Selecting Spectral Windows and Channels

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You may need to run a certain task on a specific channel or range of channels in your data: this is where the 'spw' parameter comes in. This parameter allows you to select both spectral windows (synonymous with IFs in VLA data) and channels. A colon separates spectral window information from channel information, and a tilde denotes a range. NOTE: all index numbering in CASA begins with 0, not 1!

Here are some basic examples:

spw = '1'                        # Spectral window 1 
spw = '0~3'                      # Spectral windows 0, 1, 2, and 3
spw = '1,3,5'                    # Spectral windows 1, 3, and 5
spw = '*:10~50'                  # Channels 10--50 for all spectral windows
spw = '4~7:30~40'                # Channels 30--40 for spectral windows 4, 5, 6, and 7

In most cases, to specify a single channel, you should just be able to type:

spw = '1:30'                        # Channel 30 of spectral window 1 

However, in plotms, to specify a single channel, you'll need to write it as a range:

spw = '1:30~30'                        # Channel 30 of spectral window 1 in plotms

You can also use frequency units to select spectral windows or channels:

spw = '1412~1415MHz'             # All spectral windows containing 1412--1415 MHz
spw = '*:1412~1415MHz'           # Channels in the range 1412--1415 MHz

It is also possible to specify multiple ranges:

spw = '0:3~10,1:20~30'           # Channels 3--10 from spectral window 0 and channels 20--30 from spectral window 1 
spw = '2:5~15;25~35'             # Channels 5--15 and 25--35 for spectral window 2

You can find more detail in the CASA Cookbook.

Last checked on CASA Version 3.0.0 (r9860)

--Laura Chomiuk 21:52, 10 February 2010 (UTC)

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