Performance and calibration of the NIKA camera at the IRAM 30 m telescope

15 Pages·2014·4.82 MB·English

A&A569,A9(2014) Astronomy
DOI:10.1051/0004-6361/201423557 &
(cid:2)c ESO2014 Astrophysics
Performance and calibration of the NIKA camera
at the IRAM 30 m telescope
A.Catalano1,M.Calvo2,N.Ponthieu3,R.Adam1,A.Adane4,P.Ade5,P.André6,A.Beelen7,B.Belier8,A.Benoît2,
A.Bideaud5,N.Billot9,N.Boudou2,O.Bourrion1,G.Coiffard4,B.Comis1,A.D’Addabbo2,11,F.-X.Désert3,
S.Doyle5,J.Goupy2,C.Kramer9,S.Leclercq4,J.F.Macías-Pérez1,J.Martino7,P.Mauskopf5,12,F.Mayet1,
A.Monfardini2,F.Pajot7,E.Pascale5,L.Perotto1,V.Revéret6,L.Rodriguez6,G.Savini10,K.Schuster4,
A.Sievers9,C.Tucker5,andR.Zylka4
1 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble Alpes, CNRS/IN2P3, 53 rue des Martyrs,
38026Grenoble,France
2 InstitutNéel,CNRSandUniversitédeGrenoble,38042Grenoble,France
3 InstitutdePlanétologieetd’AstrophysiquedeGrenoble(IPAG),CNRSandUniversitédeGrenoble,38041Grenoble,France
4 InstitutdeRadioAstronomieMillimétrique(IRAM),38406Grenoble,France
5 AstronomyInstrumentationGroup,UniversityofCardiff,UK
6 LaboratoireAIM,CEA/IRFU,CNRS/INSU,UniversitéParisDiderot,CEA-Saclay,91191Gif-Sur-Yvette,France
7 Institutd’AstrophysiqueSpatiale(IAS),CNRSandUniversitéParisSud,91405Orsay,CedexFrance
8 Institutd’ÉlectroniqueFondamentale(IEF),UniversitéParisSud,91405Orsay,France
9 InstitutdeRadioAstronomieMillimétrique(IRAM),18012Granada,Spain
10 UniversityCollegeLondon,DepartmentofPhysicsandAstronomy,GowerStreet,LondonWC1E6BT,UK
11 UniversitàSapienzadiRoma,00185RomaItaly
12 SESEandPhysics,ArizonaStateUniversity,Tempe,AZ,USA
Received2February2014/Accepted20June2014
ABSTRACT
TheNewIRAMKIDArray(NIKA)instrument isadual-band imaging cameraoperating withkineticinductance detectors (KID)
cooledat100mK.NIKAisdesignedtoobservetheskyatwavelengthsof1.25and2.14mmfromtheIRAM30mtelescopeatPico
Veletawithanestimatedresolutionof13arcsecand18arcsec,respectively.ThisworkpresentstheperformanceoftheNIKAcamera
priortoitsopeningtotheastrophysicalcommunityasanIRAMcommon-user facilityinearly2014. NIKAisatestbenchforthe
finalNIKA2instrumenttobeinstalledattheendof2015.ThelastNIKAobservationcampaignsonNovember2012andJune2013
havebeenusedtoevaluatethisperformanceandtoimprovethecontrolofsystematiceffects.Wediscussherethedynamicaltuningof
thereadoutelectronicstooptimizetheKIDworkingpointwithrespecttobackgroundchangesandthenewtechniqueofatmospheric
absorptioncorrection.Thesemodificationssignificantlyimprovetheoveralllinearity,sensitivity,andabsolutecalibrationperformance
ofNIKA.Thisisprovedonobservationsofpoint-likesourcesforwhichweobtainabestsensitivity(averagedoverallvaliddetectors)
of40and14mJys1/2foroptimalweatherconditionsforthe1.25and2.14mmarrays,respectively.NIKAobservationsofwellknown
extendedsources(DR21complexandtheHorseheadnebula)arepresented.ThisperformancemakestheNIKAcameraacompetitive
astrophysicalinstrument.
Keywords.methods:observational–submillimeter:general–submillimeter:galaxies–submillimeter:ISM
1. Introduction Bourrionetal.(2012a);Swensonetal.(2010)andarerelatively
simple to fabricate (see for example Day et al. 2003; Doyle
New challenges in millimeter-wave astronomy require instru- et al. 2007). This technological solution has been selected for
mentswithacombinationofhighsensitivity,angularresolution, the NIKA project (Monfardiniet al. 2010, 2011) consisting of
and mappingspeed. These goals demandthe developmentof a a dual-band millimeter camera for observations at the 30 m
new generation of arrays with large number of detectors (see Institut de RadioAstronomie Millimétrique (IRAM) telescope
for example Bintley et al. 2010;Staguhn et al. 2008; Niemack (PicoVeleta,Spain).
et al. 2008; Güsten et al. 2008). Current individual detectors, The NIKA camera is a 356-pixel instrument conceived as
such as high impedance bolometers (Tauber et al. 2012), are a resident common user instrument and an early technology
already photon-noise-limited both for space and the ground demonstratorof thecompetitivenessofKID arrays.NIKA also
observations. serves as preparation for NIKA2, a full-scale instrument to be
One of the possible technologies suitable for scaling up deployedinlate2015anddesignedtofillthefieldofview(FOV)
to larger format arrays is the kinetic inductance detector oftheIRAM30mtelescope.ThegoalsoftheNIKAinstrument
(KID). These detectors are designed to be read out with aretoperformsimultaneousobservationsintwobands(1.25mm
frequency-division multiplexing with a large multiplex factor and2.14mm)ofmilliJanskypointsourcesandtomapfaintex-
from a few hundred to several thousand per readout chain tended continuum emission with diffraction-limited resolution
ArticlepublishedbyEDPSciences A9,page1of15
A&A569,A9(2014)
and background-limited performance. Achieving these goals
will enable the NIKA instrument to be competitive in sev-
eralastrophysicalfields,suchasobservationsfromthenorthern
hemisphere of clusters of galaxies detected by Planck via the
Sunyaev-Zel’dovich(SZ)effect(PlanckandAMICollaboration
Int.II2013),observationsofhighredshiftgalaxiesandquasars,
detection of early stages of star formation in molecular clouds
in our galaxy and mapping of dust and free-free emission in
nearbygalaxies.Ina companionpaper,we presentthefirst ob-
servationofthethermalSZeffectonclustersofgalaxies(Adam
et al. 2014). The NIKA camera is now open to the public for
observationsasofJanuary2014.
Previousobservingcampaignswith theNIKA camerahave
revealedseveraltechnicalaspectsthatlimited the sensitivity of
detectors. Calvo et al. (2013) show how to linearize the KID
signal, while another important issue is the working point of
the detectors that evolves with time, owing to varying atmo-
sphericconditions,thusproducingalossinresponse.Adynam-
icaltuningofthereadoutelectronicswasdevelopedtooptimize
theKIDworkingpointbetweentwodifferentskyobservations.
