# 5.1.1 Spectral Shape Coefficients

Author(s): Josep Manel Carrasco

Colour information of the observed sources is an essential element of the photometric calibration. The required colour information is obtained from the low-resolution spectra from the BP and RP spectro-photometers from which are derived the spectral-shape coefficients (SSC). The definition of the SSCs is the same as in Gaia DR1 and Gaia DR2, and more details can be found in Carrasco et al. (2016). The usage of SSCs in the calibration model has slightly changed in this release, both in the calibration model definition and in the fact that mean calibrated SSCs computed from calibrated mean spectra are now used to evaluate the colour-dependent part of the photometric calibration (in previous releases epoch uncalibrated SSCs where calibrated and accumulated in a similar fashion to other flux parameters).

The wavelength ranges of the SSC rectangular bands are given in Table 5.1.

Although they fulfil the same role, the SSCs are not strictly pass bands. The along-scan smearing from the line-spread function (LSF) means that the SSC boundaries are fuzzy, and depend on issues such as the exact positioning of the spectrum with respect to the pixel binning of the spectrum, the local dispersion function, and small variations in the along- and across-scan rotation rates. Still, implementation has shown improved performances of the calibrations compared to using a 1-parameter colour, such as the ratio of integrated $G_{\rm BP}$ and $G_{\rm RP}$ fluxes.

Figure 5.2 shows the location of the SSC bands in the data space compared to the BP and RP simulated spectra for a sample of template stellar spectra covering the effective temperature range from $3,000$ to $50,000$ K. Figure 5.3 shows the expected SSC dependency with the colour of the star for a data set of simulated BP and RP spectra covering a wide range of spectral types.