There are several definitions of baseline in astronomy:

  • In radio astronomy a baseline is the vector connecting two radio telescopes that is used in interferometry to determine the fringe rate of a source at the nominal “phase centre” of an observation. For an interferometer with N elements there are N(N+1)/2 independent baselines. The pairs of antennae that are close together are good at measuring the large-scale structures in a source whereas the long baselines have better resolution and resolve the fine structures.
  • In astronomical data, a baseline is a reference point for the measurement of values such as the flux of a source. For example, in spectral line data, the spectral continuum can be accounted for by fitting a polynomial to the variations in the data, ignoring any emission line features. This allows for a measurement of the true height of emission lines, which would have been artificially raised or lowered if the variations in the spectral continuum had not been removed. A baseline fit can account for not just the spectral continuum, but for other effects such as telescope and atmospheric variations in astronomical data.

baseline or continuum fit to spectral line data
Example of a baseline fit to spectral line data.

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