Angular differential imaging (ADI) is a
speckle-suppression technique for high contrast imaging used along with
coronagraphy, e.g., for attempting direct imaging of
extra-solar planets.
It consists of allowing a telescope to
rotate in relation to the source so that different CCD
pixels are recording portions of the image. The noise pattern
created by the telescope can be isolated since it stays with
the telescope, whereas the source image rotates. Through stacking
images rotated so as to isolate each, then using subtraction, much
of such noise can be removed.