Keywords: imager

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## Frequency and Velocity scales

In the ALMA use of CASA, all observations are kept in the Observatory frequency frame, and only converted to a celestial reference frame (such as the Local Standard of Rest, `LSRK`) at later stages, during imaging if requested.

GILDAS is more analysis oriented, and contains a dual interpretation of the frequency axis. This axis can either be interpreted with a Velocity scale, usually in the `LSRK` frame relative to a spectral line rest frequency, or as a Rest Frequency, with the astronomical source velocity specified. The choice of representation depends on the astronomer's science objective: astronomical object study (in which case the Velocity representation is more appropriate) or chemical composition study (in which case the Rest Frequency representation is preferred).