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-spr ead function (PSF). It is shown that when frames taken simultaneously at tw o different wavelengths are subtracted from one another, the general evolut ion of the PSF profile with wavelength limits the attenuation to A(2) simil ar to 2 sigma(phi)(2)Delta lambda/lambda, where sigma(phi)(2) is the wave-f ront phase variance and Delta lambda the bandpass separation. When images t aken at three wavelengths are combined in a double difference, the residual s 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 sp ectrum of cool brown dwarfs or Jovian planets, speckle noise can be attenua ted by a factor of similar to 10(4). This technique makes it possible to re ach the photon limit in searches for methanated companions of even the brig htest stars in the sky at separations of less than lambda/D.