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Converting Tycho V and 2MASS H to Johnson V Magnitude
There are numerous stars with Tycho V ("V_T") photometry, both in the Tycho and Tycho-2 catalogs, and in the SuperWASP photometry archive. The figure above plots the (Tycho V_T minus 2MASS H) color versus the magnitude difference between Johnson V and Tycho V_T. The stars plotted as open circles are taken from the Hipparcos catalog, and represent nearby dwarf stars with negligible reddening and the smallest photometric errors.
The Johnson V photometry was taken from the Hipparcos catalog (ESA 1997), the Tycho photometry was taken from the Tycho-2 catalog (Hog et al. 2000), and the 2MASS photometry was taken from the 2MASS point source catalog of Skrutskie et al. (2006).