Wide-field broadband radio imaging with phased array feeds: a pilot multi-epoch continuum survey with ASKAP-BETA

4.8 MB·

by  I. Heywood

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed25January2016 (MNLATEXstylefilev2.2)
Wide-field broadband radio imaging with phased array feeds:
a pilot multi-epoch continuum survey with ASKAP-BETA
I. Heywood1,2⋆, K. W. Bannister1,3, J. Marvil1, J. R. Allison1,3, L. Ball1, M. E. Bell1,
D. C.-J. Bock1, M. Brothers1, J. D. Bunton1, A. P. Chippendale1, F. Cooray1,4,
6
1 T. J. Cornwell1,5 D. DeBoer1,6, P. Edwards1, R. Gough1, N. Gupta1,7, L. Harvey-Smith1,
0
2 S. Hay1, A. W. Hotan1, B. Indermuehle1, C. Jacka1, C. A. Jackson1,8,9, S. Johnston1,
n A. E. Kimball1, B. S. Koribalski1, E. Lenc1,9,10, A. Macleod1, N. McClure-Griffiths1,11,
a
J D. McConnell1, P. Mirtschin1, T. Murphy9,10, S. Neuhold1, R. P. Norris1, S. Pearce1,

2 A. Popping1,8,9, R. Y. Qiao1,12, J. E. Reynolds1, E. M. Sadler9,10, R. J. Sault1,13,
2
A. E. T. Schinckel1, P. Serra1, T. W. Shimwell1,14, J. Stevens1, J. Tuthill1, A. Tzioumis1,

]
M M. A. Voronkov1, T. Westmeier1,9, M. T. Whiting1
I 1CSIROAstronomyandSpaceScience,AustraliaTelescopeNationalFacility,P.O.Box76,Epping,NSW1710,Australia
h. 2DepartmentofPhysicsandElectronics,RhodesUniversity,P.O.Box94,Grahamstown,6140,SouthAfrica
p 3BoltonFellow
- 41–7RoweStreet,Eastwood,NSW2122,Australia
o 5TimCornwellConsulting
r 6RadioAstronomyLaboratory,UniversityofCaliforniaBerkeley,501Campbell,Berkeley,CA94720–3411,USA
t
s 7Inter-UniversityCentreforAstronomyandAstrophysics,PostBag4,Ganeshkhind,PuneUniversityCampus,Pune411007,India
a 8InternationalCentreforRadioAstronomyResearch(ICRAR),UniversityofWesternAustralia,35StirlingHighway,Crawley,WA6009,Australia
[ 9ARCCentreofExcellenceforAll-skyAstrophysics(CAASTRO)
1 10SydneyInstituteforAstronomy,SchoolofPhysics,UniversityofSydney,NSW2006,Australia
v 11ResearchSchoolofAstronomy&Astrophysics,TheAustralianNationalUniversity,CanberraACT2611,Australia
7 12SonartechATLASPtyLtd,UnitG01,16GiffnockAvenue,MacquarieParkNSW2113
5 13SchoolofPhysics,UniversityofMelbourne,VIC3010,Australia
8 14LeidenObservatory,LeidenUniversity,POBox9513,NL-2300RA,Leiden,TheNetherlands
5
0
. Accepted2016January20.Received2016January19;inoriginalform2015August20.
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2 Heywood et al.
ABSTRACT
TheBoolardyEngineeringTestArrayisa6×12mdishinterferometerandtheprototype
oftheAustralianSquareKilometreArrayPathfinder(ASKAP),equippedwiththefirstgener-
ationofASKAP’sphasedarrayfeed(PAF)receivers.Thesefacilitaterapidwide-areaimaging
viathedeploymentofsimultaneousmultiplebeamswithina∼30squaredegreefieldofview.
By cyclingthe arraythrough12 interleavedpointingpositionsand using 9 digitallyformed
beamsweeffectivelymimicatraditional1hour×108pointingsurvey,covering∼150square
degreesover711–1015MHz in12hoursofobservingtime.Threesuchobservationswere
executed over the course of a week. We verify the full bandwidth continuum imaging per-
formanceandstabilityofthesystemviaself-consistencychecksandcomparisonstoexisting
radiodata.Thecombinedthreeepochimagehasarcminuteresolutionanda1σthermalnoise
levelof 375µJy beam−1, althoughthe effectivenoise is a factor ∼3 higherdue to residual
sidelobeconfusion.Fromthiswederiveacatalogueof3,722discreteradiocomponents,us-
ing the 35%fractionalbandwidthto measure in-bandspectral indicesfor 1,037of them. A
searchfortransienteventsrevealsonesignificantlyvariablesourcewithinthesurveyarea.The
surveycoversapproximatelytwo-thirdsoftheSpitzerSouthPoleTelescopeDeepField.This
pilotprojectdemonstratestheviabilityandpotentialofusingPAFstorapidlyandaccurately
surveytheskyatradiowavelengths.
Key words: galaxies: general – radio continuum: galaxies – techniques: interferometric –
instrumentation:interferometers–astronomicaldatabases:surveys
1 INTRODUCTION numerousextragalacticdeepfields,whereradioobservationsform
onepartofapanchromaticpicture.Suchsurveyshavebeencarried
Continuumobservationsatradiowavelengthshavebeenakeycom-
out with several radio telescopes, including the (Karl G. Jansky)
ponent of observational astrophysics for many decades. The fact
VLA(e.g.Bondietal.,2003;Simpsonetal.,2006; Schinnereret
thatradioobservationsarenotaffectedbydustobscurationmeans
al.,2007;Milleretal.,2013;Heywoodetal.,2013),theWesterbork
that they lack many selection biases that exist in observations at
SynthesisRadioTelescope(WSRT;e.g.deVriesetal.,2002) the
otherwavebands, andatypicalsourceatthebrightendofthera-
GiantMetrewaveRadioTelescope(GMRT;e.g.Garnetal.,2007)
dioluminosityfunctionwillbeassociatedwitharadio-loudactive
andtheAustraliaTelescopeCompactArray(ATCA;e.g.Norriset
galacticnucleus(AGN)withamediancosmologicalredshiftof∼1
al.,2006;Middelbergetal.,2008).
