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Primary Beam Shape Calibration from Mosaicked Observations

Primary Beam Shape Calibration from Mosaicked Observations. Chat Hull Collaborators : Geoff Bower, Peter Williams, Casey Law, Steve Croft, Dave Whysong , Gerry Harp, and the rest of the ATA team GSPS 4 December 2009. The Allen Telescope Array.

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Primary Beam Shape Calibration from Mosaicked Observations

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  1. Primary Beam Shape Calibration from Mosaicked Observations Chat Hull Collaborators: Geoff Bower, Peter Williams, Casey Law, Steve Croft, Dave Whysong, Gerry Harp, and the rest of the ATA team GSPS 4 December 2009

  2. The Allen Telescope Array • Centimeter-wave LNSD interferometer in Hat Creek, CA • Commensal observing with SETI • Wide-band frequency coverage: 0.5 – 11.2 GHz (3-60 cm) • Excellent survey speed (5 deg2 FOV) • Present: ATA-42, 6.1-meter antennas • Future: ATA-350 – greater sensitivity

  3. Beam characterization • Beam: sensitivity relative to the telescope’s pointing center • Beam pattern is a sinc function (Airy disk – response of a parabolic antenna) • Central portion of the beam is roughly Gaussian • Good approximation out to the ~10% level • By that point, other effects dominate (sidelobes, reflections)

  4. Motivation • Want to make mosaics • Need to have excellent characterization of the primary beam shape • My aim: characterize it! • Using archival data from ATATS • Start with FWHM • Canonical value:

  5. Same source, multiple appearances Pointing 1 Pointing 2 Images courtesy of Steve Croft • Can use multiple matches of many sources to characterize the beam

  6. Two-point Gaussian solution

  7. Two-point Gaussian solution • Analytic solution to the Gaussian between two source appearances: • r1 , r2  distances from respective pointing centers • S1 , S2 fluxes in respective pointings

  8. Two-point Gaussian solution • Solution: • Problems: when S1 ≈ S2 and when r1 ≈ r2

  9. Problematic pairs Observed flux ratios

  10. Problematic pairs Distance ratios

  11. BART ticket across the Bay $3.65 2012 projection of UC Berkeley undergraduate fees $465,700.31 Not being able to use the best part of your data Priceless

  12. Observed flux pairs Untrimmed, uncorrected

  13. Observed flux pairs Trimmed, uncorrected

  14. Corrected flux pairs Untrimmed, corrected

  15. Corrected flux pairs Trimmed, corrected

  16. FWHM values from trimmed data

  17. Finding the best-fit FWHM

  18. Other beam characterizations • Hex-7 results • FWHM values close to canonical value • Beam holography • Slightly larger value

  19. Future work • PiGSS data • Constrain beam angle and ellipticity • Will have to contend with transformation from RA/Dec to Az/El • Compare these synthesized results with Gerry’s antenna-by-antenna results • Tweak the Gaussian approximation when solving for FWHM • Give a more rigorous statistical treatment to the data (MLE?)

  20. Conclusions • Beam has the expected FWHM! • Our value: • Telescope is producing the data we expect • Arrived at an answer with zero telescope time • Potential application to other radio telescopes needing simple beam characterization

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