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Frequency Frame Alignment

When time averaging a series of scans together, it is possible that the velocity scales are not exactly aligned. This may be for many reasons such as not Doppler tracking the observations, errors in the Doppler tracking etc. This mostly affects very long integrations or integrations averaged together from different days. Before averaging such data together, they should be frequency aligned using freq_align.

E.g.:

  ASAP> scans.freq_align()
  ASAP> av = average_time(scans)

freq_align has two modes of operations controlled by the perif argument. By default it will align each source and freqid separately. This is needed for scan tables containing multiple sources. However if scan-based Doppler tracking has been made at the observatory, each row will have a different freqid. In these cases run with perif=True and all rows of a source will be aligned to the same frame. In general perif=True will be needed for most observations as Doppler tracking of some form is made at Parkes, Tid and Mopra.

  ASAP> scans.freq_align(perif=True)

To average together data taken on different days, which are in different scantables, each scantable must aligned to a common reference time then the scantables averaged. The simplest way of doing this is to allow ASAP to choose the reference time for the first scantable then using this time for the subsequent scantables.

  ASAP> scans1.freq_align() # Copy the refeference Epoch from the output
  ASAP> scans2.freq_align(reftime='2004/11/23/18:43:35')
  ASAP> scans3.freq_align(reftime='2004/11/23/18:43:35')
  ASAP> av = average_time(scans1, scans2, scans3)