ASKAP. and phased array feeds in astronomy. David McConnell CASS: ASKAP Commissioning and Early Science 16 November 2017

Transcription

1 ASKAP and phased array feeds in astronomy David McConnell CASS: ASKAP Commissioning and Early Science 16 November 2017 Image credit: Alex Cherney / terrastro.com 1

2 Credits ASKAP Commissioning & Early Science (ACES) Aidan Hotan Aaron Chippendale Keith Bannister John Reynolds Ian Heywood Josh Marvil Lisa Harvey-Smith James Allison Max Voronkov Matt Whiting Paolo Serra Karen Lee-Waddell Bob Sault Wasim Raja and a number of others SST working groups Continuum working group Spectral working group and others Science Data Processing team Digital Group (Firmware) Entire ASKAP construction team 2

3 Antennas : Longest baseline : Frequency range : Instantaneous bandwidth: 36 x 12m diameter 6440 m MHz 300 MHz 3

4 Phased Array Feed 188 dual pol elements MHz Survey astronomers Digital receivers 12-bit direct sampling at 1536 or 1280 MHz 1 MHz channelisation Beamformers 384 MHz bandwidth Forms up to 36 dual-pol beams CASDA Archive of data products Fine channelisation 18.5/N khz; N {1,2,3,4,5,6} Up to channels ASKAPSoft Calibration and imaging pipeline Delay/phase tracking 35 other antennas Correlator 2.3 GiB/sec Pawsey Computer Centre, Perth 4

5 ASKAP Survey Science Projects CONTINUUM 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 SPECTRAL 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 5

6 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 6

7 Phased Array Feeds Practicalities 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? 7

8 ASKAP the telescope Body Level One Body Level Two Body Level Three Body Level Four Body Level Five Sky mount Roll axis 8

9 Phased Array Feeds Practicalities 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? 10

10 PAF Field-of-view Predicted field of view of 9 x 10 chequerboard phased array on ASKAP dish, at 1.25 GHz Equivalent FoV 32.8 sq deg 11

11 The wrong way

12 Rotate footprint

13 Rotate PAF (roll axis)

14 Phased Array Feeds Practicalities 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? 17

15 Beam-to-beam noise correlation Credit Ian Heywood Measurements Correlation coefficient 18

16 Optimum beam spacing Too close Sensitivity loss from correlation between beams Optimum Too wide Sensitivity loss from outer beams falling outside field-of-view 19

17 Survey strategy 1. 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 MHz - Equivalent area = 25 sq deg 1700 MHz - Equivalent area = 15 sq deg 20

18 Phased Array Feeds Practicalities 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? 21

19 Radio astronomy with PAFs with thanks to Ian Heywood

20 Example: continuum survey with BETA

21 12 m, 863 MHz

22 3 3 footprint

23 3 3 footprint

24 Interleaving

25 Interleaving

26 Tiling

27 Tiling

28 Tiling pointings beams

29 Combined mosaic = 150 3, ,29 6 deg 2 component s beams sub-bands epochs images

30 Phased Array Feeds Practicalities 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? 33

31 Beam measurement: Holography Credit Aidan Hotan 34

32 Beam measurement: Holography Degrees 35

33 Beam shape characterisation Fit ellipse to half-power level Degrees 36

34 Beam shapes across frequency Degrees 37

35 Beam shapes across antennas Degrees 38

36 Phased Array Feeds Practicalities 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? 39

37 Beam stability (apparent) Using sensitivity as beam-health indicator Three successive days Same set of beam weights 40

38 Phased Array Feeds Practicalities for astronomy 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 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 41

39 Phased Array Feed 188 dual pol elements MHz Survey astronomers Digital receivers 12-bit direct sampling at 1536 or 1280 MHz 1 MHz channelisation Beamformers 384 MHz bandwidth Forms up to 36 dual-pol beams CASDA Archive of data products Fine channelisation 18.5/N khz; N {1,2,3,4,5,6} Up to channels ASKAPSoft Calibration and imaging pipeline Delay/phase tracking 35 other antennas Correlator 2.3 GiB/sec Pawsey Computer Centre, Perth 42

40 The pipeline PKS B flux calibrator observed with each beam Flag bad data Derive bandpass calibration and antenna gains Apply calibrations Science data Flag bad data Form image Linear mosaic N times Make field source model Cray XC nodes 200 TFfops/s Self calibrate (phase only) 43

41 Field of NGC7232 Bandwidth 48 MHz 6 x 6 square footprint 2 x 12h obs, interleaved rms ~ 160μJy 5.5 deg This image extracted from CASDA archive at 45

42 46

43 Small Magellanic Cloud 6 x 6 square footprint 1 x 12h obs f = 1344MHz rms ~ 130μJy 6 deg 48

44 50

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46 ASKAP polarimetry Slide credit: Craig Anderson Fornax field sub-region: 8 hours, 30 sq. deg, 48 MHz Images: Wasim Raja, Craig Anderson 52 The magneto-ionised structure of Fornax A Craig Anderson

47 ASKAP polarimetry Slide credit: Craig Anderson ASKAP (P) 53 The magneto-ionised structure of Fornax A Craig Anderson

48 ASKAP polarimetry Slide credit: Craig Anderson ATCA (P) (Anderson+ 2015) 54 The magneto-ionised structure of Fornax A Craig Anderson

49 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 55 The magneto-ionised structure of Fornax A Craig Anderson

50 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.

51 Click to edit Master text styles Slide credit: Naomi McClure-Griffiths 57

52 Small Magellanic Cloud in HI Image by Naomi McClure-Griffiths & Helga Dénes ATCA 320 pointings, 12h/day for 8 days 13m integration on each pointing ASKAP 3 pointings, 12h/day for 3 days 12h integration on each pointing 58

53 The Fly s Eye 59

54 Slide credit: Keith Bannister

55 Slide credit: Keith Bannister

56 Slide credit: Keith Bannister 62

57 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 X PAF WORKSHOP SYDNEY SYDNEY David McConnell David McConnell

58 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 64

59 We acknowledge the Wajarri Yamatji people as the traditional owners of the observatory site. 65