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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
PAF WORKSHOP 2017 SYDNEY | David McConnell
Credits
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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
PAF WORKSHOP 2017 SYDNEY | David McConnell
Antennas : 36 x 12m diameterLongest baseline : 6440 mFrequency range : 700 - 1800 MHzInstantaneous bandwidth: 300 MHz
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PAF WORKSHOP 2017 SYDNEY | David McConnell
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
PAF WORKSHOP 2017 SYDNEY | David McConnell
ASKAP Survey Science Projects
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\
• 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
PAF WORKSHOP 2017 SYDNEY | David McConnell
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
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• 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
PAF WORKSHOP 2017 SYDNEY | David McConnell
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?
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PAF WORKSHOP 2017 SYDNEY | David McConnell
ASKAP the telescope
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•Body Level One•Body Level Two
–Body Level Three–Body Level Four
•Body Level Five
PAF WORKSHOP 2017 SYDNEY | David McConnell
Sky mountRoll axis
PAF WORKSHOP 2017 SYDNEY | David McConnell
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?
10PAF WORKSHOP 2017 SYDNEY | David McConnell
PAF WORKSHOP 2017 SYDNEY | David McConnell 11
PAF WORKSHOP 2017 SYDNEY | David McConnell
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)
PAF WORKSHOP 2017 SYDNEY | David McConnell
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?
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PAF WORKSHOP 2017 SYDNEY | David McConnell
Correlation coefficient
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Beam-to-beam noise correlationCredit Ian Heywood
Measurements
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PAF WORKSHOP 2017 SYDNEY | David McConnell
Too close
Sensitivity loss from correlation between beams
Too wide
Sensitivity loss from outer beams falling outside field-of-view
Optimum
Optimum beam spacing
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PAF WORKSHOP 2017 SYDNEY | David McConnell
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
PAF WORKSHOP 2017 SYDNEY | David McConnell
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?
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Radio astronomy with PAFs
with thanks to Ian Heywood
Example: continuum survey with BETA
12 m, 863 MHz
3 × 3 footprint
3 × 3 footprint
Interleaving
Interleaving
Tiling
Tiling
12108
pointingsbeams
Tiling
Combined mosaic
10843
1,296
beamssub-bandsepochsimages
××=
1503,72
2
deg2
components
PAF WORKSHOP 2017 SYDNEY | David McConnell
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?
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PAF WORKSHOP 2017 SYDNEY | David McConnell
Beam measurement: Holography
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Credit Aidan Hotan
PAF WORKSHOP 2017 SYDNEY | David McConnell
Beam measurement: Holography
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Degrees
PAF WORKSHOP 2017 SYDNEY | David McConnell
Beam shape characterisation
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Fit ellipse to half-power level
Deg
rees
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PAF WORKSHOP 2017 SYDNEY | David McConnell
Beam shapes across frequency
Degrees
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PAF WORKSHOP 2017 SYDNEY | David McConnell
Beam shapes across antennas
Degrees
PAF WORKSHOP 2017 SYDNEY | David McConnell
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?
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PAF WORKSHOP 2017 SYDNEY | David McConnell
Beam stability (apparent)
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Using sensitivity as beam-health indicator
Three successive daysSame set of beam weights
PAF WORKSHOP 2017 SYDNEY | David McConnell
• Data volume!
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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
Phased Array FeedsPracticalities for astronomy
PAF WORKSHOP 2017 SYDNEY | David McConnell 42
PAF WORKSHOP 2017 SYDNEY | David McConnell
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
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PAF WORKSHOP 2017 SYDNEY | David McConnell
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
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PAF WORKSHOP 2017 SYDNEY | David McConnell
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/
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PAF WORKSHOP 2017 SYDNEY | David McConnell
6 deg
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Small Magellanic Cloud6 x 6 square footprint1 x 12h obsf = 1344MHzrms ~ 130𝜇Jy
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PAF WORKSHOP 2017 SYDNEY | David McConnell
PAF WORKSHOP 2017 SYDNEY | David McConnell 51
PAF WORKSHOP 2017 SYDNEY | David McConnell
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
The magneto-ionised structure of Fornax A | Craig Anderson53
ASKAP (P)
Slide credit: Craig AndersonASKAP polarimetry
The magneto-ionised structure of Fornax A | Craig Anderson54
ATCA (P)
(Anderson+ 2015)
ASKAP polarimetry Slide credit: Craig Anderson
The magneto-ionised structure of Fornax A | Craig Anderson55
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
ASKAP polarimetry Slide credit: Craig Anderson
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.
ASKAP polarimetry Slide credit: Craig Anderson
PAF WORKSHOP 2017 SYDNEY | David McConnell
Click to edit Master text styles
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Slide credit: Naomi McClure-Griffiths
PAF WORKSHOP 2017 SYDNEY | David McConnell
Small Magellanic Cloud in HI
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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
PAF WORKSHOP 2017 SYDNEY | David McConnell
The Fly’s Eye
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Slide credit: Keith Bannister
Slide credit: Keith Bannister
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Slide credit: Keith Bannister
PAF WORKSHOP 2017 SYDNEY | David McConnell
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
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PAF 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
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PAF WORKSHOP 2017 SYDNEY | David McConnell
We acknowledge the Wajarri Yamatji people
as the traditional owners of the observatory site.