High-contrast imaging

High-contrast imaging seeks faint objects near a bright source. Residual point spread functions and speckle patterns can dominate photon shot noise; angular differential imaging and simultaneous spectral differential imaging use source motion or spectral diversity to distinguish them.

A companion or extended source contributes to the stellar reference used in differential imaging. Subtracting that reference removes part of the desired signal and distorts its shape, photometry or astrometry. Artificial-source injection and forward modelling can estimate the throughput loss.

Simultaneous spectral differential imaging compares images in nearby bands, rescaled so stellar speckle patterns approximately coincide. A companion has a different positional or spectral response. Simultaneity reduces temporal variation but does not remove chromatic and non-common-path errors.

Angular differential imaging keeps the instrument pupil fixed while the sky rotates. A stellar reference point spread function is subtracted, then the residuals are derotated and combined. Insufficient rotation and imperfect reference selection cause differential-imaging self-subtraction.