MWA Bangalore Meeting

EoR using Drift Scan StrategyN. Udaya Shankar8-December-2009

There Are Two Main Challenges In the Detection of Signal From the EoR

The foreground contamination—is 5 orders of magnitude brighter than the neutral hydrogen radio emission It’s removal with precision is needed to reveal the underlying EoR signal?

Simulations show thatHierarchical Subtraction Scheme•Bright Source Removal•Spectral Fitting•Residual Error Subtraction

Each Step subtracts foreground which gets increasingly fainter ultimately revealing the hidden EoR signal

Calibration Process

Starting Point: Billion Visibilities/ Second•The ionospheric-distortion corrected using

radio adaptive optics, • Get time variable gain, phase, and polarization of each

antenna precisely and also determine direction dependent gain.

This amounts to estimating the primary-beam of each tileThere has been no concrete demonstration of

this process.

Proposed Observing Strategy (MWA-EoR Collaboration):

Have chosen a reference measurement to be a deepobservation of a single target field using an array configuration based on the MWA.

Observing for 4 hr per day over the course of a 6 monthseason would result in approximately 720 hr of integration. Conservatively rejecting half the data for non-ideal conditions yields 360 hr of integration during the most favourable circumstances.

For getting 4 hr per day observation each tile beam is modified every ~8 mins, by using delay shifters, to ensure that the main lobe of the tail always points in the direction of the target field.

Problems of phased Arrays:a) Band-shapes change as the tile beams are steered. The band-pass ripple from feedback is most often variable with dipole delay settings.

b) The beam-shape and sidelobes are frequency dependent. This results in the total power spectra changing when the beams are steered, since the amount of power coming from different directions is modulated differently as the tile patterns are moved around the sky.

c) The locations of the half-power point and nulls in the beam pattern are frequency dependent . Thus gain VS freq for discrete sources will vary across the beam pattern,

Of course these parameters are calibrated using the sky model.

Gauribidanur T ArrayDeshDwarakaUday

The large radio telescope near Ootacamund (Ooty) was set up by TIFR radio astronomers, in the picturesque Nilgiri Hills of South India in 1970.

The Ooty Radio Telescope (ORT) is an off-axis parabolic cylinder 530 m long and 30 m wide, 24 steerable parabolic frames, operating at a nominal frequency of 326.5 MHz with a maximum bandwidth of 15 MHz at the front-end.

An array of 1056 half-wave dipoles in front of a 90 degree corner reflector forms the primary feed of the telescope.

Anantha, Ravi, Anish : Spectral Mode

The unique feature of the design is that the telescope has been constructed on a hill which has a natural slope of about 11 degrees, the same as the geographical latitude of Ooty.

This makes the long axis of the telescope parallel to the Earth's rotation axis, giving it an equatorial mount.

A celestial source in the sky can be tracked for about ten hours at a stretch by mechanical rotation of the parabolic cylinder in the east-west direction.

In the north-south direction, the telescope response is steered electronically by introducing a suitable phase and delay gradient along the dipole array.

MRT Aerial View

A Non-Coplanar Array

Problems Related to wide-field imaging : changing beams, non-redundant base-lines

Drift - Scan Strategy

In view of this we suggest Drift-Scan strategy for observing the EoR field with the MWA .

In the drift-scan strategy the system delay settings do not change leaving the set gains and the mutual couplings between the elements of a tile and hence its primary beam shape unchanged. The tiles are kept undisturbed during the observing .

An unchanging beam facilitates estimation of polarisation

leakage. In addition the w=b.s0 remains the same in a drift scan. This is likely to simplify the system calibration and make it more robust.

Observing proposal:

The advantage of calibrating drift-scans needs to be established by observations as this is a complex function of the sky, drift rates and amplitudes of system parameters and the dynamic behaviour of the ionosphere.

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We propose to carry out observations of three fields to establish or rule out the advantages of drift-scan strategy.

The three fields are:

(1)the celestial south pole as the phase centre, (2) a field close to the south pole which gives a slowly varying fringe rate,(3) a field close to the equator with a higher fringe rate

Both drift-scan and tracking modes lead to the same procedure for observations with the south pole as the phase centre.

Observations with slower and faster fringe rates will present scenarios with different impacts of system artefacts, system stability, dynamics of ionosphere.

Measured Visibilities

Derived , Processed

Input for Statistical Estimation

FourIer

Transform Sky Coordinates

Transform freq Coordinates

In tracking mode the intensity distribution in the image cube principally remains the same throughout the observations. In drift-scan mode the intensity distribution changes as the sky drifts.

Drift-Scan and EoR Signal

The statistical isotropy and homogeneity of the EoR signal allows its estimation in both the cases.

•In a tracking interferometer the ensemble average required for statistical estimation of EoR is obtained by summing over different realizations of U for the

same וUו for a given intensity distribution in the sky.

•In a Drift scan statistical average is taken over the visibilities for different intensity distributions but for the same U

•How does this affect SNR of the estimation process?

•What is the impact of the array configuration on this?

•Does non-coplanarity of the array hold a key to this answer?

•Is there room to exploit the fact that while the EoR signal is statistically homogeneous and isotropic the foregrounds are not?

Move over to Shiv for getting answers to these questions

Simulations in progress

1.u-v coverage for a drift scan2.Simulation of EoR Signal 3.Simulation of Foreground using the existing data

Study noise properties and Advantages