Stromgren and Washington photometry in Boo I

We construct two indices: m*=(C-T1)0-(T1-T2)0 and m**=(C-b)0-(b-y)0. We find that CT1by is the most successful filter combination, for individual stars with [Fe/H] < -2.0, to maintain ~0.2dex [Fe/H]-resolution over the whole red giant branch. The m**-index would be the best choice for space-based observations because the (C-y) colour is not sufficient to fix metallicity alone in an understudied system.

Our photometric metallicites of stars in the central regions of Bootes I confirm that there is a metallicity spread of at least -1.9>[Fe/H]>-3.7. The best-fitting Dartmouth isochrones give a mean age, for all the Bootes I stars in our data set, of 11.5+/-0.4Gyr. From ground-based telescopes, we show that the optimal filter combination is CT1by, avoiding the v filter entirely.

We demonstrate that we can break the isochrones' age-metallicity degeneracy with the CT1by filters, using stars with logg=2.5-3.0, which have less than a 2percent change in their (C-T1) colour due to age, over a range of 10^-14^Gyr.