Atmospheric absorptioncorrection is also required, and a ded-
icated procedurehas been developedin order to use the NIKA
instrumentasatau-meter.
Theseimprovementswereimplementedforthelasttwotech-
Fig.1.SnapshotfromtheZemaxsimulationusedtooptimizetheoptical
nical observing campaigns that took place in November 2012
systemofNIKA.Intheorderofphotontravel,theNIKAopticsconsist
and June 2013.We reporthere the overalllinearity, sensitivity, ofaflatmirroratthetopofthecryostat,anoff-axisbiconic-polynomial
andabsolutecalibrationofNIKA. curvedmirror,a300Kwindowlens,afieldstop,a4Klens,anaperture
This article is structured as follows: a review of the main stop,adichroic,a100mKlens,andtwoband-definingfiltersinfront
characteristics of the NIKA instrument that focuses on the in- of the back-illuminated KID arrays. Top panel: ray tracing from the
strumentalimprovementswithrespecttothepreviousobserving entranceofthereceivercabinofthe30mtelescope(whichissimulated
campaigns, is presented in Sects. 2 and 3. In Sect. 4, we de- but not shown in the image), view from the elevation axis (which is
symbolizedbythebigcirclesandcontainsthe2mirrorsoftheNasmyth
scribetheskyobservationsmadewithNIKAthatallowedusto
system).TheNIKAcryostatandentrancenosearerepresentedbythe
characterizethefocalplane.Section5providesashortdescrip-
rectangles.Bottompanel:zoomedplotoftheopticalsysteminsidethe
tionofthereductionpipelineusedtoanalysethedata.InSect.6 NIKA cryostat. The angular size, the beam efficiency and the overall
we describe the atmospheric absorption correction method. In opticalefficiencyofthesystemarepresentedinTable1.
Sect.7,thefinalperformanceofNIKAintermsofinstrumental
noiseequivalentfluxdensity(NEFD)iscalculatedonpoint-like
Table1.CharacteristicsoftheNIKAinstrumentcoupledtotheIRAM
sources. Finally in Sect. 8 we detail the observations of point
telescope.
sourcesandextendedsourcesofthelastNovember2012(run5)
andJune2013(run6)campaigns.
1.25mmchannel 2.14mmchannel
Pixelsize[mm] 1.6 2.3
Dualpolarization yes yes
2. TheNIKAinstrument
Angularsize(Fλ) 0.9 0.79
Cooling the two KID arrays at about 100 mK is the major Beamefficiency[%] 55 70
requirement for driving the architecture of the NIKA instru- Detectorefficiency[%] 80 80
ment. This is achieved by a 4 K cryocoolerand a closed-cycle Overallopticalefficiency[%] 30 30
3He-4He dilution. The optical coupling between the telescope Totalbackground[pW] 45 20
andthe detectorsis madebywarm aluminummirrorsandcold
Notes.Thetotalbackgroundiscalculatedperpixelwithacontribution
refractive optics. The optics consists of a flat mirror at the top
oftheatmospherederivedingoodweatherconditions.Thiscorr√esponds
ofthecryostat,anoff-axisbiconic-polynomialcurvedmirror,a toanexpectedphoton-noise levelequal√toabout5×10−17W/ Hzfor
300Kwindowlens,afieldstop,a4Klens,anaperturestop,a the2.14 mmchannel and 9×10−17W/ Hzfor the1.25 mmchannel.
dichroic, a 100 mK lens, and two band-definingfilters in front Theseparametershavea50%precisionlevel.
oftheback-illuminatedKIDarrays,whichhaveabackshortthat
matchesthecorrespondingwavelength.Aviewoftheopticsof
theNIKAinstrumentandthecouplingwiththe30mtelescope AforArea,Ωforsolidangle,uforpixelsizeinunitofFλ(λ=
is presented in Fig. 1. All the elements presented in this figure wavelengthandF = f-numberorrelativeaperture),wehave:
are real opticalelements exceptfor the verticalsegmentshow-
ingtheentranceofthereceivercabinandtheverticalsegments AΩ = AM1(uFλ)2 =(π/4)(uλ)2.
inside the cryostat(field and pupildiaphragms,filters, dichroic pixel f2
eq
anddetectorarrays).
Thethroughputofeachpixeliscalculatedusingtheknowl- where A istheeffectiveprimarymirrordiameter,and f the
M1 eq
edgeofthetelescopediameterandequivalentfocus,thefraction equivalent focal of the telescope. The effective throughput of
of unvignettedpupil defined by the cold field stop, and the re- the instrumentis then AΩeffective = N ∗AΩpixel, where N is the
ceiverpixel(KID)sizewithrespecttodiffractionpattern.Using numberofvalidreceiverpixels.
A9,page2of15
A.Catalanoetal.:PerformanceandcalibrationoftheNIKAcameraattheIRAM30mtelescope
ThegapoftheAlfilmshasbeenmeasuredthankstoabsorption
spectra taken in the lab using a Martin-Puppletinterferometer.
Themeasurementsshowthatbelowroughly110GHz,noradi-
ation is absorbed, meaning that the energy gap in the case of
18 nm Al films is approximately 0.2 meV, and the Tc around
1.45 K. The expected film impedance (with respect to the in-
coming photon able to break Cooper pairs) is actually that of
the normal film state. This has been measured to be around
2Ω/square.Thisvaluehasbeenusedwhendesigningthedetec-
tors to match their effective impedanceto that of the incoming
radiation(whichisabsorbedfromthesubstrateside,ina back-
illuminationconfiguration).
Theresultsreportedinthispapercovertwodifferentobserv-
ingcampaignsatthetelescope,withslightlydifferentproperties
ofthetwofocalplanes:
Fig.2.NIKAbandpasses.Thebandpassofthe1.25mmchannel(resp. – Campaign 11/2012. During this campaign, referred to as
2.14 mm) is shown in red (resp. blue). The bandpasses are averaged Run 5, the 2.14 mm array is made of 132 pixels with a
overallvalidpixels,withdispersion(rms)of2%at1.25mmand1%at 20nmthickAlfilmona 300μm HRsiliconsubstrate.The
2.14mm.TheATMmodelcalculatedfordifferentwatervaporcontents 1.25mmarrayconsistsof224pixelswithafilmofthesame
ispresentedingreen. thickness but on a 180 μm substrate. The cold amplifier of
the 240 GHz channel showed an unexpectedly low satura-
tionpower,sothatwecouldsimultaneouslyreadonlyeight
Therejectionofout-of-bandemissionfromtheskyandthe pixels out with the ideal excitation level or, alternatively,
telescopeis achievedby usinga seriesof low-passmetalmesh 90pixelsbutwithalowerpowerpertone,resultinginsub-
filters, placed at different cryogenic stages in order to mini- optimalperformance.The numberof valid pixelswere 100
mize the thermalloading on the detectors. This determinesthe and80forthe2.14mmand1.25mmchannels,respectively.
shape,width,andpositionofeachoftheNIKAbands.Spectral The configuration adopted during this observing campaign
characterization of the NIKA bandpass was performed using limitedthefinalsensitivityoftheinstrument.Thechannelat
a Martin-Puplett interferometer allowing recovery of the spec- 2.14 mm (changed for the 2013 campaign) was limited by
tral performanceof each pixel of the two NIKA channels with the detector noise. For the 1.25 mm channel, the detectors
uncertenties of a few percent. In Fig. 2 we present the NIKA noise was always relativelyflat. This meansthat the intrin-
bandpasses,togetherwiththeATMmodelcalculatedfordiffer- sicfrequencynoiseduetorandomvariationsoftheeffective
ent water vapor contents (Pardo et al. 2002). According to the dielectric isnegligiblecomparedto the sky noise.Thecold
Pardo model, the 1.25 mm channel is almost sensitive exclu- amplifiertemperaturenoisewasalsonegligiblecomparedto
sivelytothewatervaporemission(watervaporsecondarylineat theskynoise.
