LOFAR calibration, polarization calibration and commissioning tasks

Ger de Bruyn

ASTRON & Kapteyn Institute

MagnetismKSP - meeting, Cambridge, 25-26 Mar 2009

Outline

— Quick recap of the LOFAR calibration problem

— LOFAR beams and FOVs

— LOFAR and the ionosphere: refraction & Faraday rotation

— Some polarization calibration issues

— Some results on BBS-MIM modelling of WSRT/CS1/LOFAR data

— Commissioning timeline, tasks and manpower

Standard calibration

Calibration is needed for:

1) astrometry --> accurate positions

2) photometry --> (absolute) flux scale, spectral shape

3) image/PSF quality and image fidelity/DR

Method used:

Determine Gain/Phases (frequency) on

Stable external astrometric+flux calibrators: --> 1) and 2)

Apply selfcalibration --> 3)

Calibration/imaging software …Aperture synthesis array (users) use many different reduction packages

— AIPS : VLA, WSRT, GMRT, ATCA, VLBI,…

— Miriad : VLA, ATCA, WSRT,…

— NEWSTAR: WSRT

— AIPS++ : WSRT, VLA, …

For LOFAR, with all its novel and complicated aspects, we need to do much better. Two packages have been, and continue to be, developed:

— MeqTrees is being used to develop/simulate our understanding

— BBS will be implementing efficiently what we have learned

— and we use AIPS++/CASA for imaging

If you are not satisfied with the results blame the hardware/firmware, the software, or reconsider your understanding of the problem !

Calibration issues and overview:

Calibrating dipole-station arrays at low frequency conceptually involves

3 major unknowns:

— Sky or Global Sky Model (= GSM)

— Station beampattern: (position, frequency, polar) dependent

— Ionospheric phase screen

Qualitatively our knowledge will steadily increase stepwise

1. After some time (= MSSS !) we will know the GSM: I,Q,U,V (RA,Dec, freq, (time))

2. Improved modeling of beampatterns (expect/hope to be stable = predictable)

Quantitatively we still worry whether :

• there are enough constraints to fit for all ionosphere/beam parameters?

• it can be done in the available processing time (~ real time) ?

Many calibration issues need to be investigated…

— 3 types of effective primary beams (core, NL, Europe)

— extreme non-isoplanaticy (> 20 facets) or even scintillation conditions

— very wide frequency range MFS (>factor 1.5 -2, different beams !)

— day/night effects (thermal/flaring Sun)

— absolute flux scale

— always working with intrinsic sky fluxes (snapshots)

— deconvolution with spatially varying beams ?

— deconvolution at >> 10000:1 DR (shapelets ?)

— Galactic plane imaging and very short spacings

— intrinsic polarization calibration (Faraday rotation, RM synthesis)

— calibrating European baselines over wide FOV

This will be a major commissioning task. Especially when debugging/understanding these issues when they hit you all at once !

LOFAR and its field of view

HBA tile/station FOV & LBA dipole/station FOV

~20o

~100o

dipole (~100o)

tile (~ 20o)

24-tile station (~ 5o)

Fields-of-view in core (HBA at ~ 150 MHz)

LOFAR and the ionosphere

(some recent WSRT and CS1 results and insights)

WSRT 150 MHz image of 3C196: ’all-sky imaging needed !’

Sun

NCP

VirA

TauA

CygA

CasA

12o x12o

3C196 in last night: serious nonisoplanaticity !!

3C196

80 Jy

3 other sources

6-8 Jy

3C196 - selfcal phase solutions

6 x12h

Note the very different ionospheres !

However, these hardly affect the quality of the Q,U images

BBS and MIM-modeling on 3C196 WSRT data

Work in progress by Maaijke Mevius, Gianni Bernardi, Joris van Zwieten

Fitting 2-dimensional phase screen at altitude of 300 km

– Solving directly on UV-data (using known positions)– 2 parameters : plane– 5 parameters : 2nd order– 8 parameters : 3rd order

solving for 2 parameters

solving for 5 parameters

1) Both refraction and Faraday rotation depend on absolute TEC which changes relatively slowly with time and direction

2) Selfcalibration/imaging depend on small scale relative TEC which varies rapidly (1-10s) --> selfcal/peeling takes (partly) care

3) Ways to measure absolute TEC:

4) differential angles in large FOV images (26-Nov-08 - LSM)

5) Faraday rotation (29-Oct-08 - LSM)

6) GPS data (accurate enough ?)

