We present precise and deep optical photometry of the globular M92 .
Data were collected in three different photometric systems : Sloan Digital Sky Survey ( g ^ { \prime } , r ^ { \prime } , i ^ { \prime } , z ^ { \prime } ; MegaCam @ CFHT ) , Johnson-Kron-Cousins ( B , V , I ; various ground-based telescopes ) and Advanced Camera for Surveys ( ACS ) Vegamag ( F 475 W , F 555 W , F 814 W ; Hubble Space Telescope ) .
Special attention was given to the photometric calibration , and the precision of the ground-based data is generally better than 0.01 mag .
We computed a new set of \alpha -enhanced evolutionary models accounting for the gravitational settling of heavy elements at fixed chemical composition ( [ \alpha /Fe ] =+0.3 , [ Fe/H ] =–2.32 dex , Y =0.248 ) .
The isochrones—assuming the same true distance modulus ( \mu =14.74 mag ) , the same reddening ( E ( \hbox { \em B - - V\ / } ) =0.025 \pm 0.010 mag ) , and the same reddening law—account for the stellar distribution along the main sequence and the red giant branch in different Color-Magnitude Diagrams ( i ^ { \prime } , g ^ { \prime } - i ^ { \prime } ; i ^ { \prime } , g ^ { \prime } - r ^ { \prime } ; i ^ { \prime } , g ^ { \prime } - z ^ { \prime } ; I , B - I ; F 814 W , F 475 W - F 814 W ) .
The same outcome applies to the comparison between the predicted Zero-Age-Horizontal-Branch ( ZAHB ) and the HB stars .
We also found a cluster age of 11 \pm 1.5 Gyr , in good agreement with previous estimates .
The error budget accounts for uncertainties in the input physics and the photometry .
To test the possible occurrence of CNO-enhanced stars , we also computed two sets of \alpha - and CNO-enhanced ( by a factor of three ) models both at fixed total metallicity ( [ M/H ] =–2.10 dex ) and at fixed iron abundance .
We found that the isochrones based on the former set give the same cluster age ( 11 \pm 1.5 Gyr ) as the canonical \alpha -enhanced isochrones .
The isochrones based on the latter set also give a similar cluster age ( 10 \pm 1.5 Gyr ) .
These findings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures .