Setting the Scene For workshop 3GC-II (Portugal) On Station Beamshapes Modelling, Measurement, Application J.E.Noordam ([email protected]) J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Standing on the Shoulders of...

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Standing on the Shoulders of...

Tony Willis

His notes will be shown at the end

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

rd

3 Generation Calibration (3GC) ●

1GC: Rely on instrumental stability (1:100) ●





2GC: Selfcal (2 parameters per antenna) ●

>1:1.000.000 (WSRT/NEWSTAR)



The easiest telescope to calibrate (36 years)

3GC: Direction-Dependent Effects (DDE) ●



Enough for the great discoveries of the 70's

More parameters, more processing, more equations

4GC: Statistical analysis of the residuals

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

3GC-I (Nancay, 2009) ●

Delightfully primitive and isolated



First of a new style of workshops





Preparation/selection, 2 full weeks, continue afterwards



Encouraged by SKADS and RadioNet

The concept still needs to be tweaked: ●

Narrowed scope (just beamshapes, no ionosphere)



Proven software now exists (OMS, WSRT)



The world is more aware of the 3GC problem

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The Topic of 3GC-II: Station Beamshapes ●

Modeling (2x2 parametrized expressions) ●



Measurement (open-loop vs closed-loop) ●



Topic chair: Isak Theron Topic chair: Stephen Bourke

Application (aw-projection vs facet imaging) ●

Topic Chair: Cyril Tasse

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

I will repeat this talk once or twice for the newcomers and to remind you

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

This talk was given at ASTRON in March 2011, to plead for using the existing WSRT system for measuring the Apertif beamshapes

Measuring Actual Station Beamshapes As a function of time and frequency In full polarization J.E.Noordam ([email protected])

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Why are beamshapes important? ●





Steve Rawlings said (airily): “... once we have subtracted the foregrounds...” Unfortunately, this requires: ●

Very high Dynamic Range



Over a very wide field



Full polarization



Accurate spectral calibration

Crucial: station beam shapes (i,l,m,f,t,pol)

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Calibration: The ability to subtract bright foreground sources with high accuracy

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The EOR is hidden in the noise

The discovery of selfcal

DR 1:10.000

WSRT 3c48 RSC 1980

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

DR 1:100

Source Subtraction requires accurate knowledge of the PSF

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Wide-Field Source Subtraction The PSF varies over the field

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Courtesy Tom Oosterloo

Station Beams are NOT identical!

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Frequency Dependence

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Courtesy Ger de Bruyn

Instrumental Polarization

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Time Dependence

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

In order of Trouble ●

WSRT (no sky rotation, on-axis SPR)



ASKAP/Apertif (no sky rotation, on-axis PAF)



AAT/GMRT (sky rotation, on-axis SPR)



VLA (sky rotation, off-axis SPR)



ATA/GBT/MeerKat ●



sky rotation, widely off-axis SPR

AA (LOFAR, Embrace) ●

sky rotation, elongation, polarization

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Required Beam Accuracy

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

How are Station Beamshapes used? Source Categories (I,II,III)

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Source Categories ●

Cat “0”: The dominating source (if any) ●





Cat I: The 10-20 calibration beacons ●

Estimated and subtracted individually



Used to estimate beamshapes

Cat II: The 100-1000 fainter LSM sources ●



Used to calibrate rapidly varying errors

Subtracted collectively, using interpolated beamshapes

Cat III: The many “one-sigma” sources ●

Imaged after correcting the uv-data for beamshapes

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

So, how do we get to know our beamshapes with sufficient accuracy?

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Open-loop vs Closed-loop ●

Open-loop: ● ●



Use a theoretical beamshape model Measured the beamshapes once and for all, e.g. by scanning through a bright source

Closed-loop: ●

Continuous measurements during the observations



Using the sources in the field



Time, frequency and polarization



Like selfcal

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

TINA ●

There Is No Alternative to closed-loop ●

If we want to deliver on our promises





Fortunately, this seems to be possible ●

Enough calibration beacons (information)



Enough equations to solve for the extra unknowns





The price: It will require a lot of extra processing

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The Differential Gains Method ●



Use the dominating source to take out the rapidly varying instrumental errors (2GC) Estimate the slowly varying differential gains in the direction of 10-20 calibration beacons Integration time: >30 minutes





Estimate beamshape parameter values (i,f,t) ●

Use these to subtract Cat II sources



Use these to correct residual uv-data while imaging

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The Differential Gains Method

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Courtesy Oleg Smirnov

Differential Gains in action

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Calibration Beacons Intrinsic Source Fluxes

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Calibration Beacons Apparent Source Fluxes

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Calibration Beacons sample the beamshape

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Independent of Station Diameter (to first order)

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The Good News for SKA ●





There seem to be enough calibration beacons in a typical 21cm WSRT field To first order, this is independent of station diameter It gets better for longer wavelengths ●

Wavelengths < 21 cm could be problematic

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The Good News for SKA ●





There seem to be enough calibration beacons in a typical 21cm WSRT field To first order, this is independent of station diameter It gets better for longer wavelengths ●



Wavelengths < 21 cm could be problematic

This should be a huge relief to the SKA community

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The Bottom Line If you know your station beams (i,l,m,f,t,p) Your DR problem reduces to the WSRT problem You can have 1:000.000 too And perhaps more... J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

The WSRT as a Test Bed for prototype stations ●



Assuming the use of calibration beacons The 3km spatial resolution is needed to distinguish the beacons from each other



The more telescopes in the array, the better



The WSRT is highly stable and very well known





This allows absolute measurements



NB: The recent closure errors have been solved

It allows a gradual path to the full Apertif

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

EMBRACE/AAVP ●



We should exploit the opportunity offered by the EMBRACE-WSRT combination ●

It is there (it just needs to be a little bigger)



The software exists



It just needs an interface with the WSRT

This will give an imaging Result within a year ●



And a string of other results as by-products

It should establish AA's as the “realistic approach”

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

EMBRACE @WSRT ●



But: Please exploit (to the hilt) the opportunity offered by the EMBRACE-WSRT combination ●

It is there (it just needs to be a little bigger)



The software exists

This will give you a Result within a year ●



And a string of other results as by-products

It should establish AA's as the “realistic approach”

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Apertif ●



The same arguments apply for testing a single Apertif prototype It allows a gradual low-risk implementation of the full 14-station Apertif

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Conclusions ●



It is time to start measuring our station beams ●

For all kinds of reasons



For the first time in history

A method exists (Differential Gains) ●



available to all

It has been “demonstrated” that a typical field @21cm contains enough calibration beacons to measure the beam during the observations

J.E.Noordam

3GC-II Portugal 18-30 Sep 2011

Setting the Scene - GitHub

... equations. ○ 4GC: Statistical analysis of the residuals ... Proven software now exists (OMS, WSRT) ... Application (aw-projection vs facet imaging). ○ Topic ...

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