Fibre and seeing profiles in the plane of the detector. The y-axis is aligned with increasing altitude. The black circle represents the fibre with a radius R. The red dot and circle represent the quasar position and profile; i.e. the σ of the quasar Gaussian seeing profile. The red shaded area illustrates the light lost. To calculate the captured flux, we integrate the normalised quasar light intensity within the fibre radius. The figure is adapted from Barnes & Walsh (1988).