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Astronomical Data Simulations
Cormac Reynolds
DS2-T2 Team
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Overview
• Re-cap of the DS2-T2 goals
• Potted highlights from each of the WPs
• Simulations Framework and Collaboration
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Aperture Array and FPA Modelling,
Plus a Configuration Study
Abstracted model of telescope from Tile and Network
Simulations
Simulated Skies from a number of science groups (line,
continuum, polarization)
Produce simulated u,v data/images for scientific analysis
Telescope
based on
aspects of SKA
Reference
Design –
SKADS
Benchmark
Specification
Simulated skies
(DS2-T1)
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
AA and FPA Simulations
• Need to describe in simulations software
• Requires full “measurement equation” for phased arrrays
–
–
–
–
–
–
Pointing errors
Bandpass shape & stability [f(,)]
Sensitivity [f(,)]
Beam shape and stability [f(,)]
Polarization purity [f(,)]
ionosphere
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
AA Beam - Sundaram
• EMBRACE beam
– HPBW ~ 16 arcmin
• depends on elevation
• Pointing error – linear
rise and fall
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
PPP – Ultimate FoV Limit to
Polarization Purity - Carozzi
• There is a limit to
polarization purity
as a function of
look-direction
elevation angle
• This limit is due to
aberrations arising
from u,v projection
of low-elevation
sources
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
FPAs & Beam forming Boomsma
+
=
By changing the (complex) weights for each element, one can optimise the
beam pattern. For example: reducing sidelobes
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Configuration Studies
-Lal
Attempt to maximise the Spatial Dynamic Range
Spatial dynamic range (SDR) – the ratio between largest
and smallest adequately imaged scales – it measures,
effectively, brightness sensitivity of an array on all scales.
 SDR reflects a number of aspects of array design,
including the type of primary receiving element (antenna),
signal processing, and distribution of antennas/stations.
 Array configuration: SDR can be expressed as a function
of a „gap“, u/u, between adjacent baselines (u1,u2):
u/u = (u2 – u1)/u2
(u2 > u1)
 Uniform sensitivity is provided by u/u = const
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Figures of Merit
 Shortest spacings, a few 10s of metres  ~degree
 Longest spacings (5000m)  ~arcseconds
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Preliminaries
 An arbitrary
choice of source
model
 Observing 
1.4 GHz
Observing direction,
RA 00:00:00
Dec +90:00:00
A RUN of 12 hrs
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Methodology






Generate test array (X,Y) for logarithmic
(equiangular) spiral array configuration
Project this array on Earth’s surface and
determine (Lat, Lon, Z)
Choose an appropriate input source model
RUN glish scripts in aips++ to obtain visibilities
Import these visibilities into AIPS and perform
the mapping using IMAGR task.
Determine the “figures of merit”
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Results

The behaviour of figures of merit and hence the SDR
does not seem to have a simple dependence on u/u.
The uv-gap parameter can be used to relate the (u,v)coverage to the characteristics of the map.
 These empirical solutions can be implemented into any
proposed configuration.

 We plan to use the SDR FoM to quantify imaging
performance of:
 KAT / MEERKAT, ASKAP, SKA – Phase I
 Limitations of CLEAN deconvolution algorithm
 Need new algorithms and parallelisation.
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Ionosphere - van Bemmel
• How to design SKA so that ionospheric corruptions are
calibratable
• Determine the number and sensitivity of stations needed
so that the free parameters related to the description of
the beams and ionosphere can be determined with
sufficient signal to noise that high dynamic range maps
can be made
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Simulated source + calibration distortions
using 74 MHz data
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Solving
• Method
– Peeling produces phase corrections per array element for
several viewing directions
– Fit an Ionosphere phase screen model to these phase
corrections
– The model allows for interpolation of the phase corrections to
other viewing directions
– We adopted the polyhedron method for imaging, calculating
one phase correction per array element per time interval for
each facet within the FOV.
• First conclusions
• Encouraging first results, with some improvement over the
existing field-based calibration by Cotton et al. (2004)
•
Performance of new method is very dependent on the choice
of model functions
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
3 x 2.5 degree
VLA-B 74 MHz
field with fieldbased calibration
applied
Same field with
new calibration
method applied
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Next
• Finalize work on solver
– Investigate more base functions
– Apply to longer baselines: GMRT (150 MHz) and VLA
• Use DS2-T1 model skies
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
DS2-T2 <=> DS2-T1
• Take sky model, corrupt, return to T1 for analysis
– Sky simulations: galore!
– Turning them into a Global Sky Model (GSM)
• Arbitrary parameterizations (e.g. trees)
– Making corrupted data-sets
– Recovering the sky again (calibration)
• Tricky...
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Various simulation efforts
• LOFAR
– dipole and station beams (S. Yatawatta)
– ionosphere (M. Mevius, J. Anderson)
– Local Sky Model (LSM) (everybody...)
• WSRT (J. Noordam)
• SKADS
–
–
–
–
model skies (everybody...)
ionosphere (I. van Bemmel)
AA beams, pointing errors (S. Sundaram)
FPA beams (T. Willis, R. Boomsma)
• DIGESTIF (R. Boomsma, T. Oosterloo)
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
How to make things just fit
together?
• TDL is a good basis for exchanging trees
• The ME provides a mathematical framework
– someone makes a sky model
– someone else makes a tree for computing Jones matrices
– at least you know how to plug them together (mathematically)
• But we still have a software problem
– different styles, different conventions
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
Simulations Framework Smirnov
Configurable
catalogue
parser
Ideal
visibilities
Array config and
observation setup
Note that order of Jones terms
is significant...
Z-Jones
ionosphere
Alternate
Z-Jones
E-Jones
Beam
Alternate
E-Jones
G-Jones
Gain
Alternate
G-Jones
...
Alternate
Jones
Differential/Corrupted
Vis. (for calibration...)
2nd SKADS Workshop 10-11 October 2007
...
Cormac Reynolds
Simulations - Siamese
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds
2nd SKADS Workshop 10-11 October 2007
Cormac Reynolds