  The Integrated Sachs-Wolfe (ISW) effect is a direct signature of the presence
of dark energy in the universe, in the absence of spatial curvature. A powerful
method for observing the ISW effect is through cross-correlation of the Cosmic
Microwave Background (CMB) with a tracer of the matter in the low redshift
universe. In this paper, we describe the dependence of the obtained
cross-correlation signal on the geometry and other properties of a survey of
the low redshift universe. We show that an all-sky survey with about 10 million
galaxies, almost uniformly distributed within 0<z<1 should yield a near optimal
ISW detection, at ~ 5\sigma level. In order to achieve this level of
signal-to-noise, the systematic anisotropies in the survey must be below ~ 0.1
%, on the scale of ~ 10 degrees on the sky, while the systematic error in
redshift estimates must be less than 0.05.
  Then, we argue that, while an ISW detection will not be a good way of
constraining the conventional properties of dark energy, it could be a valuable
means of testing alternative theories of gravity on large physical scales.
