On the determination of the He abundance distribution in globular
  clusters from the width of the main sequence
  One crucial piece of information to study the origin of multiple stellar
populations in globular clusters, is the range of initial helium abundances
$\Delta{Y}$ amongst the sub-populations hosted by each cluster. These estimates
are commonly obtained by measuring the width in colour of the unevolved main
sequence in an optical colour-magnitude-diagram. The measured colour spread is
then compared with predictions from theoretical stellar isochrones with varying
initial He abundances, to determine $\Delta{Y}$. The availability of UV/optical
magnitudes thanks to the {\sl HST UV Legacy Survey of Galactic GCs} project,
will allow the homogeneous determination of $\Delta{Y}$ for a large Galactic
globular cluster sample. From a theoretical point of view, accurate UV CMDs can
efficiently disentangle the various sub-populations, and main sequence colour
differences in the ACS $F606W-(F606W-F814W)$ diagram allow an estimate of
$\Delta{Y}$. We demonstrate that from a theoretical perspective the
($F606W-F814W$) colour is an extremely reliable He-abundance indicator. The
derivative d$Y$/d($F606W-F814W$), computed at a fixed luminosity along the
unevolved main sequence, is largely insensitive to the physical assumptions
made in stellar model computations, being more sensitive to the choice of the
bolometric correction scale, and is only slightly dependent on the adopted set
of stellar models. From a theoretical point of view the ($F606W-F814W$) colour
width of the cluster main sequence is therefore a robust diagnostic of the
$\Delta{Y}$ range.
