AVERAGE

AVERAGE can perform a coherent or an incoherent average. The user must specify both an integration time (T sec, say), and an averaging method. In all methods, the input data records are binned into intervals each of length T sec (UT). All the data in each bin are averaged (by the algorithms described below) to generate one output record. The time-tag attached to the output record is always the midpoint of the interval (not the centroid). This is to avoid problems matching up data on different baselines.

In an incoherent average (METHOD=I), the averaged amplitude is the rms of the input amplitudes, and visibility phases and closure phases are averaged by averaging the sine and cosine of the input phases. WARNING: the incoherent average destroys the phase-closure relations: the averaged visibility phases are no longer consistent with the averaged closure phases.

Two alternative methods of computing a coherent average may be requested (METHOD=C or METHOD=S). In both cases the complex visibilities within each interval are averaged in the usual way, while the closure phases (if present) are averaged as in METHOD=I. This will not work if there are many fringe-periods between successive data points: it is important to "unwind" the phases as much as possible before averaging, e.g., by self-calibrating the phases. Methods C and S differ in how they compute the errors (strictly, the uncertainties) in the averaged visibilities. Method C averages the errors in the input file. Method S ignores the errors in the input file, and computes the error from the scatter of the input visibilities. Method S thus requires that there be many input points per averaging bin. Method S is useful when averaging the data output by AIPS global fringe-fitting, where the input errors are usually meaningless.

Example

% average
  INPUT  = T1.MRG
  OUTPUT = T2.MRG
  METHOD = I
  TIME = 4:00
  /

Parameters

INPUT = filename

The name of the input merge-format file (e.g., INPUT = CITSCR:[TJP.B2]T1.MRG). This parameter must be specified; there is no default.

OUTPUT = filename

A name for the output file created by AVERAGE. This parameter must be specified; there is no default.

TIMESCALE = t

The averaging time in seconds; it must be an exact number of seconds. The notation mm:ss can be used if desired; e.g., 2:30 is 2 minutes + 30 s, i.e., 150 s. This parameter must be specified: there is no default.

METHOD = "I", "C", or "S"

Select the algorithm to be used. "I" gives incoherent averaging, "C" coherent averaging with errors obtained by averaging the input errors, and "S" coherent averaging with errors obtained from the scatter in the input. This parameter must be specified: there is no default. Only the first letter is significant: e.g., you can write METHOD=INCOHERENT.

HISTORY = text

A character string (up to 78 characters) to be written in the "history" section of the output file; this is optional. (This is in addition to two history records which record the username, date, averaging time, and method.)

Limitations

2. If the input file is MERGE format 2 (i.e., it contains the u,v,w coordinates) the averaging time cannot be more than 100 times the integration time in the input file.

History