  Forthcoming photometric redshift surveys should provide an accurate probe of
the acoustic peak in the two-point galaxy correlation function, in the form of
angular clustering of galaxies within a given shell in redshift space. We
investigate the form of the anticipated signal, quantifying the distortions
that arise due to projection effects, and in particular explore the validity of
applying the Limber approximation. A single-integral prescription is presented,
which provides an alternative to Limber's equation, and produces a
significantly improved prediction in the regime of interest.
  The position of the acoustic peak within the angular correlation function
relates to the angular diameter distance to the far side of the redshift bin.
Thicker redshift bins therefore shift comoving features towards smaller angular
scales. As a result, the value of the photometric redshift error acquires a
greater significance, particularly at lower redshifts. In order to recover the
dark energy equation of state to a level of 1%, we find the total redshift
dispersion must be determined to within \Delta \sigma_z ~ 10^-3, which may
prove challenging to achieve in practice.
