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).
These source colours are needed in the determination of the daily astrometric image parameters whereas in the
cyclic processing, IDU retrieves the colour from the catalogue (Section 3.4.11).
The photometric pre-processing is described in detail in Sections 5.3 and 5.4 of Fabricius et al. (2016).