1. A system comprising: spectra means for generating a series of spectra, each spectrum in said series being constituted by plural data values taken at different wavelengths and a common respective time; orthonormal error space construction means for constructing an orthonormal spectral space from said series, said spectral space representing systematic errors in said series; and orthonormal subtraction means for subtracting from each spectrum in said series a projection of the same spectrum in said spectral space.