Chromaticity calibration for image parameters based on ν_eff from the photometric processing (PhotPipe). This is the calibration used for sources with a five-parameter solution (calculated in step 5 of Sect. 4.1). Top: preceding FoV. Bottom: following FoV. From left to right: WC0a, WC0b, WC1, WC2. Each diagram shows the development of the chromaticity term for the five CCDs indicated in the legends. The chromaticity correction in IDU was very successful for WC1 and WC2 (i.e. G ≳ 13 mag), but only partially so for brighter sources.