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Observing frequency

           

Many spectroscopic experiments have little (or no) choice of observing frequency. The line frequency is set by the chemical species and transition relevant to the astronomical problem. The observing frequency is then determined by correcting the line frequency for the systemic radial velocity and for the various motions of the Earth (see Lecture 11). For the experiment to be viable, the corrected frequency must, of course, lie within the tuning range of the receivers and feeds.

If the chemical species has many radio-frequency transitions (e.g., recombination lines), instrumental performance and astronomical criteria may again be combined when picking the observing frequency. For such projects, the instrumental criteria are similar to those for the continuum: receiver sensitivity, antenna efficiency, pointing accuracy. The astronomical criteria may include whether different transitions will be in emission, or absorption, or both.

The last factor to be considered is radio frequency interference (RFI). Some spectroscopic observations, especially those at frequencies below 2 GHz and those of objects whose red shifts move the observing frequency out of a protected band, are simply not possible because of RFI.


next up previous contents index external
Next: Spectroscopy: IF bandwidth Up: The Spectroscopist's Decision Tree Previous: The Spectroscopist's Decision Tree

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