(Condon, 1984). Moving towards fainter flux limits, radio obser-
vations become sensitive tothe radio quiet AGN population, and
The Square Kilometre Array (SKA; Dewdney et al., 2013)
an increasing fraction of galaxies whose radio synchrotron emis-
promises to revolutionise our understanding of star formation
sionisdrivenbystarformation(Condon,1992).Radiocontinuum
(Jarvis et al., 2015a) and AGN processes (Smolcˇic´ et al., 2015)
observations thus provide unique insight into black hole activity
acrosscosmictime,aswellastrulyrealisethepotentialthatdeep
(e.g.Jarvis&Rawlings,2000;Smolcˇic´ etal.,2009b;Rigbyetal.,
and wide radio continuum surveys have for answering key ques-
2011; McAlpine, Jarvis & Bonfield, 2013; Banfield et al., 2014;
tionsincosmology(Jarvisetal.,2015b).Aswemovetowardscon-
Best et al., 2014) and star formation (e.g. Seymour et al., 2008,
struction of the SKA, a new generation of large scale radio con-
Smolcˇic´etal.,2009a;Jarvisetal.,2015a)acrossthehistoryofthe
tinuum surveys are being planned and executed with new SKA
Universe.
pathfinderinstruments,aswellasthroughsignificanthardwareup-
Skysurveystypicallyhaveatrade-offbetweendepthandarea.
gradesofsomeexistingradiotelescopes.Theincreasedcapabilities
Radiosurveys withthebroadest coverage at ∼gigahertzfrequen-
ofthesemachinesovertheirpredecessors, –typicallysomecom-
ciestendtobe‘flagship’projects,occupyingasignificantfraction
binationofanexpandedfieldofview,moresensitivereceiversand
of available telescope time and covering most of the entire visi-
a huge increase in instantaneous bandwidth – will allow surveys
bleskybymeansofaverylargenumberofshortsnapshot point-
withdepthorarealcoveragethatimproveonexistingobservations
ingsto∼mJybeam−1 depths.ExamplesincludetheNRAOVLA
byordersofmagnitude.
(VeryLargeArray)SkySurvey(NVSS;Condonetal.,1998),Faint
ImagesoftheRadioSkyatTwenty-cm(FIRST;Becker,White&
AtthedeependtheMIGHTEEsurveyontheMeerKATtele-
Helfand,1995),andtheSydneyUniversityMolonglo SkySurvey
scope (Booth & Jonas, 2012) aimsto cover 35 square degrees to
(SUMSS;Bock, Large &Sadler, 1998; Mauch et al., 2003). The adepth of 1 µJy beam−1 in itsdeepest tier (Jarvis, 2012). Com-
verydeepestobservationstendtocoveronlyasingleprimarybeam
plementarytosuch deepobservations aretheall-skysurveys: the
of the instrument, for example the Lockman Hole observation of
ASKAP(Johnstonetal.,2008;deBoeretal.,2009)EMUsurvey
Condon et al. (2012) which reaches a depth of approximately 1
(Norris et al., 2011) aims to cover the entire sky south of decli-
µJybeam−1.Therearemanyexamplesthatsitsomewherebetween nation+30◦ toadepthof10µJywith10′′ angularresolution.The
thesetwoextremesthattypicallycoverafewsquaredegreesover
WODANsurvey(Ro¨ttgeringetal.,2011)willuseAPERTIF(Oost-
erloo et al., 2009; van Cappellen & Bakker, 2010), a hardware
upgrade to the WSRT,to complete the full sky coverage by con-
⋆ Email:[email protected] ductingasimilarsurveyinthenorthernhemisphere.Thesuccessor
(cid:13)c 0000RAS,MNRAS000,000–000
A pilotmulti-epochcontinuumsurveywith ASKAP-BETA 3
to NVSS is also being planned for the VLA1, and at the time of
writingisenvisagedtoconsistofanall-skysnapshotsurveyat2–
4GHz,reachingadepthof69µJybeam−1,andtakingadvantage
oftheextendedconfigurationsoftheVLAtoreachanangularres-
olution of 2.5′′ (Condon, 2015). These will be complemented by
large-areasurveys at low radiofrequencies using aperturearrays,
includingthenowlargelycomplete30–160MHzMSSSsurveyus-
ingtheLowFrequencyArray(Healdetal.,2014)andtheGLEAM
surveywiththeMurchisonWidefieldArray(Waythetal.,2015)at
80–230MHz.
TheASKAPandAPERTIFtelescopesbothfeaturephasedar-
ray feed (PAF) receivers, in which an array of many receptors is
placedinthefocalplaneofeachofthedishes.Thevoltagesfrom
thesemulti-elementreceptorsarelinearlycombinedwithmultiple
setsofcomplexweightsandsummedtogeneratemultiplebeams,
steeredindifferentdirectionswithinthefieldofviewoftheinstru-
ment. PAF beams from each dish are cross-correlated with those
sharingthesamedirectionfromallotherdishes.Thisparallelpro-
cessingofmultiplebeamsresultsinadramaticincreaseinfieldof
view (andthereforesurveyspeed) overanequivalent singlepixel
feedinstrument.
In this paper we present the results of a pilot, three epoch,
broadband (711–1015 MHz) continuum imaging survey covering
approximately 150 square degrees intheconstellationof Tucana,
andencompassingabouttwo-thirdsoftheSpitzerSouthPoleTele-
scope Deep Field(Ashby et al., 2013), using the Boolardy Engi-
neering Test Array (BETA). This is a prototype of the ASKAP
array, consisting of 6 of the 36 dishes, equipped with the first
generation (Mark I) PAF system (Schinckel et al., 2011) based
onaconnected-element ‘chequerboard’ array(Hay&O’Sullivan,
2008). A detailed description of BETA is provided by Hotan et
Figure1.Theskyareacoveredbytheobservationsisshownabove.Cover-
al.(2014).
ageisachievedthroughacombinationoftwelvearraypointingcentres(as
We describe the observations in Section 2 and the calibra-
labelled) andtheninesimultaneous beamsassociated witheachofthem,
tion and imaging procedure in Section 3. The data products are
thecirclesshowingtheapproximatehalfpowerpointofthebeamsatthe
describedinSection4.Thephotometric,astrometricandspectro- bandcentre(863MHz).Theninebeamsareplacedina3×3squarear-
scopicperformanceofBETAisexaminedindetailinSections4.3, rangement,andthoseassociatedwitheachpointingcentrearerepresented
4.4and4.5.ForthesepurposesweprimarilymakeuseofSUMSS, byacommoncolouronthisplot.
carried on using the Molonglo Observatory Synthesis Telescope
(MOST)whichiswellmatchedtotheBETAobservationsinterms
of frequency and angular resolution. We also make use of lower
frequencyobservationswiththeGMRTandhigher frequencyob- beams (Applebaum, 1976; Hotan et al., 2014). In brief, this ap-
servations from the VLA, chiefly NVSS and FIRST. Concluding proachrequiresthePAFelementstobeexcitedtohighsignificance
remarksaremadeinSection5. byastrongsignal.TheSunisappropriateforsuchapurposeona
12mdish. Thedirectionof agiven beamisenforced bysteering
theantennasothattheSunliesalongthatdirection,anddetermin-
ingcomplexweightsforeachofthe188PAFelements(94ineach
2 OBSERVATIONS
orthogonal mode of linear polarisation) to maximise signal from
ThetargetfieldwasobservedwithBETAonthreeseparateocca- theSunwithrespecttothesystemnoise. Inthecaseoftheseob-
sions as part of the commissioning and verification of theinstru- servationsaregular3×3footprintofbeamswasemployed,with
ment.Thetelescopedelivers304MHzofinstantaneousbandwidth acentralon-axisbeamsurroundedbyeightadditionalbeamsona
andfortheseobservationstheskyfrequencyrangewas711–1015 square grid withaspacing of 1.46 degrees between thecentre of
MHz, corresponding to a fractional bandwidth of 35%. The data eachbeam.
arecapturedwithafrequencyresolutionof18.5kHz,using16,416 Thespacing of thegrid waschosen tobeapproximately the
frequencychannelsacrosstheband. half-power beam widthof asingle PAFbeam at the band centre.
