The Largest Gravitational Lens: MACS J0717.5+3745 (z=0.546)
  We identify 13 sets of multiply-lensed galaxies around MACS J0717.5+3745
($z=0.546$), outlining a very large tangential critical curve of major axis
$\sim2.8\arcmin$, filling the field of HST/ACS. The equivalent circular
Einstein radius is $\theta_{e}= 55 \pm 3\arcsec$ (at an estimated source
redshift of $z_{s}\sim2.5$), corresponding to $r_e\simeq 350\pm 20 kpc$ at the
cluster redshift, nearly three times greater than that of A1689 ($r_e\simeq 140
kpc$ for $z_{s}=2.5$). The mass enclosed by this critical curve is very large,
$7.4\pm 0.5 \times 10^{14}M_{\odot}$ and only weakly model dependent, with a
relatively shallow mass profile within $r<250 kpc$, reflecting the unrelaxed
appearance of this cluster. This shallow profile generates a much higher level
of magnification than the well known relaxed lensing clusters of higher
concentration, so that the area of sky exceeding a magnification of
$>10\times$, is $\simeq 3.5\sq\arcmin$ for sources with $z\simeq 8$, making
MACS J0717.5+3745 a compelling target for accessing faint objects at high
redshift. We calculate that only one such cluster, with $\theta_{e}\ge
55\arcsec$, is predicted within $\sim 10^7$ Universes with $z\ge 0.55$,
corresponding to a virial mass $\ge 3\times 10^{15} M_{\odot}$, for the
standard $\Lambda CDM$ (WMAP5 parameters with $2\sigma$ uncertainties).
