5.3.4 Flux and LSF Calibration and Mean Spectra generation
The internal calibration of the spectra follows a similar flow as the photometric calibration. Also in this case, no external data is used in the calibration process and therefore the reference catalogue of mean source spectra needs to be established in an iterative way. A set of calibrators is selected trying to maximise the number of sources in less-populated instrument configurations and the mixing between different calibration units. For these the initialisation of the reference spectra is an iterative process starting with an identity calibration and performing at each step an update of the mean source spectra using the latest calibration and a new calibration computation based on the latest reference catalogue. The initialisation process only uses data from the INIT period, chosen to cover ranges of time when the contamination level was at its minimum.
Once the reference catalogue is established, calibrations covering all times and configurations can be computed using all observations for the calibrators. These can then be applied to all sources to generate a complete BP/RP mean spectra catalogue.
Before entering the internal calibration spectra are calibrated for the differential dispersion function to bring all sample AL locations in the same internal system, called pseudo-wavelength. The calibration model is defined as a kernel function quantifying the flux contribution at a given pseudo-wavelength from a range of surrounding pseudo-wavelengths. This enables characterising differences in response and LSF between different observations.
The mean spectra are defined as a superposition of basis functions. Hermite functions were chosen in the latest processing, but an ad hoc transformation was applied to them to ensure that the first bases were the ones describing most of the spectral features.
The integral of the function describing an internally calibrated source mean spectrum over the ranges defined for the SSCs (see Section 5.1.1) provides the colour information used in the photometric processing Section 5.4.1).
More details about the internal calibration of the BP/RP spectra will be provided in time for Gaia DR3 and their first release.