Improving the Speckle Noise Attenuation of Simultaneous Spectral Differential Imaging with a Focal Plane Holographic Diffuser

Direct exoplanet detection is limited by speckle noise in the point spreadfunction (PSF) of the central star. This noise can be reduced by subtractingPSF images obtained simultaneously in adjacent narrow spectral bands using amulti-channel camera (MCC), but only to a limit imposed by differential opticalaberrations in the MCC. To alleviate this problem, we suggest the introductionof a holographic diffuser at the focal plane of the MCC to convert the PSFimage into an incoherent illumination scene that is then re-imaged with theMCC. The re-imaging is equivalent to a convolution of the scene with the PSF ofeach spectral channel of the camera. Optical aberrations in the MCC affect onlythe convolution kernel of each channel and not the PSF globally, resulting inbetter correlated images. We report laboratory measurements with a dual channelprototype (1.575 micron and 1.625 micron) to validate this approach. A specklenoise suppression factor of 12-14 was achieved, an improvement by a factor ~5over that obtained without the holographic diffuser. Simulations of realisticexoplanet populations for three representative target samples show that theincrease in speckle noise attenuation achieved in the laboratory would roughlydouble the number of planets that could be detected with current adaptiveoptics systems on 8-m telescopes.Comment: 9 pages, 8 figure, to be published in ApJ June 20, 200