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Primary Beam Shape Calibration from Mosaicked , Interferometric Observations

Primary Beam Shape Calibration from Mosaicked , Interferometric Observations. Chat Hull Collaborators : Geoff Bower, Steve Croft, Peter Williams, Casey Law, Dave Whysong , and the rest of the ATA team URSI 5 January 2011. Outline. Motivation Beam-characterization method Results.

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Primary Beam Shape Calibration from Mosaicked , Interferometric Observations

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  1. Primary Beam Shape Calibration from Mosaicked, Interferometric Observations Chat Hull Collaborators: Geoff Bower, Steve Croft, Peter Williams, Casey Law, Dave Whysong, and the rest of the ATA team URSI 5 January 2011

  2. Outline Motivation Beam-characterization method Results

  3. The Allen Telescope Array • Centimeter-wave large-number-of-small-dishes (LNSD) interferometer in Hat Creek, CA • Present: ATA-42, 6.1-meter antennas • Wide-band frequency coverage: 0.5 – 11.2 GHz (3-60 cm) • Excellent survey speed (5 deg2 field of view) • Commensal observing with SETI

  4. Bad mosaic

  5. Good mosaic

  6. Motivation • We want to make mosaics • Need to have excellent characterization of the primary beam shape • Primary beam: sensitivity relative to the telescope’s pointing center • Start by characterizing the FWHM of the primary beam using data from ATATS & PiGSS FWHM = 833 pixels Image courtesy of James Gao

  7. PiGSSpointings Bower et al., 2010

  8. Primary-beam characterization • Primary-beam pattern is an Airy disk • Central portion of the beam is well approximated by a Gaussian

  9. Primary-beam characterization • In this work we assume our primary beam is a circular Gaussian. • Our goal: to use ATA data to calculate the actual FWHM of the primary beam at the ATATS and PiGSS frequencies.

  10. Primary-beam characterization • Canonical value of FWHM:

  11. Same source, multiple appearances Pointing 1 Pointing 2 Images courtesy of Steve Croft Can use sources’ multiple appearances to characterize the beam

  12. Same source, multiple appearances Apparent flux densities of the same source in two different pointings • We know the flux densities and the distances from the pointing centers

  13. Least-squaresminimization • Find the FWHM value that minimizes A • Benefits: • Can be extended to fit ellipticity & beam angle

  14. Observed flux pairs

  15. Corrected flux pairs

  16. Best-fit FWHM ATATS PiGSS • High A values due to systematic underestimation of flux density errors, non-circularity of the beam, mismatched sources • We see a slightly narrower beam-width, due to imperfect understanding of ATA antenna response, inadequacy of Gaussian beam model

  17. Conclusions • ATA primary beam has the expected FWHM • Our calculated value: • Results are consistent with canonical value (Welch et al.), radio holography (Harp et al.), and the Hex-7 beam characterization technique • Arrived at an answer with zero dedicated telescope time • Potential application to other radio telescopes needing simple beam characterization using science data

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