The digital beamformers of BETA (Bunton et al., 2011; For‘traditional’mosaickingofaregionofskyusingasinglepixel
Hampson et al., 2011) are capable of delivering nine dual- feedinterferometer,thearraywilltypicallyobservealistofdiscrete
polarisation beams that can be placed arbitrarily within the ∼30 positionsthathavetheprimarybeamfromonepointingsituatedat
square degreefieldof view of theinstrument. Atpresent amaxi- thehalfpowerpointoftheadjacentscan,typicallywithahexagonal
mumsignal-to-noisealgorithmisemployedtoformthecompound arrangement. In the case of ASKAP,the three-axis mount on the
antennasinthearraykeepsthedeployedbeampatternfixedonthe
skyrelativetotheantennapointingdirection(unlesstheweightsare
1 https://science.nrao.edu/science/surveys/vlass adjusted). Thus an appropriate combination of beam pattern and
(cid:13)c 0000RAS,MNRAS000,000–000
4 Heywood et al.
pointing positionscan bedevised torapidlycover alargeareaof
skytoapproximatelyuniformdepth.
ForthisprojecttheBETAarrayspentfiveminutesoneachof
twelveskypositions, repeatingthecycleforthe12hourduration
oftheobservation.Thepointingpositionsarearrangedinsixclose
pairs,withtheclosepairsusedtooffsetthefactthatthebeamspac-
ing at any given pointing is twicethe value that would generally
beusedforastandardmosaic(Bunton&Hay,2011;Hay&Bird,
2015). Theend result isthe approximately uniform sky coverage
showninFigure1,effectivelymimickingatraditional108pointing
radio survey using only twelve BETA pointings. Groups of nine
beamssharethesamecolourinthisplot.
Cycling around the twelve pointing centres with short five
minute integrations over a twelve hour observation builds up
favourableFourierplanecoverageforeachofthepointings,which
issharedbytheninebeamsassociatedwiththatpointing.The(u,v)
planecoverageofasinglepointingisshowninFigure2.Theradial
coverageaffordedbythe304MHzofbandwidthisapparent,and
thepointsarecolouredperbaseline.Notethattheshortestbaseline
hasbeenremovedforreasonsexplainedinSection3.1.
TheASKAPantennasareequippedwithanadditionalaxisof
movement(therollaxis;Forsythetal.,2009)thatkeepstheparal-
lacticanglefixedoverthecourseofanobservation,andthuskeeps
the beam pattern fixed on the sky without the need for continual
adjustment of the weights (Hay, 2011). For each of the pointing
Figure2.The(u,v)planecoverageofatypicalpointing,inthiscasefield
positionsfor thissurvey theroll axisposition wasadjusted tore- F0AfromScheduling Block1231.Notethatthis(u,v)planecoverageis
movetheparallacticangleoffsetfromscansthatoccuralonglines sharedbyallninebeamsassociatedwiththatpointing.Interleavingeachof
of fixeddeclination. Theendresult isamoreregular survey area thetwelvepointingsviatheconstantrotationoffiveminutescansbuildsup
thatdoesnottaperwithdeclination. goodFourierplanecoverage.Theradialcoverageisduetothe304MHzof
The survey area was observed withBETA on three separate bandwidth.Eachbaselinehasauniquecolouronthisplot.
occasions. Table 1 lists the start and end times of the ASKAP
Scheduling Blocks (SBs) used in the project. In addition to the
observations of the target field a calibration scan is performed Table1.Startandendtimesanddates (UT)fortheASKAPScheduling
wherebythearrayexecutesapointingpatternthatplacesthestan- Blocks(SBs)thatwereusedinthisproject. SBsmarkedwithanasterisk
(*)indicatethattheobservationsconsistofper-beamscansofthestandard
dard calibrator source PKS B1934−638 at the nominal centre of
calibratorsourcePKSB1934−638,asdescribedinthetext.NotethatSBs
eachbeamfor5minutes.Thesescans(markedwiththeasterisksin
1206and1207are partofthesameobserving runwhich interrupted for
Table1)areusedtocalibratethebandpassresponseofeachbeam
approximatelyanhour.Intotalthisfirstepochhas8h38mofdatamaking
and to set the flux density scale.2 Note that SBs 1206 and 1207
itshallowerthanthesubsequenttwoepochs.
weremergedintoasingledataset.Withatotaldurationof8h38m
thisobservation isshorterthanthoseofSBs1229and1231.Fur-
SB Date Start(UT) End(UT) Duration(h)
ther details of the calibration process are provided in Section 3.
Thebeamformingprocesswasinitiatedat05:00UTon2Decem- 1205* 02-Dec-2014 06:25:39 07:12:39 0.78
1206 02-Dec-2014 07:14:44 09:56.49 2.70
ber 2014, immediately prior to the first astronomical observation
1207 02-Dec-2014 11:03.04 16:59:44 5.94
performedaspartofthispilotsurvey,aslistedinTable1.Nofur-
1227* 07-Dec-2014 03:21:01 04:07:56 0.78
ther updates to the beamformer weights were made during these
1229 07-Dec-2014 04:16:06 16:30:46 12.24
observations.
1230* 08-Dec-2014 04:09:46 04:56:41 0.78
1231 08-Dec-2014 05:00:41 17:04:51 12.07
3 DATAREDUCTION for modern radio interferometers, albeit with a few special con-
siderationsduetothemultipleprimarybeamsthatBETAdelivers.
