There are two grisms on the OM filter wheel, named ''6-VISIBLE GRISM'' and ''10-UV GRISM'' covering the optical and the UV wavelength range, respectively (see XMM-Newton Users Handbook for details). The dispersion direction of both grisms is close to the detector Y-axis (+6 deg for the ''UV GRISM'' and -6 deg for the ''VISIBLE GRISM''). The dispersed first-order spectra of both grisms are centred with their reference wavelength (260 nm and 420 nm, respectively) on the detector location that the target would have with a non-dispersive filter in place. Each grism has a zeroth- and a first-order image. The spectra of the two grisms differ in size (in both orders). GRISM observations are only compatible with the ''Science User Defined Mode'' mode with its corresponding default window (about 2' x 13' along the dispersion direction), or with a ''FULL FRAME'' mode (17' x 17' field). For a single target or a small region it is recommended to use the ''Science User Defined'' mode with its default window. The FULL FRAME LOW RESOLUTION mode is recommended for several targets in an extended field. In this case, if the field is crowded, the spectra and zeroth orders may overlap and its extraction may be difficult. In both modes, if the dispersion direction overlaps with a bright object, it may also be problematic for extracting the spectra.