National Radio
Astronomy Observatory

3.3.1.6 Averaging Data

The Plots > Data tab enables averaging of the data in order to increase signal-to-noise of the plotted points or to increase plotting speed.The options for Averaging ar e:

• channel
• time
• all baselines or per antenna
• all spectral windows
• scalar

The box next to a given Averaging mode needs to be checked for that averaging to take effect.

For example, to average n channels together, the user would click on the box next to Channels so that an “X” appears in it, and then type the number n in the empty box. When the user next clicks on Plot, every n channels will then be averaged together and the total number of channels plotted will be decreased by a factor of n.

Time averaging is a little trickier, as it is controlled by three fields. If the checkbox next to Time under Averaging is clicked on, a blank box with units of seconds will become active, along with two additional checkboxes: Scan and Field. If averaging is desired over a relatively short interval (say, 30 seconds, shorter than the scan length), a number can simply be entered into the blank box and, when the data are replotted, the data will be time averaged. Clicking on the Scan or Field checkbox in this case will have no impact on the time averaging.

These checkboxes become relevant if averaging over a relatively long time—say the entire observation, which consists of multiple scans—is desired. Regardless of how large a number is typed into the Time averaging blank box, only data within individual scans will be averaged together. In order to average data across scan boundaries, the Scan checkbox must be clicked on and the data replotted. Finally, clicking on the Field checkbox enables the averaging of multiple fields together in time.

Clicking on the All Baselines checkbox will average all baselines in the array together. Alternatively, the Per Antenna box may be checked, which will average all baselines for a given antenna together. In this case, all baselines are represented twice; baseline 3-24 will contribute to the averages for both antenna 3 and antenna 24. This can produce some rather strange-looking plots if the user also selects on antenna—say, if the user requests to plot only antenna 0 and then averages Per Antenna, In this case, an average of all baselines including antenna 0 will be plotted, but each individual baseline including antenna 0 will also be plotted (because the presence of baselines 0-1, 0-2, 0-3, etc. trigger Per Antenna averaging to try and compute averages for antennae 1, 2, 3, etc. Therefore, baseline 0-1 will contribute to the average for antenna 0, but it will also singlehandedly be the average for antenna 1.)

Spectral windows can be averaged together by checking the box next to All Spectral Windows. This will result in, for a given channel n, all channels n from the individual spectral windows being averaged together.

Finally, the default mode is vector averaging, where the complex average is formed by averaging the real and imaginary parts of the relevant visibilities. If Scalar is chosen, then the amplitude of the average is formed by a scalar average of the individual visibility amplitudes.

When averaging, plotms will prefer unflagged data. I.e., if an averaging bin contains any unflagged data at all, only the average of the unflagged will be shown. For averaging bins that contain only unflagged data, the average of that unflagged data will be shown. When flagging on a plot of averaged data, the flags will be applied to the unaveraged data in the MS.

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

Copyright © 2010 Associated Universities Inc., Washington, D.C.

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