The Next Generation Space Telescope (NGST) will be equipped with a Multi-Object Spectrograph (MOS) 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 MEMS-based device: a micro-shutter array (MSA). In Laboratoire d'Astrophysique de Marseille, we have developed since several years different tools for the modeling and the characterization of these MEMS-based slit masks. Our models, based on Fourier theory, address two key parameters for the MOS performance: contrast and spectral photometric variation (SPV). We present in this paper our calculation for SPV evaluation. The SPV requirement is < 10%, but as SPV is strongly dependent on the object position and wavelength, the required value cannot be reached. We propose two dithering strategies able to solve this problem: blind dithering and slit reconstruction. Also, we have developed a characterization bench to measure these parameters. Preliminary contrast measurement have been carried out on the micro-mirror array (MMA) fabricated by Texas Instrument, in order to simulate the actual MEMS device for NIRMOS. Contrasts of around 500 has been measured and effects of object position on the micro-mirrors have been revealed. Further measurements with additional parameters such as the size of the source, the wavelength, and the input and output pupil size are under way.