Astronomical Data Simulations

Cormac Reynolds2nd SKADS Workshop 10-11 October 2007

Astronomical Data SimulationsAstronomical Data Simulations

Cormac Reynolds DS2-T2 Team


OverviewOverview

• Re-cap of the DS2-T2 goals• Potted highlights from each of the WPs• Simulations Framework and Collaboration


Aperture Array and FPA Modelling,Aperture Array and FPA Modelling,Plus a Configuration StudyPlus 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)


AA and FPA SimulationsAA 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


AA Beam - SundaramAA Beam - Sundaram

• EMBRACE beam– HPBW ~ 16 arcmin

• depends on elevation

• Pointing error – linear rise and fall


PPP – Ultimate FoV Limit to PPP – Ultimate FoV Limit to Polarization Purity - CarozziPolarization 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


FPAs & Beam forming - FPAs & Beam forming - BoomsmaBoomsma

+ =

By changing the (complex) weights for each element, one can optimise the beam pattern. For example: reducing sidelobes



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“, Δu/u, between adjacent baselines (u1,u2):

Δu/u = (u2 – u1)/u2 (u2 > u1)

Uniform sensitivity is provided by Δu/u = const

Configuration Studies -Lal


Figures of MeritFigures of Merit

Shortest spacings, a few 10s of metres ~degree Longest spacings (5000m) ~arcseconds


PreliminariesPreliminaries

Observing direction, RA 00:00:00Dec +90:00:00

A RUN of 12 hrs

An arbitrary choice of source model Observing

1.4 GHz


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”

MethodologyMethodology


The behaviour of figures of merit and hence the SDR does not seem to have a simple dependence on Δu/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.

ResultsResults


Ionosphere - van BemmelIonosphere - 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


Simulated source + calibration distortions using 74 MHz data


• 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

SolvingSolving


3 x 2.5 degree VLA-B 74 MHz field with field-based calibration applied

Same field with new calibration method applied


NextNext• Finalize work on solver

– Investigate more base functions– Apply to longer baselines: GMRT (150 MHz) and VLA

• Use DS2-T1 model skies


DS2-T2 <=> DS2-T1DS2-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...


Various simulation effortsVarious 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)


How to make things How to make things just fit just fit together?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


Simulations Framework - Simulations Framework - SmirnovSmirnov

Configurable catalogue

parserZ-Jones

ionosphere

E-JonesBeam

G-JonesGain

...

Ideal visibilities

Differential/Corrupted Vis. (for calibration...)

AlternateZ-Jones

...

AlternateJones

AlternateG-Jones

AlternateE-Jones

Note that order of Jones terms is significant...

Array config andobservation setup


Simulations - SiameseSimulations - Siamese




Documents

Astronomical Data Simulations