We have obtained CCD photometry in the Washington system C,T1 filters for
some 850,000 objects associated with 10 Galactic globular clusters and 2 old
open clusters. These clusters have well-known metal abundances, spanning a
metallicity range of 2.5 dex from [Fe/H]~-2.25 to +0.25 at a spacing of ~0.2
dex. Analogous to the method employed by Da Costa and Armandroff (1990, AJ,
100, 162) for V,I photometry, we then proceed to construct standard giant
branches for these clusters. The Washington system technique is found to have
three times the metallicity sensitivity of the V,I technique. Thus, for a given
photometric accuracy, metallicities can be determined three times more
precisely with the Washington technique. We find a linear relationship between
(C-T1)o (at M(T1)=-2) and metallicity (on the Zinn 1985, ApJ, 293, 424 scale)
exists over the full metallicity range, with an rms of only 0.04 dex. We also
derive methods to determine distance, reddening and metallicity simultaneously,
and note that the Washington system holds great potential for deriving accurate
ages as well.Comment: To be published in the 1999 AJ January issu
