Units, Doppler and Frequency Reference Frame

The information describing the frequency setup for each integration is stored fundamentally in frequency in the reference frame of observation (E.g. TOPO).

When required, this is converted to the desired reference frame (e.g. LSRK), Doppler (e.g. OPTICAL) and unit (e.g. km/s) on-the-fly. This is important, for example, when you are displaying the data or fitting to it. The reference frame is set on file read to the value set in the user .asaprc file.

For units, the user has the choice of frequency, velocity or channel. The set_unit function is used to set the current unit for a scantable. All functions will (where relevant) work with the selected unit until this changes. This is mainly important for fitting (the fits can be computed in any of these units), plotting and mask creation.

The velocity definition can be changed with the set_doppler function, and the frequency reference frame can be changed with the set_freqframe function.

Example usage:

  ASAP>scans = scantable('2004-11-23_1841-P484.rpf') # Read in the data
  ASAP>scans.set_freqframe('LSRK')  # Use the LSR velocity frame
  ASAP>scans.set_unit('km/s')        # Use velocity for plots etc from now on
  ASAP>scans.set_doppler('OPTICAL')  # Use the optical velocity convention
  ASAP>scans.set_unit('MHz')         # Use frequency in MHz from now on