Theprocedureforcalibratingandimagingthedatawasidenticalfor
Itwasachievedwithacustomsoftwarepipelinethatusescompo-
eachofthethreeepochs,andessentiallyfollowedstandardpractice
nents from numerous packages, as described in this section. The
procedurewasentirelyautomatic.Notethatthestepsdescribedin
Sections3.3and3.4wereexecuted twice.Followingthefirstrun
2 This approach would likely prove too costly to be executed on a per-
anautomaticsourcefinderwasused,thepositionsofthedetected
observation basis for the full 36-beam ASKAP array. The calibration of
sourceswereturnedintoamaskinordertoconstrainthedeconvo-
thebeamsforfullASKAPislikelytomakeuseofincrementalcorrections
lution.Oncethecleaningmaskswereinhand,theinitialcalibration
to the solutions derived from an initial reference calibration observation
wasdiscarded and the process restarted, withthe cleaning masks
ofastrongcalibratorsource.Theincrementalsolutionswillbederivedby
makinguseoftheon-dish calibration system,anartificial radiator atthe employedthroughoutthesecondrun.Thenecessityofthisstepis
centreoftheprimaryreflectorthatisusedtoilluminatethePAFelements discussedinSection3.5.
tohighsignificance.ThisfeatureisnotpartoftheBETAhardware. Thebespoke calibrationandimaging procedure usedfor the
(cid:13)c 0000RAS,MNRAS000,000–000
A pilotmulti-epochcontinuumsurveywith ASKAP-BETA 5
BETAcontinuumdataistheresultoftestingnumerousapproaches. relatorintegrationtimeof5seconds.Atthisstagefurtherimprove-
Themethodsoutlinedbelowarenotexpectedtobethoseadopted ments to the data quality must be made via self-calibration tech-
forthefullASKAParray.ASKAPhasadedicatedsoftwaresystem niquesasnoadditionalcalibrationscansweremade.Thisproceeds
(ASKAPsoft)thatisacrucialcomponentofthetelescope,designed as follows, for each of the twelve fields, each of which has nine
specificallytocontinuouslyprocessthedatastreamsinpseudoreal corresponding beams,givingatotalof108datasetspertargetSB
time.ASKAPsoftisitselfundergoingcommissioningtests,andin thatmustbeindependentlycalibrated.
any case it relies on many properties of the final array (e.g. low TheCASAcleantaskwasusedtomakeamulti-term,multi-
point spread function (PSF) sidelobe levels) that cannot be met frequency synthesis (MT-MFS;Rau &Cornwell, 2011) image of
byBETA.AcompletedescriptionoftheASKAPdataprocessing the data in four spectral chunks, each having 76 MHz of band-
modelisprovidedbyCornwelletal.(2011). width: 711–787, 787–863, 863–939, 939–1015 MHzrespectively
for sub-bands 1, 2, 3 and 4. As mentioned in Section 2, the roll
axis of the ASKAP telescope keeps the beam pattern fixed rela-
3.1 Pre-processing tivetothesky.Havingsidelobesthatdonotrotateoffersconsider-
able advantages in terms of using standard direction-independent
The BETA telescope produces approximately 2 TB of visibility
self-calibration techniques to deal with sources in the sidelobes
dataper 24 hoursof observing. Each Scheduling Blockproduces
(e.g. Smirnov, 2011; Heywood et al., 2013). However for a typ-
asingleCASAformatMeasurementSetthatcontainscrossandau-
ical BETA observation in this band substantial effective depth is
tocorrelation measurements for all nine beams, with distinctions
achieved through the firstsidelobe and numerous sources are de-
betweenbeamsencodedviatheFEEDtable.Allfourlinearpolar-
tectable.Theimagemustthereforebewideenoughtoallowthese
isation products are produced (XX, XY, YX and YY). The visi-
sourcestobedeconvolved.Thew-termiscorrectedforduringgrid-
bilitesforeachbeamweresplitintoastandaloneMeasurementSet
usingtheCASA3package(McMullinetal.,2007).Initialflagging ding (Cornwell, Golap & Bhatnagar, 2005) to more faithfullyre-
coverfarfieldsources.Also,sincethepurposeoftheseinitialim-
operations were also applied at this stage using the flagdata
ages is to form a sky model for refining the calibration, far field
task, namely the removal of the autocorrelations, the deletion of
theshortestbaseline4,amplitudethresholdingandasinglepassof sourcesmustalsobecharacterisedtothatend.
therflagalgorithm5asimplementedinCASA. ForasingleSBthisproduces12fields×9beams×4sub-
bands = 432 images. A sky model was constructed by using the
PyBDSM(Python Blob Detection and Source Measurement; Mo-
3.2 Bandpassandfluxscalecalibration han&Rafferty,2015)sourcefindertodecomposetheStokesIim-
ages intoaseries of point and Gaussian components. Thesource
The per-beam scans of PKS B1934−638 at full spectral resolu- finder first estimates the spatial variation in the RMS noise (σ)
tion were averaged in time, and per-channel complex gain solu- in the image by stepping a box across the image, measuring the
tionswerederivedthatbestcorrecttheobserveddatatothemodel standarddeviationofthepixelswithinitandtheninterpolatingthe
polynomial fittothespectrumof thesourcederivedbyReynolds measurements. Thisdistributionisthen subsequently usedby the
(1994). Two passes of asliding median filter wereapplied tothe sourcefinderforthresholdingpurposes.Imagepeaksthatexceeda
derived correctionstoremoveerrant channels. Thefrequency be- user-specifiedthreshold(inthiscase7σ,whereσisthelocalRMS
haviour of these solutions thus corrects the effective bandpass of value)areidentified.Thesepeaksarethengrownintoislands,con-
eachoftheformedbeams,andsincethesolutionswerenotnormal- tiguousregionswherethepixelvaluesexceedasecondarythresh-
izedtheyalsoencompassthefluxdensityscalingofthetargetdata. old(inthiscase5σ).HavingidentifiedtheseislandsPyBDSMat-
Theper-beamobservationsofthetargetfieldwerecorrectedbyap- tempts tofit them withpoint and Gaussian components, and cat-
plyingtherelevantcalibrationtablederivedfromPKSB1934−638. alogue and image products describing the fit can be exported in
Motivatedbythelargefieldofviewaffordedbythe12mdishat various formats. Manual checks when designing and refining the
these observing frequencies, we have determined via simulations calibrationandimagepipelineverifiedthatthethresholdsusedre-
thatthe∼Jy-levelsourcesinthefieldofPKSB1934−638manifest turnedalargelycompletemodelthatdidnotincludespuriousfea-
themselvesasanoise-likesignalatthe∼1%levelwhenaveraged tures. No assumptions about the primary beams of the telescope
over5minutesof data,irrespectiveof thehour angle,anddonot havebeenmadeatthisstage,thusthemodelcapturestheapparent
needtobeincludedinthecalibrationmodel. ratherthantheintrinsicbrightnessesofthesourcesinthefield.
