An iodine cell places reference absorption on the same beam as stellar lines. A template and known cell transmission constrain stellar shift and instrumental line shape together. The observed spectrum is modelled by multiplying the stellar and cell spectra before instrumental convolution.
At Lick Observatory, the Hamilton spectrograph records starlight with an imprinted iodine reference. The reference tracks instrumental wavelength and profile changes during the science exposure. Barycentric correction and a time series of relative stellar shifts then support a dynamical companion model.
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