Averaging

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8.3.5 Averaging

One can average polarizations in a scantable using the sd.scantable.average_pol function:

  averaged_scan = scans.average_pol(mask,weight)

  where:
    Parameters:
        mask:        An optional mask defining the region, where the
                     averaging will be applied. The output will have all
                     specified points masked.
        weight:      Weighting scheme. ’none’ (default), ’var’ (1/var(spec)
                     weighted), or ’tsys’ (1/Tsys**2 weighted)

    Example:

  spave = stave.average_pol(weight=’tsys’)

One can also average scans over time using sd.average_time:

  sd.average_time(scantable,mask,scanav,weight,align)

  where:

    Parameters:
        one scan or comma separated  scans
        compel:   if True, enable averaging of multi-resolution spectra.
        mask:     an optional mask (only used for ’var’ and ’tsys’ weighting)
        scanav:   True averages each scan separately.
                  False (default) averages all scans together,
        weight:   Weighting scheme.
                    ’none’     (mean no weight)
                    ’var’      (1/var(spec) weighted)
                    ’tsys’     (1/Tsys**2 weighted)
                    ’tint’     (integration time weighted)
                    ’tintsys’  (Tint/Tsys**2)
                    ’median’   ( median averaging)
        align:    align the spectra in velocity before averaging. It takes
                  the time of the first spectrum in the first scantable
                  as reference time.
    Example:

  stave = sd.average_time(scans,weight=’tintsys’)

Note that alignment of the velocity frame should be done before averaging if the time spanned by the scantable is long enough. This is done through the align=True option in sd.average_time, or explicitly through the sd.scantable.freq_align function, e.g.,

CASA <62>: sc = sd.scantable(’orions_scan20to23_if0to3.asap’,False)
CASA <63>: sc.freq_align()
Aligned at reference Epoch 2006/01/19/01:49:23 (UTC) in frame LSRK
CASA <64>: av = sd.average_times(sc)

The time averaging can also be applied to multiple scantables. For example, such data might have been taken on different days. The sd.average_time function takes multiple scantables as input. However, if they are taken at significantly different times (different days for example), then sd.scantable.freq_align must be used to align the velocity scales to the same time, e.g.

CASA <65>: sc1 = sd.scantable(’orions_scan21_if0to3.asap’,False)
CASA <66>: sc2 = sd.scantable(’orions_scan23_if0to3.asap’,False)
CASA <67>: sc1.freq_align()
Aligned at reference Epoch 2006/01/19/01:49:23 (UTC) in frame LSRK
CASA <68>: sc2.freq_align(reftime=’2006/01/19/01:49:23’)
Aligned at reference Epoch 2006/01/19/01:54:46 (UTC) in frame LSRK
CASA <69>: scav = sd.average_times(sc1,sc2)

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