2007ApJ...660..770L

Direct imaging of exoplanets is limited by bright quasi-static speckles in the point-spread function (PSF) of the central star. This limitation can be reduced by subtraction of reference PSF images. We have developed an algorithm to construct an optimized reference PSF image from a set of reference images. This image is built as a linear combination of the reference images available, and the coefficients of the combination are optimized inside multiple subsections of the image independently to minimize the residual noise within each subsection. The algorithm developed can be used with many high-contrast imaging observing strategies relying on PSF subtraction, such as angular differential imaging (ADI), roll subtraction, spectral differential imaging, and reference star observations. The performance of the algorithm is demonstrated for ADI data. It is shown that for this type of data the new algorithm provides a gain in sensitivity by up to a factor of 3 at small separation over the algorithm previously used by Marois and colleagues.