Transcript MWA Bangalore Meeting

MWA Bangalore Meeting
EoR using Drift Scan Strategy
N. Udaya Shankar
8-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 that
Hierarchical 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 tile
There has been no concrete demonstration of
this process.
Proposed Observing Strategy
(MWA-EoR Collaboration):
Have chosen a reference measurement to be a deep
observation 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 month
season 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 Array
Desh
Dwaraka
Uday
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.
.
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.
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.
Derived ,
Processed
Transform Sky
Coordinates
Measured
Visibilities
Transform freq
Coordinates
Input for
Statistical
Estimation
F
o
u
r
I
e
r
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 ‫ו‬U‫ ו‬for 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 scan
2.Simulation of EoR Signal
3.Simulation of Foreground using the
existing data
Study noise properties and Advantages