We describe the theory of diffraction in multiple-grid x-ray telescopes for use in astronomy. Convenient numerical procedures for estimating the effects of diffraction are given and illustrated for typical grid telescopes; in general diffraction limits the long-wavelength performance of such telescopes at the present state of technology. Comparisons with the measurements of Blake et al. [ Rev. Sci. Instrum. 47, 889 ( 1976)] show favorable agreement for wavelengths of order 50 Å. The effects of diffraction are similar in magnitude to the results of convolving the geometrically derived response profile with the Fraunhofer diffraction pattern expected from a slit the same size as the grid openings. The possibility of diffraction causing significant leaks which are not allowed geometrically does not seem to occur. Blake et al. measured the response of their collimators by looking at the collimated output of one diffusely-illuminated collimator with another similar collimator. We suggest a variation on their technique which entails effectvely averaging over the relative translational positions between the two collimators so as to eliminate the effect of the variable overlap of the facing grids from consideration.
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