The purpose of Tobyfit is to enable you to fit to Horace datasets taking into account the broadening of the data arising from the resolution of the instrument. Even if the excitations in your sample were infinitely long-lives you would have non-zero broadening in energy and wave vector arising from numerous contributions. The spread of energies arise from:
- the non-zero width in the moderator pulse;
- the non-zero pulse width from the Fermi or disk choppers;
- the different flight-times for the neutrons due to different distances that neutrons travel depending on the point of emission from the moderator, point of scattering ion the sampel and point of absorption in the detector.
The spread of wavevector arises from:
- spread of angle of the incident neutrons at the sample from the non-zero width of the moderator;
- the non-zero size of the sample;
- a spread of scattering angles into a given detector element because of the non-zero size of the detector element.
Resolution effects can be quite considerable. Not only do they result in an increase in the energy width of peaks (thereby giving an illusory shorter lifetime of the excitations), they can also shift the positions of peaks depending on, for example, the curvature of dispersion relation within the resolution function. This can result in incorrect values of exchange constants being extracted from the data is resolution is ignored.