  A procedure is presented to map from the spatial correlation parameters of a
turbulent density field (the radial and binormal correlation lengths and
wavenumbers, and the fluctuation amplitude) to correlation parameters that
would be measured by a Beam Emission Spectroscopy (BES) diagnostic. The inverse
mapping is also derived, which results in resolution criteria for recovering
correct correlation parameters, depending on the spatial response of the
instrument quantified in terms of Point-Spread Functions (PSFs). Thus, a
procedure is presented that allows for a systematic comparison between
theoretical predictions and experimental observations. This procedure is
illustrated using the MAST BES system and the validity of the underlying
assumptions is tested on fluctuating density fields generated by direct
numerical simulations using the gyrokinetic code GS2. The measurement of the
correlation time, by means of the cross-correlation time-delay (CCTD) method,
is also investigated and is shown to be sensitive to the fluctuating radial
component of velocity, as well as to small variations in the spatial properties
of the PSFs.
