Setting rest frequencies in plotxy

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3.3.2.12 Setting rest frequencies in plotxy

The restfreq parameter can be set to a transition or frequency and expands to allow setting of frame information. For example,

restfreq            =    ’HI’   #  a frequency quanta or transition name. see help for options
     frame          =  ’LSRK’   #  frequency frame for spectral axis. see help for options
     doppler        = ’RADIO’   #  doppler mode. see help for options

Examples of transitions include:

   restfreq=’1420405751.786Hz’  #  21cm HI frequency
   restfreq=’HI’                #  21cm HI transition name
   restfreq=’115.2712GHz’       #  CO 1-0 line frequency

For a list of known lines in the CASA measures system, use the toolkit command me.linelist(). For example:

CASA <14>: me.linelist()
Out[14]: ’C109A CI CII166A DI H107A H110A H138B H166A H240A H272A H2CO HE110A HE138B HI OH1612 OH1665 OH1667 OH1720’

ALERT: The list of known lines in CASA is currently very restricted, and will be increased in upcoming releases (to include lines in ALMA bands for example).

You can use the me.spectralline tool method to turn transition names into frequencies

CASA <16>: me.spectralline(’HI’)
Out[17]:
{’m0’: {’unit’: ’Hz’, ’value’: 1420405751.786},
’refer’: ’REST’,
’type’: ’frequency’}

(not necessary for this task, but possibly useful).

The frame sub-parameter sets the frequency frame. The allowed options can be listed using the me.listcodes method on the me.frequency() method, e.g.

CASA <17>: me.listcodes(me.frequency())
  Out[17]:
{’extra’: array([],
      dtype=’|S1’),
’normal’: array([’REST’, ’LSRK’, ’LSRD’, ’BARY’, ’GEO’, ’TOPO’, ’GALACTO’, ’LGROUP’,
       ’CMB’],
      dtype=’|S8’)}

The doppler sub-parameter likewise sets the Doppler system. The allowed codes can be listed using the me.listcodes method on the me.doppler() method,

CASA <18>: me.listcodes(me.doppler())
  Out[18]:
{’extra’: array([],
      dtype=’|S1’),
’normal’: array([’RADIO’, ’Z’, ’RATIO’, ’BETA’, ’GAMMA’, ’OPTICAL’, ’TRUE’,
       ’RELATIVISTIC’],
      dtype=’|S13’)}

For most cases the ’RADIO’’ Doppler system is appropriate, but be aware of differences.

For more information on frequency frames and spectral coordinate systems, see the paper by Greisen et al. (A&A, 446, 747, 2006) ².

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