  Unresolved Doppler velocity measurements are not homogenous across the solar
disc (Brookes et al. 1978). We consider one cause of the inhomogeneity that
originates from the BiSON instrumentation itself: the intensity of light
observed from a region on the solar disc is dependent on the distance between
that region on the image of the solar disc formed in the instrument and the
detector. The non-uniform weighting affects the realization of the solar noise
and the amplitudes of the solar oscillations observed by a detector. An 'offset
velocity', which varies with time, is observed in BiSON data and has
consequences for the long-term stability of observations. We have attempted to
model, in terms of the inhomogeneous weighting, the average observed offset
velocity.
