1. A method of measuring asymmetry in a scatterometer, the method comprising: first illuminating a target pattern with radiation and measuring a first two-dimensional angular scatter spectrum of radiation reflected or scattered by the target pattern by using the scatterometer to measure intensity at a plurality of pre-determined angular positions relative to the target pattern; relatively rotating the target pattern and the scatterometer by about 180° in a plane substantially perpendicular to the optical axis of the scatterometer; second illuminating the target pattern with radiation and measuring a second two-dimensional angular scatter spectrum by using the scatterometer to measure intensity at the plurality of pre-determined positions relative to the target pattern and rotating the second angular scatter spectrum by about 180° to produce a third two-dimensional angular scatter spectrum; and producing a sensor correction value for each of the plurality of pre-determined angular positions by, for each of the plurality of pre-determined positions, subtracting the intensity at the respective pre-determined position of the first two-dimensional angular scatter spectrum from the intensity at the respective pre-determined position of the third two-dimensional angular scatter spectrum.