Array Configuration

1. Array Configuration DivyaOberoi MIT Haystack Observatory

2. In the 512T world… • Capture order 5% of total visibilities with native correlator resolution • Assume/imagine complete flexibility about which visibilities can be captured • Question - How to choose which visibilities to capture? • 128T  1/16th of the visibilities • Get a sense for what sort of uv coverage can an optimally chosen set of 128 tiles provide

3. Characteristics of the Radio Sun Oberoi et al., 2011, ApJ, 728, L27 The “quiet” Sun is more dynamic than previously thought.

4. The “active” Sun, we know, can be fantastically dynamic. GSRBS + RSTN (San Vito) White et al., unpublished 1000 Frequency (MHz) 200 8 minutes

5. Smerd, 1970 Mercier et al, 2006, A&A, 447, 1189 Complex morphology and emission at a range of angular scales

6. The Solar Radio Imaging Problem The Sun is a challenging source to image • Time variations – sub millisec to solar cycles • Spectral variations – variety of spectral scales down to few 10s of kHz • Complicated and dynamic morphology • Emission at variety of spatial scales – from ~10” to order a degree.

7. Optimization Criterion • Optimize for • High fidelity, monochromatic, snapshot imaging • FoV size ~1° • Emphasis on maximizing the uv plane sampling

8. Implementation • Compute zenith uv coverage (300 MHz) • Divide uv plane in uv cells corresponding to 1° FoV • Compute occupancy of each uv cell • Compute a weight for each tile defined as • WTile(# of visibilities in a uv cell) Sum over all uv cells to which a tile contributes • a measure of how many uv cells does a tile contribute to, weighted by how many tiles (not baselines) contribute to that uv cell • Remove the tile with the smallest weight and iterate 1

9. Results 1. - Configuration

10. Results 2. – uv coverage

11. Nyquist grid sampling 160 MHz

12. Nyquist grid sampling 128 T 512 T

13. The Message • The optimization criterion for EoR and Solar imaging have a natural tension simply due to the characteristics of the emission they are after • 512T regime • Very large N + small foot print  significant scope for simultaneous optimization for both EoR and (Solar) imaging science • 128T regime • With only 6.25% of the baselines, not enough room to simultaneously accommodate disparate needs • A ‘compromise’ configuration will not serve any of the science objectives well

14. The VLA approach • 27 antennas • 4 configurations, switch configuration every ~4 months • Remarkably successful strategy - broad science appeal and a very capable instrument • 78.6 MUSD in 1972 Leverage the investment in the most expensive parts of the instrument (dishes + backend + infrastructure) by investing in the flexibility to move the dishes to broaden the scientific capability and returns very significantly.

15. A VLA like strategy for MWA • Expensive parts – Infrastructure, Receivers perhaps Correlator • Install more than 128 tiles (+ BFs), 128 of which can be connected up at any given time • Relieve some of the tension in array configuration optimization by providing more flexibility • Change array configuration say twice a year - EoR observing season, when the EoR fields are up at night, change to a “imaging” optimized config during the rest of the year

16. A VLA like strategy for MWA… • Use the Rx node locations based on the present 512T config, but the tile locations need not be limited to the current choices • The Rxs themselves will need to be moved to derive the most benefit from this approach • An interesting option for the MWA to examine in detail

17. Conclusion • Solar imaging performance is crucially tied to the choice of array configuration • Preserving high fidelity monochromatic, snapshot imaging capability is the single most important requirement for solar imaging science • Need the long baselines (2.5 km baseline ~3’ @ 150 MHz) • An ability to connect up different array configurations can significantly broaden the science returns from the MWA, an interesting option worth exploring in some detail