Figure 12.3: Evolution over the mission of the first and second-order colour dependence in the parameters β4 and β5 . The top two graphs refer to the linear and
quadratic colour dependence in β4 , and the bottom two graphs similarly to β5 . The
two curves in each graph refer to the following and preceding FOV. The grey lines
indicate instances of refocusing thetelescope. Only the linear colour coefficient for
β4 shows a minor dependence on focus.