183GHz)incontrasttothe2.14mmchannel,whichisslightly – Campaign 06/2013.This campaign is referred to as Run 6.
sensitive to the roto-vibrational emission line of dioxygen (at Forthe2.14mmchannel,wereplacedthearraywithanew
119GHz).ATMalsopredictsthecontributionsofminorspecies one, obtainedfrom a thinnerAl film (18 nm), in which we
likeozonetotheatmosphericabsorption.Theseadditionalcon- optimizedthecouplingfurthertothefeedline.Wealsoadded
tributions are ignored in this paper. Changing the precipitable bondings across the feedline to suppress spurious slotline
water vapor (pwv) content in the ATM model, we can derive modesthat were affecting the uniformityof the pixelprop-
the expected sky opacities integrated into the NIKA channels. erties,whichledtoanincreaseinthenumberofvalidpixels
The ratio between the opacities derived for the 1.25 mm and (Fig. 3). The overall geometry and the pitch between pix-
2.14mmchannelsisobtainednotonlyaccordingtothepseudo els was left unchanged, as was the readout chain. For the
continuumemissionoftheatmosphere(proportionaltoν2where 1.25 mm channel, the only intervention was to replace the
ν representsthe electromagneticfrequency)but also taking the cold amplifier with a new one having a higher power han-
contributionofthewatervaporemissionanddioxygenemission dling. RF filters have also beenadded on the readoutchain
intoaccount.Furthermore,betweenthe2012and2013observ- toavoidtheharmonicsofthelocaloscillatorfromreaching
ing campaignswe changed(fortechnicalreasons)the cold op- thecoldamplifierinput.Sincewesuspectedthattheoptical
tical filter chain by adding a low pass filter with a cut-off at load on the 1.25 mm array was preventing it from cooling
270 GHz. Therefore, we expect to have a different ratio in the down appropriately, we added an additional low-pass edge
derivedskyopacitiesat1.25mmand2.14mmchannelsbetween filter cutting frequencies above 270 GHz, even though this
thetwocampaigns.Fromthemodel,weobtainaratiobetween ledtothelossofafractionofthepoweravailableintheat-
the in-band sky opacities τ(2.14 mm)/τ(1.25 mm) of 0.75 and mosphericwindowofinterest.Thenumberofvalidpixelsfor
0.6forthe2012and2013observingcampaigns,respectively. thiscampaignwas125at2.14mmand190at1.25mm,for
atotalof315pixels.Thisnewconfigurationwasexpectedto
yield a muchimprovedlow noise performance,butthe bad
2.1.Detectorarrayconfigurations weatherduringtheobservingcampaignlimitedthesensitiv-
ityofourarrays.Forbothchannelsthedominantnoisecon-
The NIKA LEKID detector arrays are made of aluminum and
tribution was due to residual sky noise associated to atmo-
are sensitive to dual polarization. The need to have a symmet-
sphericturbulencesandresidualcorrelatedelectronicnoise.
ric design with a constant filling factor over the whole direct
sensitiveareadrivesustouseHilbertfractaldesign,whichisa Forthereadout,weusedthenewNIKELversion1electronics,
well-known geometry for patch antennas (Roesch et al. 2012). whichhaveworkedflawlessly.Theboards,describedindetailin
A9,page3of15
A&A569,A9(2014)
I(t)andQ(t)representsachallenge,andthisisespeciallytruein
thecaseofground-basedexperiments,asthesehavetocopewith
theeffectsinducedbythevariationsinatmosphericopacity.To
improve the photometric reproducibility, we have developed a
measurementprocesswhichisdescribedinthenextsubsection.
3.1.Modulatedreadout
For the NIKA detectors an innovative readout technique has
been developed, which has been succesfully tested during the
2011 run and adopted for all the following campaigns. The
Effect of bondings across the feedline details of this technique are described in Calvo et al. (2013).
Briefly, the underlying idea is to replace the standard excita-
tion of the detectors, which uses a fixed tone, with a new ex-
−38 citation based on two differentfrequencies.We achievethis by
m) modulatingthe localoscillator signalbetweentwo values,sep-
B
g(S) (d21−40 a(rf+ate=d bfy0 +δfLδOfL,Oin/2o)radnerdtoongeejnuesrtatbeeltowwo (tof−nes=, ofn0e−juδsftLaObo/v2e)
Ma thedetectorresonantfrequency.Themodulationiscarriedoutat
about1kHz,synchronouslytotheFPGAsamplingofthesignal.
−42 With bondings Thus,eachrawdatapoint,whichissenttotheacquisitionsoft-
Without bondings
wareatarateof23.842Hz,iscomposedofthevalues(I(t),Q(t))

1.320 F1r.3e2q5uency (GHz1).330 1.335 ofeachpixel,aswellasthecorrespondingdifferentialvalues
(cid:2) (cid:3) (cid:2) (cid:3)
Fbeigs.e3e.nIminatgheeofrfeqthueenbcoyndswinegespacdadreridedacoruotssbetfhoerefe(ebdlulien)ea(ntdopa)f.teArs(rceadn) dI(t),dQ(t) = I(f+)−I(f−), Q(f+)−Q(f−) · (1)
addingthebondings(bottom),thedepthofthedifferentresonanceshas df df δfLO δfLO
becomemuchmoreuniform,sincethecouplingoftheresonatorstothe
feedline isno longer affectedby the presence of standing waves sup- If a variation (ΔI(t),ΔQ(t)) is observed between successive
portedbytheslotlinemodes.Suchstandingwaveswerealsoresponsi- points, it is possible to estimate the corresponding shift in the
bleofthelargerdipsthatareobservedbeforeaddingthebondings,and resonantfrequency,Δf0(t),byprojecting(ΔI(t),ΔQ(t))alongthe
theydisappearafterwards. gradientfoundusingEq.(1):
(ΔI(t),ΔQ(t))·(dI/df(t),dQ/df(t))
Δfˆ(t)= ·δf · (2)
Bourrionetal.(2012b),arecapableofgeneratingupto400tones 0 (dI/df(t),dQ/df(t))2 LO
eachovera500MHzbandwidth.Thisisachievedbyusingsix
separate FPGAs: five of them generate 80 tones each over a WeusethenameRFdIdQtorefertothisestimateofΔf .
