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Wide Field Photometry of the Galactic Globular Cluster M22

We present wide field photometry of the Galactic Globular Cluster M~22 in the B, V and I passbands for more than 186,000 stars. The study is complemented by the photometry in two narrowband filters centered on H$_α$ and the adjacent continuum, and by infrared J, H and K magnitudes derived from the 2 MASS survey for $\sim$2000 stars. Profiting from this huge database, we completely characterized the evolved stellar sequences of the cluster by determining a variety of photometric parameters, including new photometric estimates of the mean metallicity, reddening and distance to the cluster. In particular, from our multi-wavelength analysis, we re-examined the long-standing metallicity spread problem in M~22. According to our dataset, we conclude that most of the observed width of the red giant branch must be due to differential reddening, which amounts to a maximum of $ΔE(B-V)\simeq0.06$, although the presence of a small metallicity spread cannot be completely ruled out. More specifically, the maximum metallicity spread allowed by our data is of the order of $Δ$[Fe/H]$\simeq 0.1÷0.2$ dex, i.e., not much more than what allowed by the photometric errors. Finally, we identified most of the known variable stars and peculiar objects in our field of view. In particular, we find additional evidence supporting previous optical identifications of the central star of the Planetary Nebula IRAS 18333-2357, which is associated with M~22.