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\noindent We have developed a method for quantitatively characterizing bi-grid Rotating Modulation Collimators that are used in a Fourier-transform X-ray imager. This method uses a beam-expanded HeNe laser to simulate the plane wave produced by a point source at infinity even though the grids are diffraction limited at 6010 angstroms. This expanded beam passes through the grids at a small angle relative to their axis of rotation and the modulated signal that is produced as the RMCs rotate is detected using a photomultiplier tube. In addition to providing a quantitative characterization of the RMCs, this method will also provide a measured point response function for the imaging system. Detailed analysis of this modulated signal to determine the effect of grid imperfections, relative twist of the grids, and other instrumental effects on the imaging capabilities of the instrument will be described.
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