0
100 MHz band, using five associated DACs. The sixth FPGA
actsasacentralunitthatcombinesthesignaloftheotherunits,
apprioprately shifting and filtering the different 100 MHz sub- 3.2.Automatedtuningprocedure
bandstofinallycoverthewhole500MHzavailableforthefre- Themodulatedreadouttechniqueisthecoreofafastandeffec-
quencycombusedtoexcitethedetectors.Ananalogous,butre-
tivemethodofretuningthe detectorsthatwe havesuccessfully
versed,processisthenappliedtothesignalacquiredbytheADC
implemented during the last two campaigns. When operating
oftheboard,whichisonceagainsplitinfivedifferentsub-bands
from ground, the variations in the background load due to the
treatedseparately.EachNIKELv1boardthusallowsustomon-
atmospherecancausetheresonantfrequencyofthedetectorsto
itorallthe400tonessimultaneouslywithamarginoftwobitsin
vary by a substantial amount, introducing shifts that can be in
the12bitsADCdynamics. some cases greater than the resonancesthemselves. This effect
Data are acquired at a 23.842 Hz rate, synchronously over
mustbeconstantlymonitoredand,ifneeded,counterbalancedby
thetwoarrays.Binaryfilesareprovidedasoutput,whichcontain
changingtheexcitationtones,inordertoalwaysmatchthereso-
therawdata,aswellastheresonantfrequencyofeachKIDused
nantfrequencyofeachpixel,thuskeepingthedetectorsnearto
tosetthecorrespondingexcitationtone.
theiridealworkingpoint.
The principal characteristics of the NIKA instrument are
The standard solution is that of performing full frequency
presentedinTable1.
sweepsbeforeeachon-skyobservation,leadingtoasignificant
lossofobservingtime. Thenewtuningmethodisbasedonthe
measurement of the angle ϕ between the vectors (I, Q) and
3. Readoutoptimizationprocedure
(dI/df ,dQ/df ),asshowninFig.4.Thus,asingledatapoint
LO LO
In the standard KID readout scheme, each pixel is excited us- isnowsufficienttoretunethedetectorswithoutrecurringtofre-
ingafixedtoneatitsresonantfrequency.Thesignaltransmitted quencysweeps.Thisleadstoacrucialadvantageintermsofob-
past the detector is compared to a reference copy of the exci- servingtime,especiallyinthecaseofmediumandpoorweather
tation signal to get its in-phase (I) and quadrature (Q) compo- conditions.Thenewprocedurereducestherequiredtimeforre-
nents.Fromtheseitisthennecessarytoestimatethecorrespond- tuningby 75%. Furthermore,in the case of altazimuthalmaps,
ingshiftinresonancefrequency f ofthedetector,becausethis whicharealwayscomposedofdifferentsubscans,thefast-tuning
0
is the physical property directly related to the incoming opti- methodmakesit,inprinciple,possibletorecenterthetonesdur-
cal power:δf ∝ δP for low valuesof δP (Swenson et al. ing the time spent by the telescope for changing the direction
0 opt opt
(2010)). Finding a reliable way to evaluate f (t) starting from ofitsmotionattheendofeachsubscan.Althoughwestillhave
0
A9,page4of15
A.Catalanoetal.:PerformanceandcalibrationoftheNIKAcameraattheIRAM30mtelescope
1 3
Low optical load
2 High optical load
1
d)
Q ϑ (ra −01
d
e
z0.5 ϕ −2
ali
Norm (dI, d 1−.349 95 1.5000 1.5005
Q) Frequency (GHz)
Fig.5. Representation of the tuning technique based on the measure-
(I, Q) ϑ = π/2 − ϕ mnaennctesofwϑhe.rAeϑsw=ee0p.T(shoelitdonleinies)fiisxecdarartiethdeocuotrrteosplooonkdinfogrfrtheqeureenscoy-
0 (diamond) and the slope evaluated (dashed line). If the optical load
increases, ϑ changes allowing the appropriate correction Δfi (black
0 0.5 1
Normalized I arrow)tobeevaluated.Thenewtone(circle)willbenearertotheac-
tualpositionoftheresonant frequency, fi,andwillbeusedtoupdate
Fig.4.RepresentationintheI-Qplaneofasweeparoundaresonance. theestimatedslopeoftheϑ(f)curve,thus0makingtheiterativetunings
Themeasureoftheanglesϕandϑcanbecarriedoutforeachacquired increasinglymoreaccurate.
point thanks to the modulated readout technique. On resonance, one
getsϑ=0.
conditions,evenverylongscanscanbecarriedoutwithouthav-
ingtochangethetones.Thisautomatictuningprocesswillallow
notmadeuseofthisapproach,itdoesnotposeanyfundamental
problem,andmightprovehighlyeffectiveespeciallyinthecase astronomerstoobservethroughoutthefuturecampaignswithno
interventionsofthesupportingNIKAteam.
of large maps and long integration,in which case the sky con-
ditionscanchangesignificantlybetweenthestartandtheendof
anobservation. 4. Field-of-view(FOV)propertiesandmainbeam
Thetuningprocesstakesplaceasfollows:fromtheangleϕ
characterization
we calculate ϑ = π/2 − ϕ, so that the new angle ϑ varies
smoothly,decreasingacrosseachresonancefromπto−π.After ThefrequencymultiplexingofNIKApreventsusfromknowing
an initial frequency scan has been performed to find the reso- a priorithe pointingdirectionof eachdetectoronthe sky.This
nances, we fix each excitation tone where ϑ = 0. At the same hastobedeterminedonastronomicalobservations.Wetherefore
time,theslopeofthecurveϑ(f),whichisapproximatelylinear scanastrongastromicalsourcewiththeentirefocalplane.The
aroundthe resonance,isdeterminedas Δϑ/Δf. Oncethe tones calibration source, typically a planet (Mars, Uranus, Neptune),
are fixed, for each tone at frequency fi it is possible to contin- issmallcomparedtoourbeamandcanbeconsideredasapoint
uously monitor the value of ϑi. If the corresponding resonant source.Indeed,duringthe2012observingcampaign,theangular
frequency fi shifts,owingtochangesintheopticalload,thisdi- diameterofUranuswas3.54arcsec.Theconvolutionofthecor-
0
rectlytranslatesintoavariationinϑi,fromwhichitispossible respondingdisk with a Gaussian beam of 13.5 and 18.4arcsec
toestimatetheactualvalueof fi(t)as FWHMwouldbroadenourbeambyonly0.17arcsecat1.25mm
0
and0.12at2.14mm.
