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Spectroscopy: IF bandwidth

         

Two different IF bandwidths enter into the observing strategy for spectroscopy.

  1. The total IF bandwidth tex2html_wrap_inline2605 that is spanned by all the line channels. tex2html_wrap_inline2605 determines the range of velocities included by the experiment, which should span not only the interesting parts of the line profile, but also an adequate number of line-free channels (allowing for any band-edge effects in autocorrelation spectrometers).
  2. The individual channel bandwidth tex2html_wrap_inline2609 . This sets the velocity resolution of the experiment. It also determines the chromatic aberration effects for the individual channel images.

As indicated in Figure 3, the choices of tex2html_wrap_inline2605 and tex2html_wrap_inline2609 are usually tightly coupled by the design of the spectral line correlator, and velocity resolution and velocity span must often be traded against one another.

The need for velocity resolution usually drives the channel bandwidth to a small enough value that individual channel images are negligibly smeared by chromatic aberration. This does not mean, however, that spectroscopists can ignore chromatic aberration in synthesis imaging. The synthesized beam pattern is different for every channel image when the channels are gridded separately with their correct frequencies. Any data reduction schemes that do not separately deconvolve the synthesized beam from each of the individual channel images may therefore ``rediscover" effects related to the frequency dependence of the beam. As Lecture 12 describes, such separate deconvolution of all the channels is not always a good strategy. If you estimate the continuum distribution by summing the ``dirty" images over a range of line-free channels, or if you difference two ``dirty" line channel images, the variation of the synthesized beam with frequency may become apparent. If you try to avoid this effect for some channels by artificially assigning them the same frequency (to make the ``dirty beam" the same for all images), you will restore the chromatic aberration corresponding to the widest channel separation tex2html_wrap_inline2615 used in such processing.


next up previous contents index external
Next: Averaging time Up: The Spectroscopist's Decision Tree Previous: Observing frequency

abridle@nrao.edu
Thu Jul 11 16:26:53 EDT 1996