The Very Small Array

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 wholel -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 Compact horn Prototype extended horns Compact Extended 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-130 30

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 • 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. VSA field positions and predicted sources at 30GHz.

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) Large Shallow Survey: • 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. 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. l(l+1)C/2 l 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.

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 ...