  A time-of-flight device was developed to characterize wavelength distribution
and uniformity of a cold neutron beam. This device is very compact -- the
distance of flight is $60$ cm -- but achieves very high resolution -- the
intrinsic resolution $\Delta \lambda/\lambda=2.4\cdot 10^{-3}$ at
$\lambda=0.89$ nm. The time-of-flight device is composed of a fixed slit, a
disk rotating up to $216$ Hz and a neutron detector with a thin spherical
conversion layer with the chopper slit in its focus. The device accepts the
complete angular divergence of the initial neutron beam. The efficiency of
neutron detection is constant over the detector area. Systematic corrections
caused by neutron scattering in air are minimized due to the reduction of the
time-of-flight length. Measurements have been performed on the beamline of the
GRANIT experiment at ILL (part of the H172 beamline) on level C, and the first
order diffraction peak of the crystal monochromator used for the GRANIT
beamline was found to be at $\lambda=0.8961(11)$ nm, and having a width of
$\sigma=0.0213(13)$ nm.
