An empirical temperature calibration for the

Context.With the $\Delta a$ photometric system, it is possible to study very distant galactic and even extragalactic clusters with a high level of accuracy. This can be done with a classical color-magnitude diagram and appropriate isochrones. The new calibration presented in this paper is a powerful extension. Aims.For open clusters, the reddening is straightforward for an estimation via Isochrone fitting and is needed in order to calculate the reddening-free, temperature sensitive, index $(g_1-y)_0$. As a last step, the calibration can be applied to individual stars. Methods.Because no a priori reddening-free photometric parameters are available for the investigated spectral range, we have applied the dereddening calibrations of the Strömgren $uvby\beta$ system and compared them with extinction models for the Milky Way. As expected from the sample of bright stars, the extinction is negligible for almost all objects. As a next step, already established calibrations within the Strömgren $uvby\beta$, Geneva 7-color, and Johnson ${\it UBV}$ systems were applied to a sample of 282 normal stars to derive a polynomial fit of the third degree for the averaged effective temperatures to the individual $(g_1-y)_0$ values. Results.We derived an empirical temperature calibration for the $\Delta a$ photometric system for A-type to mid F-type with a mean of the error for the whole sample of $\Delta T_{\rm eff}$ is 134 K, which is lower than the value in Paper I for hotter stars. No statistically significant effect of the rotational velocity on the precision of the calibration was found. Conclusions.We have derived a new intrinsically consistent, empirical, effective temperature calibration for a spectral range from early B-type to mid F-type, luminosity class V to III stars within the photometric $\Delta a$ system. The statistical mean error over the complete spectral range of about 140 to 240 K will allow to individual objects of far distant galactic be studied as well as extragalactic clusters with high accuracy