Remote sensing Fourier transform spectrometers are required to be robust and portable. Introducing moving flat mirrors instead of cube corner refractors simplifies the spectrometer design and makes it more compact. Flat mirrors, however, introduce additional angular errors. We analyze the effect of tilted beams on spectral line shape and derive a mathematic equation for this effect based on the diffraction theory. A rough estimate equation for the intensity of two interfering beams based on the amount of tilt between them is presented. A case of a varying amount of tilt during moving mirror scanning is presented and various forms of mirror misalignment are analyzed.