Spectral line-shape distortions in Michelson interferometers due to off-focus radiation source

When the radiation source in a Michelson interferometer is placed in front of or behind the focal plane of the collimator, distortions arise in the spectral line shapes. The appearance and behavior of these distortions in a cube-corner interferometer are studied. A simple and fast method to calculate the true line position of a distorted line and an easy rule of thumb to connect the off-focus shift and the amount of distortion are presented. Calculation of the phase-eror curves in the signal domain is addressed. This calculation gives a possibility of correcting the distorted interferogram.