  Differential imaging is a postprocessing method to obtain high contrast,
often used for exoplanet searches. The coherent differential imaging on speckle
area nulling (CDI-SAN) method was developed to detect a faint exoplanet lying
beneath residual speckles of a host star. It utilizes image acquisitions faster
than the stellar speckle variation synchronized with five shapes of a
deformable mirror repeatedly. By using the only the integrated values of each
of the five images and square differences for a long interval of observations,
the light of the exoplanet could be separated from the stellar light. The
achievable contrast would reach to almost the photon-noise limit of the
residual speckle intensities under appropriate conditions. The CDI-SAN can be
applied to both ground-based and space telescopes.
