askap and phased array feeds in astronomy · askap and phased array feeds in astronomy david...

ASKAP and phased array feeds in astronomyDavid McConnell — CASS: ASKAP Commissioning and Early Science16 November 2017

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Image credit: Alex Cherney / terrastro.com

http://terrastro.com

PAF WORKSHOP 2017 SYDNEY | David McConnell

Credits

2PAF WORKSHOP 2017 SYDNEY | David McConnell

ASKAP Commissioning & Early Science(ACES)

Aidan HotanAaron Chippendale

Keith BannisterJohn ReynoldsIan HeywoodJosh Marvil

Lisa Harvey-SmithJames AllisonMax VoronkovMatt WhitingPaolo Serra

Karen Lee-WaddellBob Sault

Wasim Rajaand a number of others …

SST working groupsContinuum working group

Spectral working groupand others

Science Data Processing teamDigital Group (Firmware)

Entire ASKAP construction team

…

PAF WORKSHOP 2017 SYDNEY | David McConnell 3


Antennas : 36 x 12m diameterLongest baseline : 6440 mFrequency range : 700 - 1800 MHzInstantaneous bandwidth: 300 MHz



Correlator

Digital receivers

12-bit direct sampling at 1536 or 1280 MHz1 MHz channelisation

Beamformers

384 MHz bandwidthForms up to 36 dual-pol beams

Fine channelisation

18.5/N kHz; N ∈ {1,2,3,4,5,6}Up to 16200 channels

Delay/phase tracking 35 other antennas

Phased Array Feed

188 dual pol elements700 - 1800 MHz

ASKAPSoft

Calibration and imaging pipeline

CASDA

Archive of data products

Pawsey Computer Centre, Perth

Survey astronomers

2.3 GiB/sec


ASKAP Survey Science Projects


\

• EMU - Evolutionary Map of the Universe

• VAST - An ASKAP Survey for Variables and Slow Transients

• POSSUM - Polarization Sky Survey of the Universe's Magnetism

• CRAFT - The Commensal Real-time ASKAP Fast Transients survey

• WALLABY - Widefield ASKAP L-Band Legacy All-Sky Blind Survey

• FLASH - The First Large Absorption Survey in HI

• GASKAP - The Galactic ASKAP Spectral Line Survey

• DINGO - Deep Investigations of Neutral Gas Origins

CONTINUUM

SPECTRAL


Aperture Synthesis radio astronomy

• Essentials:• COHERENCE: the means to sample

coherently the incoming radiation at a number of well spaced locations

• An ARRAY: knowledge of the relative spatial locations of the sampling points to sufficient precision

• BEAMS: knowledge of the angular “reception pattern” of each sampling point

• CALIBRATION: a means to calibrate the amplitude and phase gains of each series of samples


• and with PAFs• maintain phase centres for each beam

• coherent sampling within each PAF

• electronic beams give wonderful flexibility and corresponding hazards

• multiplicity magnifies the task


Phased Array FeedsPracticalities for astronomy

• BEAMS•Arrangement into “footprints” within the field-of-view

– what is the shape of the field-of-view?

– how best to arrange beams within that?

– what is the optimum spacing of beams?

– how should we survey large areas?

•Shapes

– what are they in practice?

– how uniform?

– how stable?



ASKAP the telescope

8

•Body Level One•Body Level Two

–Body Level Three–Body Level Four

•Body Level Five


Sky mountRoll axis








•Shapes


– how uniform?

– how stable?




Equivalent FoV

32.8 sq deg

PAF Field-of-view

Predicted field of view of 9 x 10 chequerboard

phased array on ASKAP dish, at 1.25 GHz

The wrong way

Rotate footprint

Rotate PAF (roll axis)








•Shapes


– how uniform?

– how stable?



Correlation coefficient


Beam-to-beam noise correlationCredit Ian Heywood

Measurements



Too close

Sensitivity loss from correlation between beams

Too wide

Sensitivity loss from outer beams falling outside field-of-view

Optimum

Optimum beam spacing



Survey strategy1. Choose the optimum beam separation for the chosen observing frequency.

2. Observe the field with several (2 or 3) positions, placing beam maxima on minima of previous position.

1100 MHz - Equivalent area = 25 sq deg

1700 MHz - Equivalent area = 15 sq deg








•Shapes


– how uniform?

– how stable?


Radio astronomy with PAFs

with thanks to Ian Heywood

Example: continuum survey with BETA

12 m, 863 MHz

3 × 3 footprint

Interleaving

Tiling

12108

pointingsbeams

Tiling

Combined mosaic

10843

1,296

beamssub-bandsepochsimages

××=

1503,72

2

deg2

components








•Shapes


– how uniform?

