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#### A.4.4.8 Spectrogram Layer Control

The Spectrogram Layer Control () plots a spectrum at successive (usually, but not necessarily, regularly-spaced) points in a time series. It is only available for the Time Plot Window; you can add one of these controls to the stack by using the Add Spectrogram Control () button in the control panel toolbar, or the corresponding item in the Layers menu.

Time Plot window with a Spectrogram layer

This control has layer-specific tabs Data and Style, described below. The Zone tab is described in Appendix A.4.14.1.

To control the colour map used to represent the spectral values, use the Aux fixed control.

Data Tab

Spectrogram control Data tab

The Data tab allows you to specify which values from a table will generate a spectrogram. It has the following fields:

Table
  The table supplying the data.
Time
  A table column or expression giving the epoch coordinate at which spectra are located. This should normally be a time-typed column; if it is simply of numeric type it will be interpreted as seconds since 1 Jan 1970.
Spectrum
  An array-valued table column giving the spectral data.
TimeWidth
  A table column or expression (variable or constant) giving the temporal coverage of a plotted spectrum. If not filled in, it is assumed to be the most common (median) difference between time points.
Row Subset
  The subset for which the spectrum should be plotted. To plot multiple subsets (not usually useful with this kind of plot) you would need multiple spectrogram layer controls in the stack.

Style Tab

Spectrogram control Style tab

The Style tab configures the plotting style. Options are:

Scale Spectra
  If this option is checked (the default), an attempt will be made to plot the spectra on a vertical axis that represents their physical values. This is only possible if the column or table metadata contains a suitable array that gives bin extents or central wavelengths or similar. An ad hoc search is made of column and table metadata to find an array that looks like it is intended for this purpose.

  If this option is unchecked, or if no suitable array can be found, the vertical axis just represents channel indices and so is labelled from 0 to the number of channels per spectrum.

  This configuration item is somewhat experimental; the details of how the spectral axis is configured may change in future releases.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk