# 5.5.1 Relationships with other photometric systems

Author(s): Josep Manel Carrasco, Michele Bellazzini

This section includes some transformations between Gaia photometry and other common photometric systems using Gaia DR3 data. Hipparcos (ESA 1997), Tycho-2 (Høg et al. 2000), SDSS12 (Alam et al. 2015), Johnson-Cousins (Stetson 2000), 2MASS (Skrutskie et al. 2006) and GSC2.3 (Lasker et al. 2008) are included here.

Except for the Johnson-Cousins system, for which all available sources were used due to its intrinsic quality, for the other systems only those sources with small magnitude error and small excess flux were used for the fitting. In order to minimise the effect of the photometric noise on the derived relationships, only Gaia DR3 sources with $G<13$ mag were considered for the cross-match, and only those with available photometry in all three Gaia passbands and in the external photometric systems were used. Nevertheless, this dataset is not appropriate for SDSS12 transformations, as SDSS12 sources brighter than 14 mag are saturated. To avoid this problem, for SDSS12 transformations some Gaia DR3 sources with $\sigma_{G}<0.01$ and only SDSS12 magnitudes fainter than 15 mag were used. For the GSC2.3 transformations, two different fittings were done, one using sources with $\delta\geq 0$, and the other with sources at $\delta<0$, as the photometric systems in the two celestial hemispheres are somewhat different. In order to obtain cleaner fittings, some filtering criteria were adopted in the production of the colour-colour diagrams. Detailed information on the filtering to produce the photometric relationships is given in Table 5.6 and Table 5.7.

The polynomial coefficients obtained with the resulting sources are listed in Table 5.8 and Table 5.9. The validity of the relationships derived from these fittings is, of course, only applicable within the colour intervals used to perform the fittings (see Table 5.10). The photometric relationships derived between Gaia and Hipparcos, Tycho-2, SDSS12, Johnson-Cousins, 2MASS and GSC2.3 can be seen in Figure 5.34 to Figure 5.43. The relationships presented here were obtained using preliminary data. Thus, some sources used here for the fitting might have been filtered out in the final production.

The purpose of these relationships is to provide a tool applicable to as wide as possible a population of stars, in order to obtain a crude estimation of their photometry when transforming from one system to another. There are cases in which different populations (particularly M type giants and dwarfs) show different behaviours in the colour-colour diagram. In these cases a single fitting was chosen, namely the one reproducing the most populated group for which the validity of the relationship holds over the widest colour range (M giants in this case). Thus, for many relationships shown here the extension to red colours is only valid for M giants and not for M dwarfs.