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C.J. Salter, T. Ghosh (NAIC, Arecibo Observatory)
When a radio position-switched spectrum is taken in which there is a continuum imbalance between the relevant ON and OFF positions, the resultant (ON-OFF)/OFF spectrum frequently shows a significant standing-wave ripple. This has been found to be the case with Arecibo searches for HI emission and absorption lines against radio-loud AGNs. This effect can be dramatically reduced with a "double-switched" observing mode, using a continuum passband calibrator. Examples are presented, the improvement quantified, and the effect on the signal-to-noise ratios considered.
"The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation".