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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