– how stable?



Beam measurement: Holography


Credit Aidan Hotan


Beam measurement: Holography


Degrees


Beam shape characterisation


Fit ellipse to half-power level

Deg

rees



Beam shapes across frequency

Degrees



Beam shapes across antennas

Degrees








•Shapes


– how uniform?

– how stable?



Beam stability (apparent)


Using sensitivity as beam-health indicator

Three successive daysSame set of beam weights


• Data volume!


Image credit: Alex Cherney / terrastro.com

•188 sensing elements per PAF

• PAF/beamformers produce 36 dual-pol beams

• Beams formed for 300 x 1MHz channels

• Up to 16200 fine frequency channels

• 5s correlator integration time

• Visibility data to disk : 2.3 gigabytes /sec

• Beam multiplicity:

•36 x 2 x 36 = 2592 beams across array

•2592 x 300 = 777,600 sets of beam weights

•ASKAP has over 2 million monitor points


http://terrastro.com



Correlator

Digital receivers

12-bit direct sampling at 1536 or 1280 MHz1 MHz channelisation

Beamformers

384 MHz bandwidthForms up to 36 dual-pol beams

Fine channelisation

18.5/N kHz; N ∈ {1,2,3,4,5,6}Up to 16200 channels

Delay/phase tracking 35 other antennas

Phased Array Feed

188 dual pol elements700 - 1800 MHz

ASKAPSoft

Calibration and imaging pipeline

CASDA

Archive of data products

Pawsey Computer Centre, Perth

Survey astronomers

2.3 GiB/sec



Flag bad data

Derive bandpass calibration

and antenna gains

Apply calibrations

PKS B1934-638 flux calibrator

observed with each beam

Science data

Flag bad data

Form image

Make field source model

Self calibrate (phase only)

N times

The pipeline

Cray XC30 - 472 nodes200 TFfops/s

Linear mosaic



5.5 deg

Field of NGC7232Bandwidth 48 MHz6 x 6 square footprint2 x 12h obs, interleaved

rms ～ 160𝜇Jy

This image extracted fromCASDA archive at

https://data.csiro.au/dap/

PAF WORKSHOP 2017 SYDNEY | David McConnell 46PAF WORKSHOP 2017 SYDNEY | David McConnell


6 deg


Small Magellanic Cloud6 x 6 square footprint1 x 12h obsf = 1344MHzrms ～ 130𝜇Jy

The magneto-ionised structure of Fornax A | Craig Anderson52

ASKAP polarimetry

Fornax field sub-region: 8 hours, 30 sq. deg, 48 MHz

Images: Wasim Raja, Craig Anderson

Slide credit: Craig Anderson


ASKAP (P)

Slide credit: Craig AndersonASKAP polarimetry


ATCA (P)

(Anderson+ 2015)

ASKAP polarimetry Slide credit: Craig Anderson


ATCA (P)

Many polarised and unpolarised sources observed in multiple beams and multiple interleaves – use the sky itself as a probe of the instrumental polarisation response


ATCA (P) Obvious when polarised source properties are examined as a function of position relative to their surrounding beam centres, providing a slew of sensitive tests for calibration errors.



Click to edit Master text styles


Slide credit: Naomi McClure-Griffiths


Small Magellanic Cloud in HI


ATCA 320 pointings, 12h/day for 8 days13m integration on each pointing

ASKAP 3 pointings, 12h/day for 3 days12h integration on each pointing

Image by Naomi McClure-Griffiths & Helga Dénes


The Fly’s Eye


Slide credit: Keith Bannister

PAF WORKSHOP 2017 SYDNEY | David McConnell 62PAF WORKSHOP 2017 SYDNEY | David McConnell

Slide credit: Keith Bannister


ASKAP - current state

• 33 of 36 antennas equipped with PAFs• 16 antennas incorporated into array• 7 (8) other antennas available for

single-dish observing• Routine use of 36 beams• Bandwidth 240 MHz

• One antenna available for fledgling

XPAF WORKSHOP 2017 SYDNEY | David McConnell


Conclusions

• ASKAP works

• Phased Array Feeds offer enormous flexibility

• Phased Array Feeds add hugely to system complexity

• ASKAP will work better•with mastery of PAF (on-dish) calibration

•with application of more advanced beam forming methods

•with further development of control and operating procedures

•with further development of data processing methods (software)

An exciting future




We acknowledge the Wajarri Yamatji people

as the traditional owners of the observatory site.

askap and phased array feeds in astronomy · askap and phased array feeds in astronomy david...

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