Time average

Time averaging highlights using mstransform and split

mstransform and split are both able to perform a weighted time average of data.  In mstransform time averaging is accessed by setting timeaverage = True and controlled by the resulting sub-parameters of timeaverage. Additionally, mstransform is able to perform a baseline dependent time average as described in the paper Effects of Baseline Dependent Time Averaging of UV Data by W.D. Cotton [1] .

# mstransform time average parameters


timeaverage         = True # Average data in time.
     timebin        = '0s' # Bin width for time averaging.
     timespan       = ''   # Span the timebin across scan, state or both.
     maxuvwdistance = 0.0  # Maximum separation of start-to-end baselines that can be included in an average(meters)

In split time averaging is available via top level parameters timebin and combine.  The functionality is very similar to that in the old version of split (still available for a period of time as oldsplit).  However, there are some differences as explained below.

# split time average parameters


timebin   = '0s'   # Bin width for time averaging
combine   = ''     # Span the timebin across scan, state or both

  

  • Whereas oldsplit uses exclusively the WEIGHT column to perform the weighted average, mstransform and split use both FLAG and spectral weights (when present). To be specific WEIGHT_SPECTRUM is used when averaring CORRECTED_DATA, and SIGMA_SPECTRUM is used when averaging the DATA column.
  • Also mstransform and split are able to transform the input WEIGHT/SIGMA_SPECTRUM according to the rules of error propagation that apply to a weighted average, which result in an output weight equals to the sum of the input weights. For a detailed reference see, Data Reduction and Error Analysis by Bevington & Robinson [2].
  • When mstransform and split process an ALMA MS and timebin is greater than 30s, timespan is automatically set to state, to overcome a limitation of the ALMA ASDM binary dumps.
  • As of version 4.5, mstransform and split both allow timespan field in addition to scan and state.
  • maxuvdistance: In the case of mstransform, when maxuvdistance is greater than 0 this parameter controls the maximum uv distance allowed when averaging data from the same baseline. It works in conjunction with the timebin parameter in the sense that the averaging process is finalized when either timebin is completed or maxuvdistance is reached. The details of the baseline dependent averaging algorithm are available here:

    ftp://ftp.cv.nrao.edu/NRAO-staff/bcotton/Obit/BLAverage.pdf

Citation Number 1
Citation Text Effects of Baseline Dependent Time Averaging of UV Data by W.D. Cotton (OBIT No. 13, 2008). PDF
Citation Number 2
Citation Text Data Reduction and Error Analysis by Bevington & Robinson (3rd Ed., McGraw Hill, 2003)