7) snapshot all-sky observation sequences (e.g. 10s every 120s) and combining absolute+relative delays

Ionospheric TEC modeling

Ionospheric refraction at LBA/HBA frequencies

Differential effects are computed with TMS2000 ‘analytic’ model. Results are shown for ionospheres with a

‘high TEC’ (p = 10 MHz) and a ‘low TEC’ (p = 5 MHz)

LBA HBA

BBS - G/Ph solutions to CasA + CygA ~ 50 MHz

Simultaneous selfcal solutions for a 24h dataset.

Date: 27 Jul 2007

L3463 dataset (one 0.2 MHz subband

Note how the differential phase solutions drift apart by ~100o when CygA and CasA rise in the sky. Probably due to large scale ionospheric TEC

Tracks of CasA + CygA in a zenith sin-projection

West

South

LOFAR polarization calibration strategy + wish list

The stations beams and the ionosphere are the most important unknowns

1) Measuring beams and implementing them in the calibration and imaging software will be the 1st important goal of the calibration group

2) Modeling the ionosphere is the 2d most important goal.

The MKSP, EoR and Survey KSP should have a great interest in this

This requires coordination ! --> May 18+19 at project status meeting

We need to know both its large scale absolute TEC and the small-scale differential TEC

Therefore we need TEC models, preferably in 3-D

Then we need BBS + MeqTrees software to validate those models against data

Data should come from bright unresolved, isolated, sources all around sky

and, finally, we need a significant number of polarized sources

LOFAR commissioning in early phase

May 2009: first 2 stations --> a single ~ 5 km baseline

There is a whole range of commissioning tasks that comes up in ~ May when the 1st interferometer should become available for observing/testing. Mapping those tasks onto the specific expertise found in the various KSPs is important (May 18/19 meeting)

The tasks that I believe to be well suited to MKSP efforts are those related to the ionosphere:

1) Determining the large scale absolute TEC distribution

- through differential refraction between a suite of bright sources

- check against GPS

- fit using 2-D and 3-D models (simulate ?)

2) Monitor temporal variations in the ionospheric RM using polarized sources

e.g. (PSR0531+21,PSRB1937+21, PSRJ0218+42, others ?)

Do all these observations in a dedicated MKSP busy week ?

When? Who joins ?

July 2009: 6 stations + 2 Europe ?

Some imaging will be possible on dominant (=bright) sources

Which ones and why ?

September 2009: 10-15 stations + 3 Europe?

serious calibration+ imaging commissioners needed

Start building up data on MKSP test fields, e.g.:

- FAN area (+66) (no bright source, but lots of polarized power !)

- TauA +PSR0531+21 (very bright source + polarized pulsar)

- B1834+62 ( ~4’ DDRG) (simple field with extragalactic AGN source )

- NGC891 + 3C66A/B + PSRJ0218+42 (tough field !)

- North Celestial Pole (+ 3C61.1, polarized ?)

Commissioning tasks in later phases

Priorities for commissioning targets: some issues ..

Choice of commissioning fields proposed by LOFAR KSP’s and WGs

Probably to be decided by LOFAR TAC _ calibration PM ?

Hopefully we receive guidelines on May18+19 2009 meeting .

When to observe: before, during or after MSSS ?

Targets may have to be challenging in one or more of the crucial parameters:

- low declination (dec < 0o) (short syntheses --> poor uv-coverage)

- very complex Galactic plane (--> requires excellent uv coverage!)

- how many stations are needed for which targets ?

- very high dynamic range requirements (e.g. near CygA, CasA,..)

- limiting sensitivity (how close do we get to thermal noise. ?.)

Note: manpower probably must be available in order to work on data !