Although the BP and RP data are treated from scratch again in the photometric processing (see
Chapter 5), a pre-processing of these data within IDT is needed in order to derive
instantaneous source colour information (which may differ from the mean source colour). The source
colours are needed in the determination of the astrometric image parameters
(Section 2.4.8). The photometric pre-processing is described in detail in Sections 5.3 and 5.4 of Fabricius et al. (2016).