We build differential-reddening maps for 66 Galactic globular clusters ( GCs ) with archival HST WFC/ACS F606W and F814W photometry .
Because of the different GC sizes ( characterised by the half-light radius R _ { h } ) and distances to the Sun , the WFC/ACS field of view ( 200 \arcsec \times 200 \arcsec ) coverage ( R _ { obs } ) lies in the range 1 \la R _ { obs } / R _ { h } \la 15 for about 85 \% of the sample , with about 10 % covering only the inner ( R _ { obs } \la R _ { h } ) parts .
We divide the WFC/ACS field of view across each cluster in a regular cell grid , and extract the stellar-density Hess diagram from each cell , shifting it in colour and magnitude along the reddening vector until matching the mean diagram .
Thus , the maps correspond to the internal dispersion of the reddening around the mean .
Depending on the number of available stars ( i.e .
probable members with adequate photometric errors ) , the angular resolution of the maps range from \approx 7 \arcsec \times 7 \arcsec to \approx 20 \arcsec \times 20 \arcsec .
We detect spatially-variable extinction in the 66 globular clusters studied , with mean values ranging from \mbox { $ \langle { \mbox { $ \delta \mbox { $E ( B - V ) $ } $ } } \rangle$ } \approx 0.018 ( NGC 6981 ) up to \mbox { $ \langle { \mbox { $ \delta \mbox { $E ( B - V ) $ } $ } } \rangle$ } \approx 0.16 ( Palomar 2 ) .
Differential-reddening correction decreases the observed foreground reddening and the apparent distance modulus but , since they are related to the same value of E ( B - V ) , the distance to the Sun is conserved .
Fits to the mean-ridge lines of the highly-extincted and photometrically scattered globular cluster Palomar 2 show that age and metallicity also remain unchanged after the differential-reddening correction , but measurement uncertainties decrease because of the reduced scatter .
The lack of systematic variations of \langle { \mbox { $ \delta \mbox { $E ( B - V ) $ } $ } } \rangle with both the foreground reddening and the sampled cluster area indicates that the main source of differential reddening is interstellar .