The MeqTrees package (Noordam & Smirnov, 2010) was
used to predict model visibilities based on the per sub-band sky
3.3 Self-calibration
modelsderivedfromtheimagebyPyBDSM.Asetofper-antenna
Followingbandpassandfluxscalecorrectionthetargetdatawere phase-onlycomplexgaincorrectionswerethensolvedforbycom-
averagedfrom16,416×18.5kHzchannelsto304×1MHzchan- paringtheobservedvisibilitiestothemodeldata.Anindependent
nels.Notimeaveragingwasperformed,preservingthedefaultcor- correctionwasderivedforboththeXXandYYpolarisations,al-
though no polarisationinformation wasincluded inthemodel. A
singlesolutionwasderivedforeachfiveminutescanandforeach
3 http://casa.nrao.edu sub-band.
4 AntennasAK01andAK03areseparatedbyonly37m.Includingthis
baselinetendstodegradethequalityoftheimageduetotheintroduction
ofemissiononlargespatialscalesthatisnotfaithfullyreproduceddueto 3.4 Imaging
thelackofspacingsbetweentheshortesttwobaselinesoftheBETAarray.
Followingthecorrectionof thedatausing theself-calibrationso-
ThissituationwillimproveasmoreantennasinthecoreofASKAPcome
lutions each of the 108 individual Measurement Sets were re-
online.
5 rflagidentifiesandexciseserrantvisibilitypointsbasedontheirdevi- imagedanddeconvolvedinfourspectralsub-bandsusingtheCASA
ationfromstatisticscomputedinslidingtimeandfrequencywindows. cleantaskinasimilarwaytotheprocessthatwasusedtoderive
(cid:13)c 0000RAS,MNRAS000,000–000
6 Heywood et al.
theskymodel.Forthefinalimagesdeconvolutionwasperformed ularlyatthefaintend)canbestronglyaffectedbythedeconvolu-
intwopasses,adeep6cleanwiththemaskinplaceandasecondary tion,likelyduetothehigh(∼15%ofpeak)PSFsidelobelevelsof
shallow7passwiththemaskremoved.AcommonGaussianrestor- the six dish array. Thus we only apply shallow deconvolution to
ingbeamof70′′×60′′(PA=0◦)wasenforcedforallimages. the residual data following the deconvolution step that employed
PrimarybeamcorrectionwasachievedusingthestandardAiry masks.Thisresultsinaccuratefluxdensitymeasurements(Section
patternfora12mdishappropriateforeachsub-band,ascomputed 4.3)andreliableautomaticpipeliningofthedata,howevertheprice
bytheCASAimager(seealsoAppendixA).OnequirkoftheBETA wepayforthisisaneffectivelyhighernoisefloor(Section4.2).
system8isthateachbeamrecordsvisibilitiesthatarefringestopped The symptoms introduced by these biases are temporary, in
toacommonreferencedirection,typicallythatofthearraypoint- that they are expected to be greatly reduced as more antennas in
ingcentre, which inthiscase iscoincident withthecentreof the theASKAParraycomeonline.Theself-calibrationprocedurewill
on-axis formed beam. Thus when imaging off-axis beams witha bebetterconstrainedandthedeconvolutionprocesswillbemuch
standard imaging package the region of maximal sensitivity will more robust as the PSF side lobes are significantly reduced due
beoffsetfromtheimagecentreatthenominalcentreofthebeam to the improved (u,v) plane coverage. It is pleasing to note that
thathasbeenimaged.Theprimarybeamimagesmustbeshiftedto themosttroublesomeaspectsofprocessingthedatafromthisnew
thecorrect position beforethecorrectional divisionoperation oc- telescopeoccurredsimplybecauseweonlyhadsixantennasatour
curs. Corrected images were cut at the radius where the primary disposal.
beamgaindroppedbelow30%.Linearmosaickingwasdonewith
the Montage9 software, using the assumed variance patterns as
weightingfunctions.
4 RESULTSANDDISCUSSION
3.5 Anoteoncalibrationanddeconvolutionbiases 4.1 Widefieldradioimages
Withtheremovaloftheshortestbaseline,self-calibrationofeach TheautomatedcalibrationandimagingproceduredescribedinSec-
BETA beam involves solving for six unknowns (the per-beam tion 3 was applied to each of the three observations of this field,
complex gain terms) using only 14 equations (one per base- resulting in 432 primary beam corrected images per epoch. Four
line, e.g. Cornwell & Wilkinson, 1981; Ekers, 1984). The self- sub-bandmosaicswereproducedfromthe4×76MHzchunksof
calibration problem for BETA is therefore relatively poorly con- calibrateddataacrossthe304MHzband:.Thefullbandmosaics
strainedwhencomparedtoanarraysuchastheVLA(27antennas, wereformed by linearly mosaicking all 432 images per epoch in
351baselines),ASKAP(36antennas,630baselines)orMeerKAT ordertoincludea(somewhatcoarse)frequency-dependentprimary
(64antennas,2016baselines).Acarefulandconservativeapproach beamcorrection.Thesefullbandmosaicsformthebasisofthein-
mustbeadoptedinordertoavoidself-calibrationbiases,whereby strumental verification presented in Sections 4.3 and 4.4, as well
the contribution to the visibility measurement made by sources asthevariabilitystudypresentedinSection4.6.Acombined,deep
that are not inthe sky model can besubsumed into thegain cor- imagewasgeneratedbystackingthemosaicsformedfromthethree
rections causing that set of (typically fainter) sources to be sup- epochs.Thisisusedtoproduceacatalogueofthecomponentsin
pressed.Incompleteskymodelscanalsoimpartsubtleandhighly theimageasdescribedinSection4.7,aswellastomeasurethedif-
non-intuitivefeaturesintoaradioimage,seeforexampleGrobler ferentialsourcecountsat863MHz(Section4.8).Combinedthree-
etal.(2014). epochsub-band mosaicswerealsoproduced. Thesewereusedto
Furthermorethephenomenonofcleanbias(orsnapshotbias) examinethein-bandspectralperformanceofBETAinSection4.5,
must also be considered. This also typically manifests itself as a aswellasproduceestimatesofthesourcespectralindicesforthe
systematicunderestimationofthesourcefluxdensitiesabovesome catalogue. Priortocombining theimages, astrometriccorrections
threshold,andhasbeenshowntoexhibitflux-dependent suppres- were applied in order to correct minor offsets in the coordinate
sion of the fainter sources, as well as affect the flux densities of frames,asdescribedinSection4.4.
sources below the thermal noise limit (White et al., 2007). Most Tosummarise: each of thethree epochs resulted infivemo-
investigationsintocleanbiashavebeenconductedusingVLAdata saics(foursub-bandandonefullband)andthereareanadditional
(where strong linear features in the snapshot PSF are thought to five mosaics for the combined data, twenty images in total. The
exacerbatetheproblem)viatheinjectionofsyntheticsourceswith combinedepoch,fullbandradiomosaicisshowninFigure3.The
knownbrightnessintothedata(Condonetal.,1998;Becker,White PyBDSMsourcefinderusingapixelthresholdof5σandanisland
& Helfand, 1995). A thorough understanding of the problem in thresholdof3σ(whereσisthesourcefinder’sownestimateofthe
the context of broadband radio continuum imaging is yet to be localnoise,seeSection3.3)wasusedtoproducecomponent lists
achieved,howeverHelfand,White&Becker(2015)notethatbias for each of these imagesfor cross-matching and verificationpur-
appearstocorrelatewithinterferencelevels. poses.
