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#### A.4.4.4 Spectrogram Layer Control

This control is experimental. As currently implemented it lacks some important features. It may be improved or withdrawn in a future version.

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 experimental Time Plot Window.

Time Plot window with a Spectrogram layer

This control has two tabs, Data and Style, described below.

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.

Spectrogram control Style tab

The Style allows you to configure the way the spectrogram is plotted. It has the following fields:

Spectral Shader
  Select the colour map from a list of options.
Shader Clip
  Select a sub-range of the full colour map above. By default the whole colour map is used, but if you want the range of colours in the plot to be formed from only a part of the colour band shown in the Shader control, you can move the handles in from the end of this slider.
Shader Flip
  Whether the spectral values should map forwards or backwards into the colour map.
Shader Quantise
  Allows the colour map to be quantised. By default, the colour map is effectively continuous. If you slide the slider to the right, or enter a value in the text field, the map will be split into the map will be split into a decreasing number of discrete colours. This can be used to generate a contour-like effect, and may make it easier to trace the boundaries of regions of interest by eye.
Scaling
  Determines the function used to map the range of spectral data values onto the colour map. Options are linear, logarithmic, square and square root.
Spectral Subrange
  Adjusts the range of the spectral coordinate over which the colour map is applied. By default the colour map is scaled using limits found from the spectral data values themselves, but you can restrict the range using this slider.
Null Colour
  What colour should be used to represent spectral samples with a null value. If the associated Hide option is selected, then those points will not appear in the plot at all (they will be transparent).

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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@bristol.ac.uk