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Spectral Window Selection

A Miriad visibility dataset can contain simultaneous observations at multiple spectral bands, or spectral windows. These are called `IF-bands' in AIPS. When there are multiple spectral windows, channels are still numbered from 1 to N, where channel 1 is the first channel of the first spectral window, and channel N is the last channel of the last spectral window. That is, channel numbering takes no regard of spectral window boundaries.

When there are multiple spectral windows, it is often convenient to be able to select based on the spectral window, rather than a channel range. This is achieved with select=window, which selects data from a set of spectral windows. The general form is

select=window(w1,w2,...)

where w1, w2, etc, are spectral window numbers. For example, selecting the data from spectral windows 1, 2 and 3 would be achieved with

select=window(1,2,3)

Spectral window selection and velocity/felocity linetypes are quite complementary. The different spectral windows might correspond to different spectral lines with different rest frequencies, and so it is desirable to select only a subset of spectral windows (those corresponding to a given spectral line) in forming the output velocity channels.

Spectral window selection is not particularly complementary to channel linetype processing - both select a range of input channels. When line=channel and select=window are used together, the total apparent number of input channels is equal to the number of channels in the selected spectral windows, and the start channel number of the line parameter is relative to the first selected spectral window. For example, if there are multiple spectral windows, then

select=window(2)
line=channel,1

would select just the first channel of the second spectral window. Similarly

select=window(2)
line=channel,10,8,1,2

would select 10 channels, being every second channel starting at the 8th channel of the second spectral window.

There are some restrictions on the use of spectral window selection.

• For channel linetype, the selected spectral windows must be contiguous. For example, you cannot select just windows 1 and 3. This restriction is relaxed by some tasks, most notably uvcat.
• For channel linetype, window selection cannot be used with or selection.
• For velocity/felocity linetype, if both window and or selections are used, then there should be a window selection within each or clause. Additionally only one of the or clauses should be satisfiable for a given visibility. For example, consider the following:

     select=time(0:00,1:00),window(1,2),or
            time(0:00,2:00),window(3,4)
  

  Here data in the times 0:00 to 1:00 could have been selected by either of the two clauses. There is no guarantee as to which of the two window subcommands will be used during this time interval.