BlindcleaningoftheBETAdatawithvaryingnumber ofit-
erationsshowsthattherecoveredfluxdensitiesofsources(partic-
4.2 Sensitivityandconfusion
6 Fortheinitialdeepdeconvolutionthecleancycleisterminatedwhenthe Estimates of the local background noise produced by PyBDSM
peakresidualisbelow5timestheRMSnoiseofthecorrespondingStokes
serve to deliver a source catalogue that has very high reliability,
Vimage(seeSection4.2),orat5,000iterations,whicheveroccursfirst.
7 Theshallowcleaningoperationconsistsonlyof100blinditerationson andtheresultingRMSmapsareveryusefulforcomputingthevis-
ibilityareasatdifferentfluxthresholds(Section4.8).Theymaynot
theresidualsremainingfollowingtheinitialdeepclean.
8 ThiswillnotbethecaseforanyASKAParraythatisequippedwithMark howeverbeareliableindicatorofthethermalnoiseperformanceof
IIPAFs. theobservation,andmoreaccuratelyquantifythe‘effective’noise
9 http://montage.ipac.caltech.edu/ of the image. In the case of the BETA observations the effective
(cid:13)c 0000RAS,MNRAS000,000–000
A pilotmulti-epochcontinuumsurveywith ASKAP-BETA 7
Figure3.Totalintensitymosaicformedfromcombiningtheimagesfromallthreeepochs.TherectanglemarkstheareapresentedinFigure9andSection4.5,
andtheareaofoverlapwiththeSpitzerSouthPoleTelescopeDeepFieldisshown.ThesinglesignificantlyvariablesourceJ223825−511419(Section4.6)is
alsomarked.Thegreyscaleislinearandrunsfrom−30to80mJybeam−1.
(cid:13)c 0000RAS,MNRAS000,000–000
8 Heywood et al.
noiselimitisdominatedbythreeeffects.Thefirstistypicalofra-
diointerferometer mapsinthat theregionsaround bright sources
tendtohaveelevatedartefactlevelsduetocalibrationdeficiencies.
The other two effects at play here are deconvolution related: the
conservative approachtocalibrationanddeconvolution employed
in order to minimise biases (Section 3.5) results in only shallow
cleaningofthefaintsourcesintheimage.Residualsidelobesthus
contribute significantly to the image background. The second ef-
fect comes about by our use of image-plane combination of the
dataacrosstheband.Asthetruenoisefloorispusheddown,fainter
sourcesarerevealedthathavenotbeencleanedatall.
Classical source confusion is not expected to be contribut-
ing to the effective noise in these images. The level of classical
confusion can be estimated by using the extragalactic radio con-
tinuumsimulationofWilmanet al.(2008; 2010). Thesimulation
uses observed and extrapolated luminosity functions to generate
mockgalaxypopulationsincludingradioloudandquietAGN,qui-
escentstarforminggalaxiesandGigahertzPeakedSpectrum(GPS)
sources, the clustering properties of which are determined by a
modeloftheunderlyingdarkmatterdistribution.Theresultisacat-
alogueof∼260millioncomponentsover400squaredegreeswith
afluxlimitof10nJy,witheachcomponenthavinganestimateof
itsradiofluxdensityatfivefrequencies.
Welinearlyinterpolatethesimulated610and1400MHzflux
densitymeasurementsto863MHz,anddeterminethefluxlimitat
whichthenumber of sourcesper unitareaexceeds athreshold at Figure4.Thepinkhistogramsofthepixelintensitiesofthepseudo-Stokes-
Vmosaicsprovideeffectivemeasurementsofthethermalnoiseoftheob-
whichtheobservations aredeemed tobe confused. Fromthiswe
servations,the±1σvaluesofwhichareindicatedbytheupperbarsoneach
estimatethat the classical confusion limit for theBETA observa-
subplot.Theeffectivenoiselevel(estimatedbyPyBDSMfromtheStokes
tionsis337µJyform=10,and180µJyform=5,wheremisthe
Imosaics)iselevated byafactorof∼3,primarilyduetoincompletede-
numberofbeamspersource,thecriteriontypicallyusedtodefine
convolution,asindicatedbythelowerbarsoneachsubplot.Thecolumns
classicalconfusion(Wilson,Rohlfs&Hu¨ttemeister,2013).Simu- arethethree epochs andthecombined data, andtherows showthe four
lations based on the faint source count measurements of Condon sub-bandsplusthefull-banddata.Refertothetextforfulldetails.
et al. (2012) estimate the classical confusion limit to be 158 and
202µJybeam−1 at711and1015MHzrespectivelyforaresolu-
tionof70′′×60′′(J.Condon,privatecommunication). astronomicalsignal).Alsoshownontheplotviathebluebarsare
We can estimate the true thermal noise performance of the themedianvaluesofthenoisemeasurementsfromthecorrespond-
ingPyBDSMStokesIRMSmap.Theeffectivenoiseistypicallya
data by forming Stokes V images and reproducing the mosaics.
Moreaccuratelythesemapswillbepseudo-Stokes-V,asalthough factorof∼3higherthantheexpectedthermalnoise.
useoftheunpolarisedcalibratorPKSB1934-638leadstoaccurate Thisisundesirable,butisapenaltythatispaidinexchangefor
calibration of the XX and YY gains, no further polarisation cali- theautonomous pipelining ofthedata.Human-guided calibration
brationhastakenplace,andsomeinstrumentalleakagefromother and imaging of BETA data can successfully deal with this issue,
Stokes parameters into V will be present. Thus even intrinsically primarilybysuperviseddeconvolutionandidentificationofspuri-
unpolarisedsourceswillleavesomefractionoftheirStokesIflux ousimagefeatures,andthemanualconstructionofdeepskymod-
intheStokesVimage,thelevelofwhichiscoupledtothediffer- els. We note that sidelobe confusion, as with the calibration and
encebetweenthetrueXXandYYbeamsandtheassumedprimary deconvolution biases, will be much reduced in future versions of
beamusedtocorrecttheimagesatthepositionofthesource.The theASKAParray.
