163 research outputs found
On Measuring Accurate 21-cm Line Profiles with the Robert C. Byrd Green Bank Telescope
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)
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
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
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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