We have obtained CCD photometry in the Washington system C,T _ { 1 } 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 ] \sim - 2.25 to +0.25 at a spacing of \sim 0.2 dex .
Two independent observations were obtained for each cluster and internal checks , as well as external comparisons with existing photoelectric photometry , indicate that the final colors and magnitudes have overall uncertainties of \buildrel < \over { \sim } 0.03 mag .

Analogous to the method employed by Da Costa and Armandroff ( 1990 , AJ , 100 , 162 ) for V,I photometry , we then proceed to construct standard ( M _ { T _ { 1 } } , ( C - T _ { 1 } ) _ { 0 } ) giant branches for these clusters adopting the Lee et al .
( 1990 , ApJ , 350 , 155 ) distance scale , using some 350 stars per globular cluster to define the giant branch .
We then determine the metallicity sensitivity of the ( C - T _ { 1 } ) _ { 0 } color at a given M _ { T _ { 1 } } value .
The Washington system technique is found to have three times the metallicity sensitivity of the V,I technique .
At M _ { T _ { 1 } } = -2 ( about a magnitude below the tip of the giant branch , roughly equivalent to M _ { I } = -3 ) , the giant branches of 47 Tuc and M15 are separated by 1.16 magnitudes in ( C - T _ { 1 } ) _ { 0 } and only 0.38 magnitudes in ( V - I ) _ { 0 } .
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 - T _ { 1 } ) _ { 0 } ( at M _ { T _ { 1 } } = -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 metallicity calibrations for M _ { T _ { 1 } } = -2.5 and -1.5 , as well as for two other metallicity scales .
The Washington technique retains almost the same metallicity sensitivity at faint magnitudes , and indeed the standard giant branches are still well separated even below the horizontal branch .
The photometry is used to set upper limits in the range 0.03 – 0.09 dex for any intrinsic metallicity dispersion in the calibrating clusters .
The calibrations are applicable to objects with ages \buildrel > \over { \sim } 5 Gyr – any age effects are small or negligible for such objects .

This new technique is found to have many advantages over the old two-color diagram technique for deriving metallicities from Washington photometry .
In addition to only requiring 2 filters instead of 3 or 4 , the new technique is generally much less sensitive to reddening and photometric errors , and the metallicity sensitivity is many times higher .
The new technique is especially advantageous for metal-poor objects .
The five metal-poor clusters determined by Geisler et al .
( 1992 , AJ , 104 , 627 ) , using the old technique , to be much more metal-poor than previous indications , yield metallicities using the new technique which are in excellent agreement with the Zinn scale .
The anomalously low metallicities derived previously are undoubtedly a result of the reduced metallicity sensitivity of the old technique at low abundance .
However , the old technique is still competitive for metal-rich objects ( [ Fe/H ] \buildrel > \over { \sim } –1 ) .

We have extended the method developed by Sarajedini ( 1994 , AJ , 107 , 618 ) to derive simultaneous reddening and metallicity determinations from the shape of the red giant branch , the T _ { 1 } magnitude of the horizontal branch , and the apparent ( C - T _ { 1 } ) color of the red giant branch at the level of the horizontal branch .
This technique allows us to measure reddening to 0.025 magnitudes in E ( B - V ) and metallicity to 0.15 dex .
Reddenings can also be derived from the blue edge of the instability strip , with a similar error .

We measure the apparent T _ { 1 } magnitude of the red giant branch bump in each of the calibrating clusters and find that the difference in magnitude between the bump and the horizontal branch is tightly and sensitively correlated with metallicity , with an rms dispersion of 0.1 dex .
This feature can therefore also be used to derive metallicity in suitable objects .
Metallicity can be determined as well from the slope of the RGB , to a similar accuracy .
Our very populous color- magnitude diagrams reveal the asymptotic giant branch bump in several clusters .

Although M _ { T _ { 1 } } of the red giant branch tip is not as constant with metallicity and age as M _ { I } , it is still found to be a useful distance indicator for objects with [ Fe/H ] \buildrel < \over { \sim } –1.2 .
For the 6 standard clusters in this regime , < M _ { T _ { 1 } } ( TRGB ) > = -3.22 \pm 0.11 ( \sigma ) , with only a small metallicity dependence .
This result is found to be in very good agreement with the predictions of the Bertelli et al .
( 1994 , A & AS , 106 , 275 ) isochrones .
We also note that the Washington system holds great potential for deriving accurate ages as well as metallicities .