# 5.3.7 Photometric relationships with other photometric systems

Author(s): Josep M. Carrasco

This section includes some photometric transformations from Gaia passbands to other common photometric systems using Gaia DR2 data. Hipparcos (ESA (1997)), Tycho-2 (Høg et al. (2000)), SDSS12 (Alam et al. (2015)), Johnson-Cousins(Landolt (2009)), 2MASS (Skrutskie et al. (2006)) and GSC2.3 (Lasker et al. (2008)) are included here.

Only those sources with small magnitude error and small excess flux (see Sec. 5.5.2) were used for the fitting. Gaia DR2 sources were cross-matched with $G<13$ mag (in order to limit the influence of the photometric noise in the derived relationships), considering those having available photometry in the external photometric systems. 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 we used some Gaia DR2 sources with $\sigma_{G}<0.003$ and SDSS12 magnitudes fainter than 15th. For GSC2.3 transformations, two different fittings were done, one using sources with $\delta>0$, and another with $\delta<0$, as the photometric system is not equal in the two celestial hemispheres.

In order to obtain cleaner fittings, some filtering criteria were added to produce each colour-colour diagram. The data filtering to produce the photometric relationships can be checked in Table 5.5 and Table 5.6. The polynomial coefficients obtained with the resulting sources are included in Table 5.7 and Table 5.8. The validity of these fittings is, of course, only applicable in the colour intervals used to do the fitting (see Table 5.9).

Photometric relationships derived between Gaia and Hipparcos, Tycho-2, SDSS12, Johnson, 2MASS and GSC2.3 can be seen in Figures from  5.11 to 5.21.