Lick Observatory is an astronomical observing site associated with optical spectroscopy and precision measurements of stellar motion. In the Lick Observatory iodine-cell radial-velocity method, a cell places known iodine absorption on the stellar beam, allowing instrumental changes and a stellar Doppler shift to be fitted together.
This apparatus illustrates why a spectral shift cannot be read independently of calibration. The reference and stellar spectra pass through the same optical system, and their combined image must be modelled with the instrumental line profile. Repeated measurements then test an orbital signal rather than merely identifying one shifted spectral line.
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