pupil oversizing, wavelength, and dithering strategy. Evaluation of the encircled energy is also presented, including the spectrograph aberrations.