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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
GSPS4 December 2009
4 December 2009
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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 sensitivity4 December 2009
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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 December 2009
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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:
4 December 2009
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Same source, multiple appearances
4 December 2009
Images courtesy of Steve Croft
Pointing 1 Pointing 2
• Can use multiple matches of many sources to characterize the beam
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Two-point Gaussian solution
4 December 2009
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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
4 December 2009
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Two-point Gaussian solution
• Solution:
• Problems: when S1 ≈ S2 and when r1 ≈ r2
4 December 2009
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Problematic pairs
4 December 2009
Observed flux ratios
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Problematic pairs
4 December 2009
Distance ratios
114 December 2009
BART ticket across the Bay
2012 projection of UC Berkeley undergraduate fees
Not being able to use the best part of your data
Priceless
$3.65
$465,700.31
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Observed flux pairs
4 December 2009
Untrimmed, uncorrected
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Observed flux pairs
4 December 2009
Trimmed, uncorrected
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Corrected flux pairs
4 December 2009
Untrimmed, corrected
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Corrected flux pairs
4 December 2009
Trimmed, corrected
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FWHM values from trimmed data
4 December 2009
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Finding the best-fit FWHM
4 December 2009
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Other beam characterizations
• Hex-7 results– FWHM values close to canonical value
• Beam holography– Slightly larger value
4 December 2009
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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?)
4 December 2009
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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
4 December 2009