Sub-picosecond electron pulses become increasingly important for the development of next generation light sources to study chemical and biological processes. Such processes occur on picosecond or sub-picosecond time scales and the ability to observe such reactions in real time is of great scientific interest. These and other research opportunities fuel large scale efforts toward the generation of femtosecond electron pulses. A critical part in this endeavor is the ability to measure and correctly analyze femtosecond pulses which is beyond the capabilities of even the most advanced streak cameras leaving a spectroscopic method as the presently most efficient way to measure such short pulses. This method is based on an autocorrelation technique performed in a Michelson interferometer. The technique, however, suffers from limited suitable optical materials available for the far infrared spectral regime. In this work the effects of such limitations and other problem areas have been investigated and their impact on femtosecond pulse measurements is discussed in detail both qualitatively and quantitatively. Furthermore, this technique can also be applied to nondestructive coherent diffraction radiation which displays its own limitations and problems as will be discussed.