The Very Small Array

Angela Taylor & Anze Slosar

Cavendish Astrophysics

University of Cambridge

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...

The VSA Site - Tenerife

•Observatorio del Teide, Tenerife

•2400 m altitude

•Transparency at 30 GHz 98 %

•Excellent weather (< 10% data lost)

VSA

CBI

DASI

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

VSA Correlator

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)

Calibration on Jupiter

• 80 mins integration time

• Dynamic Range ~500:1

• Noise ~ 240 Jy/(baseline x sec)1/2

Observation of the Cygnus Loop

• VSA 30 GHz contours superimposed on Green Bank 15 GHz data.

• Test of VSA ability to map known structure.

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.

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)

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.

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

Source subtraction antenna in enclosure,

Tenerife, April 2000

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

80 days total observing on one field with

VSA Compact Array

• 80 x 5 hours

• rms noise ~ 60 mJy

•31 x 25 arcmin resolution

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 ...