ϑi(t) Thesourceisraster-scannedat35arcsec/s,andeachsubscan
fi(t)(cid:6) fi− · (3)
0 Δϑ/Δf is 420arcseclong,centeredonthe sourceasthe latter is being
trackedbythetelescope.Thisscanningspeedprovidesabout10
Although this relationship is not exact, mainly because the and12pointsperFWHMat1.25mmand2.14mmresp.,hence
changesin the optical load affect the ϑ(f) relationship and the providingexcellentbeamsampling.Tohaveacleandetermina-
corresponding value of Δϑ/Δf, the results are very accurate, tionofthebeamparameters(position,width,andorientation)of
provided that the shift in the resonance frequency does not eachKID,weproceedintwosteps.
exceedtheresonancewidth.
Theresultscanbefurtherimprovedbyiteratingtheprocess. 1. Weapplyamedianfilterofapproximately5FWHMofwidth
For this reason every time that the telescope is not observing, tothedetectortimelinestosubtractmostoftheatmospheric
we activate an automatedtuning procedure.This procedurere- signal and the low-frequency, correlated electronic noise
peatedlyestimatesthecurrentvalueof fi(t)foreachdetectorus- while preserving most of the planet signal (less than 1%
0
ingEq.(3),adjuststhecorrespondingtoneaccordingly(Fig.5), lost at scales smaller than 2.5 × FWHM). These timelines
andupdatesthecoefficientΔϑ/Δf bymeasuringthevalueofϑi are then projected onto individual maps per detector, and
just before and just after changing the frequency fi. Thus tak- a Gaussian ellipse is fitted on the source.Centroid position
ingthechangesintheslopeofϑ(f)intoaccount,theexcitation providesa first estimate of the pointingparametersof each
tone fi rapidlyconvergetothecorrectresonantfrequency fi(t). pixel. The amplitude of the Gaussian gives a calibration in
0
Theprocedureisthenhaltedassoonasanewobservationstarts. Jy/Hz, where Hz represents the shift in resonant frequency
In preparationfor the NIKA open scientific pools, we will add foreachdetector.Iftheatmosphericabsorptionisknownand
an automaticcheckof the distanceof eachtone fromits corre- accountedfor,thisbecomesadeterminationoftheabsolute
spondingresonance.Ifthisdistanceincreasestoomuch(some- calibrationofeachdetector.Ifnot,thiscanstillbeconsidered
thingthatwouldleadtoincorrectdata),thetoneisautomatically as a cross-calibration of KIDs all at once, and they can be
recenteredontheresonance.Theprocessisthusnotcontinuous combinedaccordinglyintoasinglemapofthesource.This
but only carried out in case of need. Under good sky weather mapprovidesthepositionofthesourceinskycoordinates.
A9,page5of15
A&A569,A9(2014)
Fig.7.BeamFWHMdistributionat1.25(blue)and2.14(red)mmfor
the2012(left)and2013(right)observingcampaigns.Thedistribution
ofFWHMswasoflowerqualityduringthe2013observingcampaign
owingtopoorweatherconditions.
feed line and the pixel are recovered. The variation in reso-
nancefrequencyforeachKIDinthearrayisobtainedbyapply-
ingtheoptimizationprocedure,RFdIdQ,discussedinSect.3.1.
We developed a dedicated reduction pipeline to calibrate, fil-
ter, and process data onto sky maps. The main steps of the
processingare:
– Read raw: raw data and the main instrumental parameters
Fig.6. Detector sampling of the FOV at 1.25mm (left)and 2.14 mm
including FOV reconstruction and atmospheric opacity for
(right) for the 2012 (top) and 2013 (bottom) observing√campaigns.
Beampatterncontourshaveadiameterof1σ=FWHM/ 8ln2. each observational scan. The data are ordered per kid and
regularlysampledwithtime.Thisiscalledtimeorderedin-
formation(TOI).
2. Withthisinformationinhand,wecanflagoutthesourcein – Flags: bad detectors are flagged based on the frequency
allKIDtimelinesandbuildatemplateofthelowfrequency rangenotbeinglargeenoughtohavealowcross-talklevel.
part of the signal (mostly sky noise and electronics noise) Noisy or badly systematic, affected detectors are flagged
using,ateachtime,alldetectorsthatarefarfromthesource depending on the statistical properties of their noise (such
(typically furtherthan a few beam FWHM, e.g. 30 arcsec). as Gaussianity, stationarity, noise jumps). Saturated and
This template is then subtracted from each KID timeline. off-toneKIDsarealsoflaggedout.
Thisleadstoacleandeterminationoftheplanetsignalwith – Filtering: frequency lines produced by the vibration of the
nofiltering.Timelinesarethenprojectedasecondtimeonto cryostatpulsetubeareflaggedandremovedwithdedicated
individual maps per KID, and a Gaussian elliptical fit pro- Fourierspacefiltering.
videsthefinalpointingparametersandFWHMestimates. – Cosmic rays: we detect glitches in the raw data. Since
1.25 mm and 2.14 mm KID arrays are different (in terms
For illustration, Figs. 6 and 7 show maps of the 1.25 mm and ofsizeofthepixelsandthicknessofthesubstrate),therate
2.14 mm detector images and the distribution of the deter- ofobservedglitchesvariesbetweenthem.Weobserveabout
minedbeamwidthsforthe2012and2013observingcampaigns. sixglitches/mininthe2.14mmarrayandaboutfourforthe
Several scans on calibration sources are performed during the 1.25mmarray.Theobservedglitchrateisingoodagreement
campaigntoimproveonthereconstructionofthepointing.For with the expected cosmic ray flux at the IRAM telescope,
the 2012 observing campaign, with typically six such scans, whichisessentiallycomposedbymuonswitharateofabout
we observed a median variation of the detector pointing direc- 2events/cm2/minRameshetal.(2012).Thetimeresponse
tions of 3.4 arcsec at 1.25 mm and 3.2 at 2.14 mm. Although oftheKIDsishundredsofμs,whichisnegligiblecompared
beams were observedto be a bit larger and asymmetric during to the NIKA acquisition rate, thereforeeach cosmic ray hit
2013 observing campaign because of the weather conditions1, onlyaffectsonlyasinglesample.Glitchesareremovedfrom
the number of valid pixels increased and the FOV was almost the TOIbyflaggingpeaksat5σlevel.Thenthe TOIislin-
fully sampled. For the 2012 observing campaign, the average early interpolated in order not to perturb the decorrelation
FWHMwere12.3±0.2arcsecat1.25mmand18.1±0.3arcsecat method.Flaggedsamplesarenotprojectedontothesky.