errorintroducedbythesedifferencestypicallyincreaseswithdis-
tance from the centre of the beam, thus spurious emission in the
4.3 Photometry
StokesVmosaicsiseffectivelysuppressedbythemosaicweight-
ingscheme.Sourcesthatarebothbrightandstronglyintrinsically Aswitha traditional single pixel feedinterferometer, regular ob-
polarisedwillalsoinprincipleremainintheimage,typicallyata servationsofacalibratorsourcearerequiredtodeterminetheabso-
levelthatisafewpercentoftheirStokesIbrightness. lutefluxdensityscaleinthepresenceoftemporalinstrumentalgain
Figure4showshistogramsofpixelintensityasmeasuredfrom drifts.WithaPAFsystemhowevertheprimarybeamsareformed
thepseudo-Stokes-Vmosaicsoverregionsthathavemorethanone fromweightedsumsofthesignalsfrommanyindividualelements.
beamcontributingtothem.Thecolumnsarethethreeepochsand Whilethedirectional responsesoftheindividual elementscanbe
thecombineddata,andtherowsshowthefoursub-bandsplusand assumed to be stable (as can to first order the primary beam re-
the full-band data. The sensitivity gain afforded by averaging in sponseofasinglepixelfeedtelescope),thetemporaldriftsinthe
time(firstthreecolumnsintothefinalcolumn)orfrequency(first electronic gains of these elements have the potential to modulate
four rowsintothefinalrow) isclear.Thelabeledpinkbarsshow the shape of the compound beams (Smirnov & Ivashina, 2011).
the±1σvalues,whereσisthestandarddeviationofthepixelhis- Whilethe absolute fluxdensity response of thearray can becor-
tograms.Weassumethatthesemeasurementsgiveanaccuratemea- rected by visiting aknown calibrator, correcting variations in the
surement of the thermal noise (from the effective absence of any off-axis response due to element gain drifts can only be done by
(cid:13)c 0000RAS,MNRAS000,000–000
A pilotmulti-epochcontinuumsurveywith ASKAP-BETA 9
making compensatory adjustments of the beamformer weights10.
Forthesurveypresentedinthispaper,theBETAbeamformerswere
loaded withappropriate weightsprior tothefirstobservation and
theseweightswerenotadjustedfortheweekoverwhichtheobser-
vationswereconducted.
Weexaminethephotometricaccuracyandstabilityofourob-
servationsbycrossmatchingthelistofsourcesdetectedinthefull
band, per-epoch radio mosaics, as well as in the final combined
mosaic,withtheSUMSScatalogue.The843MHzobservingfre-
quencyofSUMSSisslightlyoffsetfromour863MHzobserving
frequency, however the fractional change in the flux density of a
typical α = −0.7 (S ∝ να) source between the observing fre-
quencies of SUMSS and these observations is less than 2%. We
do not attempt to correct for this. Components were matched by
searchingforthepairwiseminimumseparation,andrequiringthat
thisseparationbelessthan20′′,orroughlyonethirdoftheBETA
synthesised beam width. In addition, contiguous islands of emis-
sionintheBETAcataloguesthatcontainedmorethanasinglefit-
tedcomponentwererejected.Thishastheeffectofonlyselecting
isolatedsourcesforthecrossmatching,avoidingregionsofemis-
sionthatmaybefittedbydifferent multiplecomponents between
thesurveys,althoughwiththesomewhatlowangularresolutionof
theseobservationssuchcomplexsourcesarerare.
Figure5.PeakfluxdensitiesoftheBETAcomponentscomparedtothose Figure 5 shows a log-log plot of the peak flux densities of
ofmatchedcomponents fromtheSUMSScatalogue forthethreeepochs
theSUMSScomponentsagainstthoseofthematchingBETAcom-
andthecombineddata.Theredpointsindicatecomponentsthatappearun-
ponents for the three epochs, as well as the combined epoch as
resolvedinSUMSS(andshouldthereforebeunresolvedintheBETAdata).
marked above each panel. The diagonal line is the 1:1 relation-
Thehardloweredgeonthecombineddataplot(lowerright)isduetothe
6mJybeam−1peakfluxlimitofSUMSS.Thenumberofmatchedcompo- ship. Increased scatter in the measurements with decreasing flux
nentsforthefourpanelsare(lefttoright,toptobottom)1,841;2,282;2,241 densityistobeexpectedasthenoiseoftheobservationsbecomes
and 2,927. The relative shallowness of the first epoch and the increased anincreasinglysignificantfractionofthecomponentbrightness.In
depthofthecombineddataarereflectedinthesecounts.Thecolouredbars addition tothisfeature, thedistributions inFigure5show anon-
onthecombinedpaneldenotetherangesofthefluxdensitybinsusedto linearshiftfromanexcessintheBETAmeasurementsatthebright
constructFigure6. endtoSUMSSexcessatthefaintend.Thisshiftisvisualisedand
quantifiedinFigure6whichshows histogramsof thecomponent
counts as a function of their BETA to SUMSS peak flux density
ratios.Thesearegroupedintologarithmically-spacedbinsaccord-
ingtotheirBETApeakfluxdensitymeasurements,asindicatedon
Figure6,andbythecolouredbarsinthelowerrightpanelofFigure
5.ThenumbersonFigure6showthemedian,meanandstandard
deviationoftheratiosforeachfluxdensitybin,movingfromafew
percent excess in the BETA measurements at the bright end to a
∼15%decrementinthefaintestbin.
Such shifts away from the 1:1 relationship are often seen
whencomparingfluxdensitymeasurementsfromsurveyswithmis-
matched angular resolution and depth. For example, Franzen et
al. (2015) compare ATCA flux density measurements to deeper
VLA data for a sample of unresolved sources, noting an excess
intheATCAmeasurements that approaches afactor of 1.5times
the VLA flux density for the faintest sources. Thisis ascribed in
part to Eddington bias (Eddington, 1913) skewing the measure-
mentshighintheshallowerATCAdata.Allison,Sadler&Meekin
(2014) compareSUMSSdatatomatchedsourcesfromthesingle
dishContinuum HI ParkesAllSkySurveycatalogue (Calabretta,
Staveley-Smith&Barnes,2014),resultinginadistributionthatis
Figure 6. Histogram of component counts as a function of their BETA
qualitatively very similar to those in Figure 5, and Hodge et al.
toSUMSSpeakfluxdensitymeasurements.Thesourcesaregroupedinto
logarithmically-spaced binsaccordingtotheirBETApeakfluxdensity,as
indicatedbythekeyaboveandbythestripesonthelowerrightpanelofFig-
ure5.Thefaintestbinisrepresentedbytheblacklinehistogramforclarity. 10 SuchasystemwillbeimplementedforASKAP’sMarkIIPAFsystem
Valuesonthefigureshowthemedian,themeanandthestandarddeviation
throughtheuseoftheon-dishcalibrationsystem,allowinganoisesourceto
oftheratiosperbin.
beradiatedontothePAFfromthecentreoftheprimaryreflector.Direction
dependentcalibrationschemesareanotherviablemethodfordealingwith
theseeffects.
