NGC 6819 is a richly populated , older open cluster situated within the Kepler field .
A CCD survey of the cluster on the uvbyCa H \beta system , coupled with proper-motion membership , has been used to isolate 382 highly probable , single-star unevolved main-sequence members over a 20 \arcmin field centered on the cluster .
From 278 F dwarfs with high precision photometry in all indices , a mean reddening of E ( b - y ) = 0.117 \pm 0.005 or E ( B - V ) = 0.160 \pm 0.007 is derived , where the standard errors of the mean include both internal errors and the photometric zero-point uncertainty .
With the reddening fixed , the metallicity derived from the same 278 stars is [ Fe/H ] = -0.116 \pm 0.101 from m _ { 1 } and -0.055 \pm 0.033 from hk , for a weighted average of [ Fe/H ] = -0.06 \pm 0.04 , where the quoted standard errors of the mean values include the internal errors from the photometric scatter plus the uncertainty in the photometric zero points .
If metallicity is derived using individual reddening values for each star to account for potential reddening variation across the face of the cluster , the analogous result is unchanged .
The cluster members at the turnoff of the color-magnitude diagram are used to test and confirm the recently discovered variation in reddening across the face of the cluster , with a probable range in the variation of \Delta E ( B - V ) = 0.045 \pm 0.015 .
With the slightly higher reddening and lower [ Fe/H ] compared to commonly adopted values , isochrone fitting leads to an age of 2.3 \pm 0.2 Gyr for an apparent modulus of ( m - M ) = 12.40 \pm 0.12 .