1. The Very Small Array Angela Taylor & Anze Slosar Cavendish Astrophysics University of Cambridge

2. 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...

3. The VSA Site - Tenerife Observatorio del Teide, Tenerife 2400 m altitude Transparency at 30 GHz  98 % Excellent weather (< 10% data lost)

4. VSA

5. CBI

6. DASI

7. Compact vs Extended Array Mirror Size /mm143322 Prim. Beam (30 GHz)5.4  2.4  Synth. Beam (30 GHz)  34'  11' l - Range 100 - 700 300 - 1800  S (28 x7hr) /mJy24 4  T (28 x7hr) /  K beam -1 3030 C ompact Extended Compact hornPrototype extended horns

9. VSA Correlator

10. 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)

11. Calibration on Jupiter 80 mins integration time Dynamic Range ~500:1 Noise ~ 240 Jy/(baseline x sec) 1/2

12. Observation of the Cygnus Loop VSA 30 GHz contours superimposed on Green Bank 15 GHz data. Test of VSA ability to map known structure.

13. 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.

14. 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)

15. 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.

16. 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

17. Source subtraction antenna in enclosure, Tenerife, April 2000

18. 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 

19. 80 days total observing on one field with VSA Compact Array 80 x 5 hours rms noise ~ 60 mJy 31 x 25 arcmin resolution

20. 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...