163 research outputs found

    On Measuring Accurate 21-cm Line Profiles with the Robert C. Byrd Green Bank Telescope

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    We use observational data to show that 21 cm line profiles measured with the Green Bank Telescope (GBT) are subject to significant inaccuracy. These include ~10% errors in the calibrated gain and significant contribution from distant sidelobes. In addition, there are ~60% variations between the GBT and Leiden/Argentine/Bonn 21 cm line profile intensities, which probably occur because of the high main-beam efficiency of the GBT. Stokes V profiles from the GBT contain inaccuracies that are related to the distant sidelobes. We illustrate these problems, define physically motivated components for the sidelobes, and provide numerical results showing the inaccuracies. We provide a correction scheme for Stokes I 21 cm line profiles that is fairly successful and provide some rule-of-thumb comments concerning the accuracy of Stokes V profiles.Comment: 39 pages, 20 figures, accepted for publication in PAS

    A New Technique for Heterodyne Spectroscopy: Least-Squares Frequency Switching (LSFS)

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    We describe a new technique for heterodyne spectroscopy, which we call Least-Squares Frequency Switching, or LSFS. This technique avoids the need for a traditional reference spectrum, which--when combined with the on-source spectrum--introduces both noise and systematic artifacts such as ``baseline wiggles''. In contrast, LSFS derives the spectrum directly, and in addition the instrumental gain profile. The resulting spectrum retains nearly the full theoretical sensitivity and introduces no systematic artifacts. Here we discuss mathematical details of the technique and use numerical experiments to explore optimum observing schemas. We outline a modification suitable for computationally difficult cases as the number of spectral channels grows beyond several thousand. We illustrate the method with three real-life examples. In one of practical interest, we created a large contiguous bandwidth aligning three smaller bandwidths end-to-end; radio astronomers are often faced with the need for a larger contiguous bandwidth than is provided with the available correlator.Comment: 37 pages, 8 figure

    Our Local Microcosmos

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    UM Department of Astronomy, UM Student Astronomical Society, UM Exhibit Museum of Natural History, Michigan Center for Theoretical Physicshttp://deepblue.lib.umich.edu/bitstream/2027.42/57277/3/our_local_microcosmos.MP3http://deepblue.lib.umich.edu/bitstream/2027.42/57277/2/our_local_microcosmos.pd

    Crosscorrelation Spectropolarimetry In Single-Dish Radio Astronomy

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    Modern digital correlators permit the simultaneous measurement of all four Stokes parameters using auto and crosscorrelation. We briefly outline the fundamental requirements and some practical details of performing such measurements and refer to two additional papers that provide overview and cover calibration issues in detail.Comment: 8 pages, no figures. Accepted by PAS
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