The main characteristics of NSE spectrometers is their exceptional energy resolution. As for other spectrometers, the instrumental resolution can be determined measuring a sample which is static in the window probed by the spectrometer. By increasing the Fourier time the polarization of the scattered beam decreases, not because of the interaction with the sample (which is, in this case, static) but because of the unavoidable inhomogeneities in the magnetic field. Such inhomogeneities depolarize the neutron beam more and more as the field is increased and determine the maximum Fourier time which can be probed by the spectrometer.
Since, the Fourier time is a function of the third power of the incoming beam wavelength, the most effective strategy to access longer times is by using longer wavelength, at the price of a lower neutron flux. This figure below shows the experimentally determined resolution of the NSE instrument at the NCNR at three different wavelengths, which are most commonly used.