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).