Array Configuration

Divya OberoiMIT Haystack Observatory

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

Characteristics of the Radio Sun

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

8 minutes

200

1000

Freq

uenc

y (M

Hz)

GSRBS + RSTN (San Vito)White et al., unpublished

The “active” Sun, we know, can be fantastically dynamic.

Smerd, 1970

Complex morphology and emission at a range of angular scales

Mercier et al, 2006, A&A, 447, 1189

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.

Optimization Criterion

• Optimize for – High fidelity, monochromatic, snapshot imaging– FoV size ~1°

– Emphasis on maximizing the uv plane sampling

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

Results 1. - Configuration

Results 2. – uv coverage

160 MHz

Nyquist grid sampling

Nyquist grid sampling128 T

512 T

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

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.

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

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

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