1.3.1.0 ms.statwt2

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ms.statwt2 - Function

1.3.1 THIS APPLICATION IS CURRENTLY FOR TESTING ONLY! USE AT YOUR OWN RISK! Compute and set weights based on variance of data.

Description

THIS APPLICATION IS UNDER DEVELOPMENT AND CURRENTLY FOR TESTING ONLY! USE AT YOUR OWN RISK!

THIS APPLICATION MODIFIES THE WEIGHT, WEIGHT SPECTRUM, FLAG, AND FLAG_ROW COLUMNS OF THE INPUT MS. IF YOU WANT A PRISTINE COPY OF THE INPUT MS TO BE PRESERVED, MAKE A COPY OF IT BEFORE RUNNING THIS APPLICATION.

This application computes weights for the WEIGHT and WEIGHT_SPECTRUM (if present) columns based on the variance of values in the CORRECTED_DATA column. If the MS has no CORRECTED_DATA column, the application will fail. The following algorithm is used:

1. For unflagged data, create two sets of values, one set is composed solely of the real part of the data values, the other set is composed solely of the imaginary part of the data values. 2. Compute the variance of each of these sets, vr and vi. 3. Compute veq = (vr + vi)/2. 4. The associated weight is just the reciprocal of veq. The weight will have unit of (data unit)-2), eg Jy-2).

Data are aggregated on a per-baseline, per-data description ID basis. Data are aggregated in bins determined by the specified values of the timebin and chanbin parameters.

TIME BINNING

If the value of timebin is an integer, it means that the specified value should be multiplied by the representative integration time in the MS. This integration is the median value of all the values in the INTERVAL column. Flags are not considered in the integration time computation. If either extrema in the INTERVAL column differs from the median by more than 25%, the application will fail because the values vary too much for there to be a single, representative, integration time. The timebin parameter can also be specified as a quantity (string) that must have time conformant units.

The time bins are not necessarily contiguous and are not necessarily the same width. The start of a bin is always coincident with a value from the TIME column, So for example, if values from the time column are [20, 60, 100, 140, 180, 230], and the width of the bins is chosen to be 110s, the first bin would start at 20s and run to 130s, so that data from timestamps 20, 60, and 100 will be included in the first bin. The second bin would start at 140s, so that data for timestamps 140, 180, and 230 would be included in the second bin. Also, time binning does not span scan boundaries, so that data associated with different scan numbers will always be binned separately; changes in SCAN_NUMBER will cause a new time bin to be created, with its starting value coincident with the time of the new SCAN_NUMBER. Similar behavior can be expected for changes in FIELD_ID and ARRAY_ID.

CHANNEL BINNING

The width of channel bins is specified via the chanbin parameter. Channel binning occurs within individual spectral windows; bins never span multiple spectral windows. Each channel will be included in exactly one bin.

The default value ”spw” indicates that all channels in each spectral window are to be included in a single bin.

Any other string value is interpreted as a quantity, and so should have frequency units, eg ”1MHz”. In this case, the channel frequencies from the CHAN_FREQ column of the SPECTRAL_WINDOW subtable of the MS are used to determine the bins. The first bin starts at the channel frequency of the 0th channel in the spectral window. Channels with frequencies that differ by less than the value specified by the chanbin parameter are included in this bin. The next bin starts at the frequency of the first channel outside the first bin, and the process is repeated until all channels have been binned.

If specified as an integer, the value is interpreted as the number of channels to include in each bin. The final bin in the spectral window may not necessarily contain this number of channels. For example, if a spectral window has 15 channels, and chanbin is specified to be 6, then channels 0-5 will comprise the first bin, channels 6-11 the second, and channels 12-14 the third, so that only three channels will comprise the final bin.

OTHER CONSIDERATIONS

Flagged values are not used in computing the weights, although the associated weights of these values are updated.

If the variance for a set of data is 0, all associated flags for that data are set to True, and the corresponding weights are set to 0.

Because data are modified in the input MS, the nomodify parameter must be set to False when opening the associated MS tool.

Arguments


Inputs
timebin	Length for binning in time to determine statistics. Can either be integer to be multiplied by the representative integration time, a quantity (string) in time units.
	allowed:	any
	Default:	variant 1
chanbin	Channel bin width for computing weights. Can either be integer, in which case it is interpreted as number of channels to include in each bin, or a string ”spw” or quantity with frequency units.
	allowed:	any
	Default:	variant spw

Returns
bool

Example

            # update the weights of an MS
            ms.open("my.ms", nomodify=False)
            # compute weights, using time bins of 300s
            if ms.statwt(timebin=("300s")):
                print "Successfully updated weights"
            else:
                print "Updating weights failed"
            ms.done()

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