  Stellar coronagraph performance is highly sensitive to optical aberrations.
In order to effectively suppress starlight for exoplanet imaging applications,
low-order wavefront aberrations entering a coronagraph such as tip-tilt,
defocus and coma must be determined and compensated. Previous authors have
established the utility of pupil-plane masks (both
non-redundant/sparse-aperture and generally asymmetric aperture masks) for
wavefront sensing. Here we show how a sparse aperture mask (SAM) can be
integrated with a coronagraph to measure low-order, differential phase
aberrations. Starlight rejected by the coronagraph's focal plane stop is
collimated to a relay pupil, where the mask forms an interference fringe
pattern on a subsequent detector. Our numerical Fourier propagation models show
that the information encoded in the fringe intensity distortions is sufficient
to accurately discriminate and estimate Zernike phase modes extending from
tip-tilt up to radial degree $n=5$, with amplitude up to $\lambda/20$ RMS. The
SAM sensor can be integrated with both Lyot and shaped pupil coronagraphs (SPC)
at no detriment to the science beam quality. We characterize the reconstruction
accuracy and the performance under low flux/short exposure time conditions, and
place it in context of other coronagraph wavefront sensing schemes.