2.14mm.Forthe2013observingcampaign,theywere13.5±0.9 – Calibration of the TOI: the absolute calibration is applied
at 1.25 mm and 18.4± 0.8 arcsec at 2.14 mm. The combined to these TOIs and an atmospheric absorption correction
beamislargerthanindividualdetectorbeamsbecauseofapossi- performed(seeSect.6.2).
blesmalloffsetinthereconstructeddetectorpositionintheFOV. – Atmospheric and electronic noise decorrelation:depending
onthescientifictarget,twobasicdecorrelationmethodshave
beentested:
5. Datareduction
1) Dual-band decorrelation: spectral decorrelation is per-
As discussed in Sect. 3.1, the NIKA data are acquired at formed to recover the diffuse thermal SZ effect (see Adam
23.842 Hz, and each pixel is fed a single tone. For each sam- etal.2014formoredetail)inclustersofgalaxies.Wecapital-
ple andforeachKID, thein-phase(I),the quadrature(Q),and izeonthesignificantdifferencebetweenthethermalSZas-
their derivatives (dI and dQ) of the transfer function of the sociatedsignalandthetwofrequencychannels.Inparticular,
at240GHzthethermalSZemissionisnegligibletofirstor-
1 Underturbulentsky,theanomalousrefractionislargerandspreads der,andwecanusethischanneltoobtainatemplateofthe
theincominglight,henceenlargingtheimage. atmosphericemission.
A9,page6of15
A.Catalanoetal.:PerformanceandcalibrationoftheNIKAcameraattheIRAM30mtelescope
Table2.DifferentcontributionstothetotalcalibrationerroroftheNIKAdata.
Systematics 1.25mmChannelerror 2.14mmChannelerror
Secondarybeamsfraction∗(cutsat30(cid:7)(cid:7),60(cid:7)(cid:7),90(cid:7)(cid:7)) 25%,41%,43% 9%,30%,33%
Primarycalibrator 5% 5%
Elevationdependantgaincorrection∗∗ 20% 10%
Spectralresponse 2% 1%
FOVreconstruction 3.4arcsec 3.2arcsec
Opacitycorrection 5% 6.5%
Datareductionfiltering(onpointsources) 5% 5%
Overallcalibration 15% 10%
Notes. (∗) Is estimated by measuring the main beam volume (2πσ2) over the integral of the beam volume up to a considered angular radius.
(∗∗)Typicallyfor1.25mmthegaincorrectionisabout8%atanelevationof30deg(3%at2.14mm).
2) Single-band decorrelation: a sky noise TOI template is isdeducedfromMoreno(2010);PlanckCollaborationVIII
producedbyaveragingallTOIsofasinglearray.Thistem- (2014) using a frequency-dependent model of the planet
plateisscaledforeachdetectorandsubtractedfromindivid- brightnesstemperatureandintegratingovertheNIKAband-
ualTOIs.Variationsonthismethodhavebeentested.They passes. We obtained brightness temperatures of 113 K at
involvemaskingthepointsourceifbrightenough,ormask- 2.14 mm and 94 K at 1.25 mm. This model is accurate at
inganextendedsourceifdetectedinapreviousiterationof thelevelof5%.
the map-makingprocessorfilteringtheskynoise template. – Elevation dependentgain: the IRAM 30 m telescope has a
Inparticular,weconsideraniterativeprocedure.Afirstsky gain that changes with the elevation. This is because the
map is constructed as discussed above. From this map we antenna was not designed in a complete homology way.
construct a mask of the source in the TOI. We construct a According to the design and measurements (Greve et al.
new TOI sky noise template, avoiding samples within the 1998; Kramer et al. 2013), the peak of the elevation gain
mask.Onlyafewiterationsareneededtoconverge. curve is obtained for an elevation of 43 degrees. Elevation
So far,the electronicnoisehasnotbeendealtwith.In gen- gainvariationshavebeencorrectedforinthedataanalysis.
eral, the cross-talk can be due either to resonator coupling – Spectralresponse:thebandpassesasdescribedinSect.2are
ortoelectronicscross-talk.Theresidualestimatedcross-talk obtained as an average of all pixels for each channel. This
in the data is about 2%. Decorrelation process reduces, or leadstoacontributiontothetotalcalibrationerrorduetothe
in some case completely removes, the cross-talk. This can dispersion of the measured bandpasses and to the slightly
limit the sensitivity of the final sky maps. We are currently differentresponseofthepixels.
investigatinghowtocorrectforthisbyrelyingontheuseof – FOVreconstruction:torecoverthepointingdirectionofeach
measurementsatfrequenciesthatareoff-resonance. pixel, a focal plane reconstructionvia planet scans is used.
TheaccuracyofthistechniquehasbeendescribedinSect.4.
– mapmaking:weprojectandaveragetheTOIsfromallKIDs – Secondarybeam fraction:planetobservationswere used to
of a same frequency band on a pixelized map. We do not measure individual pixel primary beams as well as for the
project flagged data, and each detector sample is weighted FOV reconstruction. The beam width is obtained by fit-
bytheinversevarianceofthedetectortimeline.Thelatteris ting a Gaussian on the planetmaps. We haveobservedthat
estimated far fromthe source.This weightingscheme does variations in the atmospheric conditions lead to changes
notaveragedownanyresidualcorrelatednoisecomingfrom on the beam width. Since we calibrate assuming a fix
thesky,forinstance.Theerrorbaronfluxmeasurementthat beamwidth,thesevariationsinduceasystematicerrorinthe
we quote is thereforenotcomputedanalytically from these finalcalibration.
weightsbutratherfromthenoiseonthemapdirectly. – Opacitycorrection:theskymapsarecorrectedfortheatmo-
Uncertaintiesonfinalmapsarealsocomputed. spheric contribution rescaling the observed signal by what
that would be obtained in the absence of atmosphere. This
is achieved via the elevation scan technique (skydip). The
6. Photometriccalibration
NIKA skydip procedure was successfully tested during the
ThevarietyofNIKAscientifictargetsgoingfromthermalSZob- lasttwoNIKAobservingcampaigns,anditproducedalow-
servations to dust polarization properties requires an accurate leveldispersionofthederivedopacityatdifferentelevations.
calibrationprocessabletodefinetheimpactofsystematicerrors – Datareductionfiltering:themainstepsinthedataprocess-
on the final sky maps as well as possible. The overall calibra- inghavebeendiscussedinSect.5.Theestimatedsystematic
tion uncertainty for point sources on the final data at the map errorintroducedbythedatareductionfiltering(inparticular
levelisestimatedtobe15%forthe1.25mmchanneland10% due to the common modes subtraction) is estimated at 5%
for 2.14 mm channel. In the following, a list of the principal for both channels. This was computed from the dispersion
error sources, which have a direct impact on the total calibra- betweendifferentprocessingmodes.
tion uncertainties, is presented. In Table 2, we quantify these
Inthefollowingwediscussthesecondarybeamfractioncontri-
contributions.
butionandtheatmosphericabsorptioncorrectioninmoredetail.
– Primarycalibrator:uranuswasselectedastheprimarycali-
6.1.Secondarybeamfraction
bratorfortheNIKAabsolutecalibration.Theabsolutecali-
brationfactorisderivedbyfittingaGaussianoffixedangular The secondary beam fraction needs to be estimated and ac-
sizeonthereconstructedmapsofUranus.ThefluxofUranus counted for in the case of extended sources. This is known by
A9,page7of15
A&A569,A9(2014)
Fig.8.Toppanel:radialprofileoftheNIKAbeampattern(reddots)includingthesecondarybeamcontributionforthe1.25mmchannel(left)
and2.14mmchannel(right)inthecaseofUranusobservations.Weareabletomeasurethebeampatternprofileuptoscalesofabout100arcsec.
