Efficient speckle noise attenuation in paint companion imaging

Numerical simulations are used to evaluate a technique to attenuate speckle noise and enhance faint companion images buried in a bright-star point-spread function (PSF). It is shown that when frames taken simultaneously at two different wavelengths are subtracted from one another, the general evolution of the PSF profile with wavelength limits the attenuation to A(2) similar to 2 sigma(phi)(2)Delta lambda/lambda, where sigma(phi)(2) is the wave-front phase variance and Delta lambda the bandpass separation. When images taken at three wavelengths are combined in a double difference, the residuals caused by speckle evolution are very strongly damped and attenuations A(3) similar to A(2)(2) are obtained. Very strong attenuation of speckle noise can thereby be achieved in images taken with adaptive optics (sigma(phi)(2) < 1). With three filters spanning the CH4 bandhead at 1.59 mu m in the spectrum of cool brown dwarfs or Jovian planets, speckle noise can be attenuated by a factor of similar to 10(4). This technique makes it possible to reach the photon limit in searches for methanated companions of even the brightest stars in the sky at separations of less than lambda/D.