Converting frequency and velocity frames

For details on the various frequency and velocity definitions and rest frames used in JCMT heterodyne observations, please see our Velocity Considerations page.

Common requests:

  1. to convert processed data from the JCMT Science Archive, which is in Barycentric frequency, into radio line-of-sight velocity () in LSRK. You can use the following Starlink commands to do this, assuming you have started with a file named ‘archive_file.fits’. These would be copied into a terminal, after setting up the Starlink software within that terminal. The lines starting with a # symbol are comments.
    # Load the convert package to enable conversion from FITS to NDF
    # Convert your archive file into Starlink Data Format (NDF)
    fits2ndf archive_file.fits archive_file.sdf
    # Load the KAPPA package (contains many useful routines)
    # Set the 3rd axis of the coordinate system to be in units of radio velocity
    wcsattrib ndf=archive_file.sdf mode=set name=system\(3\) newval=vrad
    # Set the standard of rest to be the LSRK
    wcsattrib ndf=archive_file.sdf mode=set name=StdofRest newval=LSRK
    # OPTIONAL: if fits output is required, convert the file back to fits
    ndf2fits archive_file.sdf archive_file_radiovelocity.fits
  2. to add together spectral data from moving targets (eg comets) onto a ‘source’ (‘cometocentric’) velocity scale:
    makecube <rawdatafile00099> system=GAPPT alignsys=TRUE out=out99.sdf
    wcsattrib out99.sdf set alignoffset 1
    wcsattrib out99.sdf set skyrefis origin
    wcsattrib out99.sdf set sourceVRF topocentric
    wcsattrib out99.sdf set stdofrest source
    wcsattrib out99.sdf set alignstdofrest source
    wcsattrib out99.sdf set SourceVel <velocity>
    wcmosaic out*.dat . . .


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