Atlargerdistanceswedonothaveenoughsignal-to-noise.ForillustrationwealsoshowtheNIKAmeasuredprimarybeam.Bottompanel:map
ofthefarsidelobesfor1.25mmchannel(left)andfor2.14mmchannel(right).ThemapsarederivedusingobservationofSaturn.Thesemaps
arecompatiblewiththe2DstructureofthebeampatternoftheIRAM30mtelescopedescribedinGreveetal.(1998);Krameretal.(2013).The
diffractionringseenat1.25 mmatabout 2arcminradius,corresponds tothediffractionringduetopanel buckling. Thespider supporting the
secondarymirrorofthetelescopeisvisibletoalevelofabout−30dB.
comparing the NIKA beam pattern with the beam pattern of procedure that uses the NIKA instrument itself as a tau-meter.
the 30 m telescope as measured by Kramer et al. (2013, K13) Forthelast2012and2013observingcampaigns,theNIKAat-
on the Lunar edge using the EMIR receiver. For practical pur- mosphericcalibrationconsistedinmeasuringthevariationinthe
posewehavedividedthebeamintothreeregions:shortangular resonance frequencies of the detectors versus the airmass via
scalescorrespondtothemainbeam,intermediateangularscales elevationscans(skydipDickeetal.1946)from65to20degrees
corresponding to the first error beam, and large angular scales above the horizon. The procedureis based on the idea that the
assimilatedtofarsidelobesofthe30mtelescope.Tocompute KIDs response (the change in resonant frequency for a given
themainbeamweperformedaGaussianfittothefullobserved change in absorbed power) is a constant property of each de-
NIKA beam pattern. The best Gaussian fit is shown in yellow. tector.Thishasbeendemonstratedin laboratoryunderrealistic
TheNIKAmainbeamisconsistentwiththeoneofK13because conditions(with an optical load changing between about 50 K
we expect the main beam to be defined by the diameter of the and300K,Monfardinietal.2014)
30 m telescope. In the same way far side lobes, which are ex- During Skydip the telescope performs ten elevation steps
pected to come mainly from the second and third error beams each corresponding to variations of 0.18 in air mass. We per-
ofthe 30 mtelescope causedbysmall adjustmenterrorsof the formatuningofthereadoutelectronicsbeforeacquiringauseful
panelsandtheirframes,arealsoconsistentbetweenNIKAand signal.Foreachstepwehave22sofusefulsignalatagivenel-
K13.However,thefirsterrorbeamobservedintheNIKAbeam evation.ThisisshowninthetopplotofFig.9wherewepresent
patternislargerthantheonemodeledbyK13.Thisisnotunex- the observedelevation as a function of time during the Skydip
pectedsincetheobservationswerecarriedoutatadifferenttime procedure.
ofdayandunderdifferentweatherconditions.InFig.8,weshow Weexpecttheacquiredusefulsignalperdetectortorespond
profilesand mapsof the beam emphasizingthe contributionof totheairmassas:
thesecondarybeamforbothNIKAchannels.Theresultsonthe
estimationofitscontributionarepresentedinTable2. FsGkryoduinpd = F0+CTatm[1−e−xτskydip]· (4)
Here, FGround is the acquiredsignal correspondingto the abso-
6.2.AtmosphericabsorptioncorrectionusingaSkydip skydip
lutevalueoftheshiftinthefrequencytoneforeachpixel,F is
calibration 0
the instrumentaloffsetcorrespondingto the frequencytone ex-
In the previous observing campaigns, the atmospheric ab- citationfortheconsideredpixelforzeroopacity,C thecalibra-
sorption correction was made using the IRAM tau-meter that tionconversionfactorinHz/K,T (inKelvin)istheequivalent
atm
performs elevation scans continuously at a fixed azimuth at temperatureoftheatmosphere,τ theskyopacityduringthe
skydip
225 GHz. To derive the opacity at the exact position of the skydip (at the wavelength of the fit, either 1.25 or 2.14 mm),
scan and at the same frequencies as NIKA we implemented a xtheairmasswithx=sec(δ)whereδistheaverageelevationof
A9,page8of15
A.Catalanoetal.:PerformanceandcalibrationoftheNIKAcameraattheIRAM30mtelescope
Fig.10. Atmospheric opacity evolution for the NIKA 2012 and 2013
observing campaigns calculated from the Skydip analysis. The error
barswereestimatedbyanalysingdifferentSkydipobservations.During
the 2013 observing campaign, the agreement with 225 GHz IRAM
tau-meterisgood.
Ainitialadvantageofthismethodisthatwedonotneedtoper-
form the Skydip at the exact time of the source observations
to properly correct the atmospheric contribution in the consid-
ered scan. A second advantage is that we can estimate the at-
Fig.9. Top panel: telescope positions during an elevation scan proce- mospheric opacity at the same (azimuth-elevation) position of
dure:10stepsinelevationhavebeenperformedwithoutchangingthe thesource,insteadoftheaverageskyopacity.Finally,withthis
azimuthal position. Data for absolute calibration are taken in the re- methodofcorrectingfortheatmosphericabsorption,theNIKA
gion between the blue and the red lines. Middle panel: signal in Hz bandpassesaredirectlytakenintoaccount.Thismethodisonly
ofeachvalidKID(bluefor1.25mmdetectors,redfor2.14mm)asa limitedbythevalidityoftheair-massscalinglawwithelevation
functionofairmass,togetherwiththefittedmodel(blackdottedlines).
(secant model) and by the degeneracyof the atmospheric tem-
Bottom panel: deduced integrated in-band opacities for the 1.25 mm
peraturewiththeopacityinourmodel.Toavoidanexcessiveim-
and2.14mmchannels.
pactofthisdegeneracy,weperformedafewSkydipprocedures
perdayofobservation.
InFig.10wepresentthemeasuredopacitiesforallthemap-
thetelescopeduringthescan.Byperformingafitofthethreepa-
rameters,F ,C,andτ,forallvaliddetectorsatthesamewave- ping scans of the NIKA observing 2012 (top) and 2013 (bot-
0
tom) campaigns. During the 2012 campaign on 22 and 23 of
lengths, we obtain a common sky opacity at zenith during the
November,theopacitywaslessthen0.05forthe1mmchannel.
skydip.Wecanrerunthefitoneachdetector,assumingthecom-
mon τ value, and getthe coefficients F andC per detector. In During the June 2013 campaign, the weather was worse, only
0
permittingproperobservationswith the1.25mmand2.14mm
themiddleandbottompanelsofFig.9wepresentthemainre-
channelsforaboutafewhoursattheendwhentheopacitywas
sults of the data analysis of one Skydip performed during the
about0.1forthe1.25mmchannel.
