Hanning smoothing of uv data (hanningsmooth)

NRAO Home > CASA > CASA Cookbook and User Reference Manual

4.7.3 Hanning smoothing of uv data (hanningsmooth)

For strong spectral line sources (like RFI sources), the Gibbs phenomenon may cause ringing across the frequency channels of an observation. This is called the Gibbs pheomenon and a proven remedy is the Hanning smoothing algorithm. Hanning smoothing is a running mean across the spectral axis with a triangle as a smoothing kernel. The central channel is weighted by 0.5 and the two adjacent channels by 0.25 to preserve the flux. Hanning smoothing significantly reduces Gibbs ringing but there’s no gain without a penalty and here it is the loss of a factor of two in spectral resolution.

In CASA, the hanningsmooth task will apply Hanning smoothing to a spectral line uv data measurement set. The inputs are:

#  hanningsmooth :: Hanning smooth frequency channel data to remove Gibbs ringing
vis                 =         ’’        #  Name of input visibility file (MS)
datacolumn          =      ’all’        #  the name of the MS column into which
                                        #   to write the smoothed data
outputvis           =         ’’        #  name of the output visibility file
                                        #   (MS)
async               =      False        #  If true the taskname must be started
                                        #   using hanningsmooth(...)

hanningsmooth will operate on the input measurement set if no outputvis file name is provided. This option will keep the disk usage of large datasets under control. But one should be aware that the data is overwritten. If outputvis is provided, the task will copy the input MS to a new file with that name and operate there. The datacolumn parameter determines which of the data columns is to be Hanning smoothed: ’all’, ’corrected’ or ’data’. ’all’ refers to both, the CORRECTED_DATA and the DATA column. If ’corrected’ is specified but does not exist in the MS, hanningsmooth will create this column for your convenience.

ALERT: We intend to make the kernel size a user supplied parameter. In the longer term we intend to offer other varieties of spectral smoothing as well.

[next] [prev] [prev-tail] [front] [up]

More information about CASA may be found at the CASA web page

This code is available under the terms of the GNU General Public Lincense