primary beam shape calibration from mosaicked observations chat hull collaborators : geoff bower,...
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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
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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