The derivation of the reference fluxes was carried on a restricted time period where the effects of contamination by water ice on the mirrors and CCDs (Gaia Collaboration et al.2016) was at a low level. This period was selected after the second decontamination, when the throughput loss was much lower and stable (see Fig. 8 in Riello et al. (2018) ), and it spans 354 days. This period was long enough for the scanning law
to ensure that the entire sky was covered multiple times. We refer to this as the INIT dataset.
The first stage in the determination of the reference fluxes is the accumulation of the raw data to form a first set of reference fluxes. This was done using the INIT dataset only. The first step in the calibration process is the Time Link Calibration, see Section 5.3.3.
Sources that lie in the colour range where the calibration model is defined (0.0 mag to 4.0 mag in raw colour ) are referred to as Gold sources. Sources that lie outside this colour range will be calibrated via an ad hoc procedure and are referred to as Silver sources.
Following this, the Gate Window Class link calibration (Section 5.3.4) was carried out using reference fluxes that had been formed from time link-calibrated fluxes. A preliminary large-scale calibration was also carried out in order to reduce the noise level on the photometry source.
Using the full link–calibrated fluxes, a new set of reference fluxes were accumulated. These were then used in the first of the large-scale calibrations. These generally have a time scope of one day. This calibration has the biggest effect on reducing the overall systematics in the fluxes.
The large-scale calibrations and accumulation process are now iterated until the calibrations are reasonably stable. This is assessed using an L1 norm metric as described in
Evans et al. (2017). For Gaia DR2, five iterations were carried out.
The small-scale calibrations were then calculated using the large-scale-calibrated reference fluxes. The time scope for the small-scale calibrations covered the entire initialization
period. Two sets of iterations were then carried out between the large and small-scale calibrations to ensure that the minimum standard deviations for the solutions were achieved and
that these calibrations were stable. No accumulation process was carried out between these iterations, i.e., the reference fluxes remained the same throughout this process. Using these
large and small-scale calibrations, the source photometry for the INIT period was derived.
This was then used to generate a series of large and small-scale calibrations covering the entire period considered for Gaia DR2. The time scopes for the large-scale calibrations was one day, as in the initialization of the reference system, and for
the small-scale calibrations, two time scopes were used. These calibrations were then applied to calibrate the epoch fluxes which were then accumulated to generate the weighted-mean fluxes that are released in the catalogue.
Not all sources could be calibrated with the described procedure. Some were excluded due for being outside the colour range
where the time link calibration is defined. Others dropped out during the iterative process due to low number of observations and/or problematic calibrations. These were recovered by an iterative process, where the existing calibrations are applied to raw fluxes initially to generate a better set of source fluxes. This process was stopped after six iterations.
Finally, some other source did not have complete colour information. For these sources a default set of SSCs were used in the application of the calibrations. These default colours were derived covering the whole sky. These are known as Bronze sources.
In the catalogue, the Bronze sources are flagged as having used default colours in the application of the calibrations and will only have G-band magnitudes published.