We present new UV ( F218W ) data for stars in the central region of the Galactic globular cluster NGC2808 , collected with the WFPC2 camera on board of the Hubble Space Telescope .
These data together with F439W and F555W-band data and previous ground based observations provide a multifrequency coverage of the cluster stellar population extending up to a distance of 1.7 times the cluster core radius .
We discuss this complete sample of stars , which includes 764 Red Giant Branch ( RGB ) stars brighter than the Horizontal Branch ( HB ) luminosity level , 1239 HB stars , 119 Asymptotic Giant Branch ( AGB ) , and 22 AGB-manqué stellar structures .
As already known , we find that blue HB stars separate into three distinct groups .
However , our multiband photometry indicates that several stars in the two hotter HB groups show a flat spectrum , thus suggesting the binarity of these objects .
Artificial star experiments suggest that at most 50 % of them might be photometric blends .
Moreover , at variance with previous claims one finds that canonical Zero Age Horizontal Branch ( ZAHB ) models do reach effective temperatures typical of observed hot HB stars .
We also show that the ratio between HB and RGB stars brighter than the HB luminosity level steadly increases when moving from the cluster center to the periphery , passing from R= 1.37 \pm 0.14 in the cluster core to R= 1.95 \pm 0.26 in the outer cluster regions .
We discuss the possible origin of such a radial gradient in the context of the Blue Tails phenomenon , advancing some suggestions concerning the clumpy stellar distribution along the HB .