= Solution
Both methods reduce structured stellar residuals, rather than removing all noise. In <angular differential imaging>, changing <atmospheric turbulence>, imperfect <adaptive optics>, flexure and thermal drift change the <point spread function> between frames. The reference then fails to represent the instantaneous stellar field. Small sky rotation makes close companions contaminate their own reference, causing <differential-imaging self-subtraction>. Extended disks are especially susceptible; subtraction can alter shape as well as total flux. More images help independent <photon shot noise>, but do not necessarily average away correlated residuals.
<Simultaneous spectral differential imaging> avoids the time delay, but different channels have non-common-path <wavefront errors>. Chromatic <optical aberrations>, wavelength-dependent amplitude errors and out-of-pupil propagation prevent a perfect radial rescaling of <speckle patterns>. Filter throughput, detector calibration, image registration and <atmospheric dispersion> also leave subtraction residuals. Nearby bands align the stellar field better but give less positional diversity; wider separation gives more displacement but larger chromatic mismatch. A smooth-spectrum companion, or one too close for appreciable rescaled displacement, can undergo severe <differential-imaging self-subtraction>.
Artificial-companion injection through the complete processing pipeline and forward modelling can calibrate lost throughput and photometric or astrometric biases. They do not guarantee that every correlated residual is a real source. Independent epochs or spectral evidence remain valuable.
\b[Reference mismatch produces residual speckles; source contamination produces self-subtraction and biased photometry.]
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