High-flux output from concentrators and solar simulators can determined by a direct measurement of focal region flux distribution. This is done at ANU by placing a Lambertian, i.e. uniform diffusely reflecting, white planar target in the focal region, and capturing digital images of the concentrated light on its surface. For the SG4 Big Dish, due to the high peak concentrations of the sun (over 10,000 x), flux measurement was carried out at night using a full moon as the light source. Parameters such as dish peak and geometric concentration and mean surface slope error were obtained after with appropriate corrections for the small difference in the solid angles subtended by the sun and the moon.