2013campaign.
The F , C coefficients only depend on the response of the
0
detectors.Sincethenon-linearitiesoftheKIDfrequencysignal
6.2.1. Consistencywithmodels
arenegligibleintheconsideredrangeofbackgrounds,thecoef-
ficientscanbeappliedtoalltheobservingcampaigntorecover TheconsistencyoftheSkydiptechniquecanbevalidatedbyus-
theopacityoftheconsideredscan.Thisisobtainedbyinverting ingtheATMmodel(Pardoetal.2002).Wederivedtheexpected
Eq.(4)as: opacities integrated into the actual NIKA bandpasses over a
range between 0.04 and 20 mm of precipitable water vapor. In
⎛ ⎞
τscani = x1 ln⎜⎜⎜⎜⎝1− FsGcraConuinTd−F0i⎟⎟⎟⎟⎠, (5) fFoigr.11.215wmempreasnednt2t.h1e4cmommpacrhiasonnneblestwanedenththeeAoTpMacimtieosddeel.riTvhede
scan i atm plotsatthe toppresentthe resultsforthe 2012observingcam-
whereτ istheopacityoftheconsideredscanasmeasuredby paignandthebottomplotsresultsfor2013observingcampaign.
scani Theagreementbetweenthemeasuredopacitiesandthemodelis
agivendetector,x correspondstotheairmassattheelevation
scan goodforbothcampaigns.
oftheconsideredscan,FGroundisthemeasuredabsolutevalueof
scani
the detector resonance frequency during the scan, and F and
C are the coefficients derived from the Skydip technique0.i The
i 7. Noiseequivalentfluxdensity(NEFD)
opacityτ isdeducedbyaveragingτ forallvaliddetectors
scan scani
atagivenwavelength.Thebrightnessoftheobservedscanmap The flux of a point source is measured via a 2D Gaussian fit
SGround can be rescaled onto the scale that would be obtained inthefinalaveragemapwithafixedwidthandafixedposition.
usingdetectorsoutsidetheatmosphere(SStar)with Thefitislinearandinvolvesaconstantbackground,evaluatedin
asquareareawithasizeoftentimestheFWHMoftheGaussian
SStar =SGround·exτscan. (6) distribution,inourcase 13arcsec (resp.18arcsec)at1.25mm
A9,page9of15
A&A569,A9(2014)
Fig.12.NEFDdistributionforthetwoarrays(1.25mmtop,2.14mm
bottom) during the 2012 observation campaign, for the MM18423
scans. The detector sensitivity is presented as a function of the
KID number, sorted according to the resonance frequency. The av-
erage NEFD of the two arrays are 48 and 23 mJy s1/2 at 1.25 and
2.14mm,respectively,fortheseobservations. Ifweconsider onlythe
best20%detectorsofeacharray,thenwegetanaverageNEFDof39
and15mJys1/2.
Fig.11.Comparisonbetweentheatmosphericopacitiesmeasuredwith 2)) 200
tThheeStkoypdipplottecishnfioqrueth(ebl2u0e12pocinamts)paaingdn,thteheAbToMttommodpelolt(pfuorrptlheeli2n0e1s)3. s^(1/ 180 1212....21215454 mmmmmmmm ((((rrrruuuunnnn 5555))))
campaign. mJy 160 11..2255 mmmm ((rruunn 66))
D ( 140 22..1144 mmmm ((rruunn 66))
F
NE 120
(resp.2.14mm).Aphotometriccorrection(oftheorderof5%)
100
is applied to the flux measurement to account for the filtering
effectinducedbytheTOIprocessing. 80
Thenoiseequivalentfluxdensity(NEFD)iscomputedasthe 60
array-averagedsensitivitytopointsourcesi.e.thefluxrootmean
40
square(rms)obtainedinonesecondofintegration.Theaverage
20
isdoneviathereciprocalofthesquareoftheNEFD.Ittakesthe
timeeffectivelyspentbythearrayonthesourceintoaccount. 00 0.1 0.2 0.3 0.4 0.5 0.6
Figure12presentstheNEFDforeachvalidKIDofthearray τ/sin(δ)
at1.25mm(top)and2.14mm(bottom)obtainedduringtheob- Fig.13.Array-averagedNEFDasafunctionoftheline-of-sightopac-
servationofMM18423+5938.Inthiscase,theaveragesensitiv- ity τ/sin(δ). Black dots (resp. red squares) refer to the 1.25 mm ar-
itiesontheskyare∼48and∼23mJys1/2 at1.25and2.14mm, ray(resp.2.14mm)forthe2012observationcampaign,duringwhich
respectively. We note that the sensitivity distribution variation the trend at 2.14 mm is compatible witha exp(τ/sin(δ)) behavior (as
acrossthearrayisquitelargewiththemajorityofdetectorscon- shownwiththeredcurve),whileadeparturefromthisbehaviorisob-
served at 1.25 mm. The latter can be attributed to the degradation of
centrated at the better sensitivity end. If we consider only the
theKIDresonancequalityfactorwithincreasingpowerload.Opencir-
best20%detectorsofeacharray,thenwegetanaverageNEFD
of39and15mJys1/2. cles(resp.opensquares)refertothe1.25mmarray(resp.2.14mm)for
the2013observationcampaign.Veryfewpointshavebeenmeasured,
Figure 13 presents the array-averaged NEFD of the NIKA
buttheachievedsensivitiesaremuchbetteranddofollowanexpected
camera at 1.25 and 2.14 mm as a function of the line-of-sight exponentialtrend.
opacity τ/sin(δ), where τ is the zenith opacity and δ the eleva-
tion. The NEFD measurements are obtained during the obser-
vations described in Table 3. At zero zenith opacity, the array- Thesensitivityat1.25mmwaslimitedbyasaturationeffect
averagedNEFDare∼40and∼23mJys1/2at1.25and2.14mm, on the readoutelectronics, which has been corrected since. By
respectively, which give an indication of the performance of onlyusingeightdetectors,wehavebeenabletodetectthefaint
the 2012 NIKA camera in optimal weather conditions. As ex- sourceMM18423.Inthatcase,themeasured,averaged1.25mm
pected, the sensitivities degradewith increasing opacity,corre- NEFD is 27 mJy s1/2 to be compared with 48 mJy s1/2 for the
spondingtoworseweatherconditions.At2.14mm,weobserve, full-arrayobservation(Fig.12).
as expected, a degradation due to the opacity effect, follow- SXDF1100.001has been observedtwice during this obser-
ing exp(τ/sin(δ)), while the degradationseems to be enhanced vation campaign, with two different integration times (68 and
at 1.25 mm. Nevertheless, the sky noise decorrelation seems 258 min), and we used it to have a first estimate of the evolu-
to work in those cases as well. We therefore confirm that the tionofNEFDandnoisewithintegrationtime.Wenotethatthe
increasedpoweronkidssomehowreducestheirperformance. noiseintegratesdownbecausetheNEFDisstable,andthenoise
A9,page10of15