a) Survey completeness as a function of off-axis angle and limiting flux. The 10%, 50% and 90% levels are indicated. The dashed red line indicates the restriction put on the off-axis angle. b) Cumulatively sensitivity-weighted surface areas as a function of limiting flux, integrated over all off-axis angles. The short-dashed red line indicates the deduced limiting flux of 2.5 × 10^-15ergcm^-2   s^-1 at which 50% completeness is reached as indicated in a).