(cid:13)c 0000RAS,MNRAS000,000–000
10 Heywood et al.
(2011)seeatrendtowardsexcessbrightnessintheirVLAA-array
measurements compared to thelower resolution FIRSTmeasure-
mentswhichtheyinterpretintermsofresolutionbiases.
Due to projection effects, the resolution of SUMSS is
45′′×(45csc|δ|)′′whereδisthedeclination,increasingthenorth-
southsizeofthePSFfrom50′′ to60′′ overtheextentofthecom-
monarea.HoweverforallpositionsthePSFoftheSUMSSsurvey
is(afactorof1.1–1.5times)smallerthanthatofBETA.SUMSS
resolves∼10%ofsources(∼25%atitssouthernmostdeclinations,
∼2%towardsthecelestialequator).Nocleartrendemergesinex-
aminationsofthedifferencebetweentheBETAandSUMSSmea-
surementsasafunctionofthedeconvolvedsourcemajoraxis.Se-
lectingsourcesthatareunresolvedinSUMSSbyrequiringapeak
tointegratedfluxratioofbetween0.98and1.02resultsinthesub-
set of objects denoted by the red points on Figure 5, and a two-
sampleKolmogorov-Smirnovtestrejectsthehypothesisthatthese
aredrawnfromdistinctdistributions(p-value=15%).Ifresolution
biasisatplayhereitisnotlikelytobethedominantcauseofthe
observedtrend.
Serraetal.(2015)used30hoursofBETAtimetoconductHI Figure7.Thedifferences inthepeakcomponentfluxdensitiesforapair
imagingofanearbygalaxygroupoverthefrequencyrange1.4025– ofepochsexpressedasafractionofthemeanvalueforthatpair.Thecen-
1.4210MHz(1.3%fractionalbandwidth).Crossreferencingofthe tralepoch(SB1229)ischosenasareferenceepochandtheplotshowsthe
sources detected in the corresponding continuum image with the SB1231toSB1229valuesagainsttheSB1229toSB1206–1207values,
NVSScataloguerevealedamedianandmeanexcessof7%inthe i.e.thefirstandsecondepocharecomparedonthex-axis,andthesecond
andthirdepocharecomparedonthey-axis.Thematchedcomponentsare
BETAmeasurements.Infractionaltermstheseapproximatelycor-
placedintologarithmically-spaced binsdependingontheirpeakfluxden-
respondtothesourcesaboveourthirdfluxdensitybininFigure6,
sityvalueinthereferenceepoch,SB1229,asindicatedbythecolourscale.
sothediscrepancyisconsistentwithourfullbandwidthcontinuum
Theincreasedscatterduetothedecreasingsignal-to-noiseratioisclear.
measurements.Serraetal.(2015)remarkthatinaccuraciesinmod-
elsoftheprimarybeamsmaybethecause,andsucheffectswould
certainlyalsobepresentinthedatapresentedhere.Calibrationand increasingly inaccurate over time, and this slight degradation in
deconvolutionbiasesmayalsobepresentinbothdatasets,despite theflux accuracy may be a manifestation of such ‘ageing’ of the
effortstominimisethem. weights.
Epoch to epoch stability is demonstrated in Figure 7. This Howdoesthis∼2–3%fluxscaleoffsetcomparetomeasure-
shows the differences in the peak component flux density values mentsmadewithothersynthesisarrays?Thisoffsetisnotanoma-
forpairsof epochs, expressedasafractionofthemeanvaluefor louslydamaging.Perley&Butler(2013)claimthatatL-bandthe
the pair. The x-axis compares the first and the second epoch and VLAwillexhibitascatterinmeasuredfluxdensitiesofbetterthan
the y-axis compares the second and the third epoch. Again, the 1%solelyduetotheaccuracyoftheflux-scaletransferfromthepri-
points are colour coded by their peak flux density, as measured marycalibrator,risingto3%athigherfrequencies, althoughthey
in the second epoch (SB 1229). The increased spread due to de- notethatnon-optimalcalibrationmethodsandobservingconditions
creasing signal-to-noise ratio is evident. Note that the catalogues candegradethisaccuracysignificantly.TheATCAexhibitssimilar
exclude sources that have apeak flux density of lessthan 5σ, so levelsof accuracy from the flux-scaletransfer, better than 3% (J.
atwoepochcomparisonofasourceatthedetectionthresholdfor Stevens,privatecommunication).Croftetal.(2010;2011)presenta
twoobservationsofequaldepthwillbydefinitionexhibitapparent multi-epochtransientsurveyusingthe42-elementAllenTelescope
variabilityatthe20%level.Thediagonal biasinFigure7results Array,andnotefluxscaleoffsetsofuptoafactorof2betweentheir
naturally from making this type of plot from noisy data. Making epochs.Theyascribethistothesignificantlydiffering(u,v)plane
threemeasurementsofasourceoffixedbrightnessinthepresence coveragesbetweensomeoftheir60secondsnapshotobservations
of independent Gaussian noise will result in a light curve that is introducing variable levels of clean bias. The PAF beams appear
eitherrising,falling,orpeaksordipsinthecentre.Thesefoursce- to be stable enough to conduct astronomical measurements over
nariosplacetheobjectinoneofthemarkedquadrantsonFigure7. timescalesoforder oneweek,andinanycaseitislikelythatthe
Foralargenumberofmeasurementstheupperleftandlowerright beam weights will berefreshed or adjusted on shorter timescales
(peaking ordipping) quadrants willcontaintwiceasmanypoints for the full ASKAP system, and increasing the calibration of the
as therising or falling quadrants. It is trivial toprove thiswitha beamgainamplitudeswould likelyremovetheoffsetweidentify
simpleMonteCarlosimulation. here.
Thebroadeningofthedistributioninthex-directionislikely
duetothedecreasedeffectivedepthofthefirstepoch.Thereisalso
4.4 Astrometry
an offset from the zero line in the y-direction, suggesting that a
∼2–3%offsetinthemeasuredvaluesispresentforthefinalepoch. Theuncertaintyinthepositionofaradiosourceasmeasuredfrom
ExaminationofthecalibratedspectraofPKSB1934−638forthese aninterferometricimagegenerallyhastwocomponents.Theseare
twoepochsshowsnosuchoffset,suggestingthatthisiseitherdue a statistical component that is related to the signal to noise ratio
toasystem-widegaindriftassociatedwithSB1231thatthephase- ofthedetectionandtheangular resolution oftheinstrument,and
onlyself-calibrationdoesnotcorrect,oranimage-planeeffect.Ele- asystematiccomponentthatisassociatedwitherrorsintheastro-
mentgaindriftswillcausederivedbeamformerweightstobecome metric reference frame that are generally calibration related. The
(cid:13)c 0000RAS,MNRAS000,000–000