Simulated image for a source at − 1.5 degree off-axis in the direction of increasing incident angle on the grating. The projected photons near Z = 400 (the narrow lines) are double reflections from the mirror. The fan like shape is due to single or double reflections from the mirror and a single reflections from the grating. In the RFC coordinate frame position (0,0) is de center of CCD5 which is the position of the grating first order blaze wavelength. The coordinates are in units of 0.01 mm.