The Very Small Array Angela Taylor & Anze Slosar Cavendish Astrophysics University of Cambridge.
Transcript of The Very Small Array Angela Taylor & Anze Slosar Cavendish Astrophysics University of Cambridge.
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The Very Small Array
Angela Taylor & Anze Slosar
Cavendish Astrophysics
University of Cambridge
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Overview of the VSA
• Collaboration between the Instituto de Astrofisica de Canarias, Jodrell Bank (Manchester University), Mullard Radio Astronomy Observatory (Cambridge University)
• 14-element interferometer.
• Observes at 26-36 GHz, from Observatorio del Teide, Tenerife
• Will image and measure power spectrum of CMB anisotropies for 100 < l < 1800
• Single sideband, single polarisation.
• Dual (scaled) array configuration ensures constant temperature sensitivity across whole l -range...
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The VSA Site - Tenerife
•Observatorio del Teide, Tenerife
•2400 m altitude
•Transparency at 30 GHz 98 %
•Excellent weather (< 10% data lost)
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VSA
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CBI
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DASI
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Compact vs Extended Array
Mirror Size /mm 143 322
Prim. Beam (30 GHz) 5.4 2.4
Synth. Beam (30 GHz) 34' 11'
l - Range 100 - 700 300 - 1800
S (28 x7hr) /mJy 24 4
T (28 x7hr) /K beam-1 30 30
Compact Extended
Compact horn
Prototype extended horns
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VSA Correlator
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VSA - Instrument Calibration
Geometry (+pointing) calibration:
• Use model of telescope and calibration observation of unresolved point source in Maximum Likelihood analysis to determine 300-400 parameters.
Flux calibration:
• Cas A, Cyg A, Crab, Jupiter• Based on Mason et al. 1999• Both primary (daily) and secondary
(weekly) calibrations.• ‘Rain gauge’ (noise injection system)
reject/ correct for atmospheric excess signal.
Phase calibration:• Good phase stability (~weeks)
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Calibration on Jupiter
• 80 mins integration time
• Dynamic Range ~500:1
• Noise ~ 240 Jy/(baseline x sec)1/2
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Observation of the Cygnus Loop
• VSA 30 GHz contours superimposed on Green Bank 15 GHz data.
• Test of VSA ability to map known structure.
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Observing Program
VSA field positions and predicted sources at 30GHz.
• Routine CMB observing since September 2000.
• Compact array used both for wide shallow survey and smaller mosaiced regions to produce sensitive measurements up to l = 700.
• 3 evenly spaced regions plus calibrators observed each day.
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Current Status (1)
Small Mosaiced Regions:
• Mosiacing increases l -resolution and reduces sample variance.
• Size of mosaiced area is limited by speed of source subtraction survey.
• all mosaiced fields integrated to sample variance ( 80 days on each)
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Current Status (2)
• Reduce sample variance and measure first
peak.
• all fields (2 days on each) completed.
• Emphasis on low l means source subtraction
not a serious problem.
Large Shallow Survey:
All Compact Array observations complete now.
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Source Subtraction for the VSA
The Problem...
• At 26-36 GHz we expect extragalactic radio sources to be a major contaminant.
• There exists no suitable high-frequency all-sky survey.
• Sources are known to be variable.
And The Solution...
• Survey VSA fields at 15 GHz with Ryle telescope to a sensitivity of 4mJy.
• Simultaneously with the VSA 30GHz observation, monitor each Ryle source with subtraction interferometer at 30GHz
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Source subtraction antenna in enclosure,
Tenerife, April 2000
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Effect of Sources on the CMB Power Spectrum
• Source power spectrum at 30GHz based on the 15GHz survey .
• Simulated source power spectrum after VSA subtraction strategy implemented.
l
l(l+1)C/2
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80 days total observing on one field with
VSA Compact Array
• 80 x 5 hours
• rms noise ~ 60 mJy
•31 x 25 arcmin resolution
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Conclusions
• The VSA will measure CMB anisotropies over 100 < l < 1800.
• Compact array observations are now complete. ( 100 < l < 700).
• Extended array observations will begin in October (300 < l < 1800).
• Point source subtraction is essential and is integral to the experiment.
• Analysis going well and first results anticipated soon ...