1. A method for detecting a target in a scene pixel of a hyperspectral scene, the target having a spectrum similar to a known spectral reference, the method comprising: in a target detector provided with hyperspectral imaging data for a plurality of scene pixels of a hyperspectral scene and one or more spectral references each corresponding to a target, wherein the hyperspectral imaging data being provided from a hyperspectral sensor and a given spectral references includes a full set of terms representing spectrum whitened by transforming the spectrum using a spectral covariance of the hyperspectral scene, and wherein each term is indexed by an eigenvector and eigenvalue of the spectral covariance used to calculate that term: for a given spectral reference: computing a signal-to-clutter ratio (SCR) including a full set of SCR terms, the SCR represents the probability of detecting a target corresponding to the spectral reference in the hyperspectral scene; ranking the full set of SCR terms based on the contribution of each of the SCR terms to the computed SCR; based on the ranking, selecting indices of eigenvalues that correspond to a portion of the full set of SCR terms; with respect to the given spectral reference, for each scene pixel in the plurality of scene pixels: calculating a partial whitened scene pixel spectrum including terms with the selected eigenvalue indices; computing an intermediate target score using a target score equation including the terms of the partial whitened scene pixel spectrum and terms of the whitened spectral reference spectrum with the selected eigenvalue indices; comparing the intermediate target score to a threshold; identifying scene pixels having intermediate target scores less than the threshold as empty scene pixels that do not include targets; if a number of empty scene pixels is less than a specified fraction of a total number of scene pixels and a number of intermediate target scores computed is less than a counter, then: updating the partial whitened scene pixel spectrum calculation to include a term with an eigenvalue index previously not selected; updating the target score equation to include terms of the updated partial whitened scene pixel spectrum and terms of the whitened spectral reference spectrum to form an updated target score equation; for each scene pixel remaining in the plurality of scene pixels, computing an updated intermediate target score using the updated target score equation; comparing the updated intermediate target scores to an updated threshold, the updated threshold being greater in value than the threshold; identifying scene pixels having updated intermediate target scores less than the updated threshold as empty scene pixels that do not include targets; if the number of empty scene pixels is greater than or equal to the specified fraction of the total number of scene pixels or the number of intermediate target scores computed is greater than or equal to the counter, for each scene pixel remaining in the plurality of scene pixels, computing a full dimension target score based on a full dimension target score equation including a full set of terms; and declaring which scene pixels include targets corresponding to the given spectral reference based on comparisons of the computed full dimension target score to a full dimension target score threshold, the full dimension target score threshold being greater in value than the threshold and the updated threshold.