Synchronous interferometric speckle subtraction (SISS): A concept to remove speckle noise in adaptive optics imaging and interferometry

In PSFs obtained by AO systems, a cloud of speckles is surrounding the central diffraction core. These speckles don't average in time, and are extremely difficult to calibrate by the observation of a reference star. This "speckle noise" is setting the detection limit of faint companions around bright stars far above the photon noise. Speckles are coherent with the central diffraction core of the PSF, while a faint companion is not. By inducing interferences between the speckle cloud and a reference wave extracted from the central core of the PSF, it is possible to take advantage of this fundamental difference to identify and subtract the speckles. A time-variable phase delay is introduced in the reference wave in order to rapidly modulate the intensity of the speckles. The amplitude of this modulation leads to an accurate measure of the speckle intensity, which can then be subtracted to the image. This technique, which is compatible with spectroscopy, allows efficient detection of companions about 1000 times fainter than the speckles, and is especially attractive for exoplanet searches. The same technique can also be used on interferometers.