The NIKA2 instrument, a dual-band kilopixel KID array for millimetric astronomy

1.1 MB·

by  M. Calvo

JournalofLowTemperaturePhysicsmanuscriptNo.
(willbeinsertedbytheeditor)
M. Calvo1, A. Benoˆıt1, A. Catalano1,2,
J.Goupy1,A.Monfardini1,2,N.Ponthieu1,3,
E.Barria1,G.Bres1,M.Grollier1,G.Garde1,
J.-P.Leggeri1,G.Pont1,S.Triqueneaux1,
R.Adam2,O.Bourrion2,J.-F.Mac´ıas-Pe´rez2,
M.Rebolo2,A.Ritacco2,J.-P.Scordilis2,
D.Tourres2,C. Vescovi2,F.-X. De´sert3,
A. Adane4, G. Coiffard4, S. Leclercq4,
S.Doyle5,P.Mauskopf5,6,C.Tucker5,P.Ade5
6
P.Andre´7,A.Beelen8,B.Belier9,A.Bideaud5,
1
0 N.Billot10,B.Comis2,A.D’Addabbo11,
2 C. Kramer10, J. Martino8, F. Mayet2,
F.Pajot8,E.Pascale5,L.Perotto2,V.Reve´ret7,
n
a L.Rodriguez7,G.Savini12,K.Schuster4,
J A.Sievers10,R.Zylka4

2
1 The NIKA2 instrument, a dual-band

kilopixel KID array for millimetric
]
M
astronomy
I
.
h
p thedateofreceiptandacceptanceshouldbeinsertedlater
-
o
r
t Abstract NIKA2 (New IRAM KID Array 2) is a camera dedicated to millime-
s
a terwaveastronomybaseduponkilopixelarraysofKineticInductanceDetectors1
[

1 1InstitutNe´el,CNRSandUniversite´deGrenoble,France
v 2 Laboratoire de Physique Subatomique et de Cosmologie, Universite´ Grenoble-Alpes,
4 CNRS/IN2P3,53,ruedesMartyrs,Grenoble,France
7 3 Institut de Plane´tologie et d’Astrophysique de Grenoble (IPAG), CNRS and Universite´ de
7 Grenoble,France
4InstitutdeRadioAstronomieMillime´trique(IRAM),Grenoble,France
2
5AstronomyInstrumentationGroup,UniversityofCardiff,UK
0
. 6ArizonaStateUniversity,Tempe,AZ,USA
1 7LaboratoireAIM,CEA/IRFU,CNRS/INSU,UniversitParisDiderot,CEA-Saclay,91191Gif-
0 Sur-Yvette,France
6 8Institutd’AstrophysiqueSpatiale(IAS),CNRSandUniversite´ParisSud,Orsay,France
1 9Institutd’ElectroniqueFondamentale(IEF),Universite´ParisSud,Orsay,France
: 10InstitutdeRadioAstronomieMillimetrique(IRAM),Granada,Spain
v
11 Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Gran Sasso, Assergi (AQ),
i
X Italy
12 UniversityCollegeLondon,DepartmentofPhysicsandAstronomy,GowerStreet,London
r
a WC1E6BT,UK
E-mail:[email protected]
2
(KID).Thepathfinderinstrument,NIKA2,hasalreadyshownstate-of-the-artde-
tectorperformance.NIKA2buildsuponthisexperiencebutgoesonestepfurther,
increasing the total pixel count by a factor ∼10 while maintaining the same per
pixel performance. For the next decade, this camera will be the resident photo-
metricinstrumentoftheInstitutdeRadioAstronomieMillimetrique(IRAM)30m
telescopeinSierraNevada(Spain).Inthispaperwegiveanoverviewofthemain
componentsofNIKA2,anddescribetheachieveddetectorperformance.Thecam-
erahasbeenpermanentlyinstalledattheIRAM30mtelescopeinOctober2015.
Itwillbemadeaccessibletothescientificcommunityattheendof2016,aftera
one-year commissioning period. When this happens, NIKA2 will become a fun-
damentaltoolforastronomersworldwide.
Keywords KineticInductanceDetectors,mm-waveastronomy,
1 Introduction
The IRAM 30m antenna is on of the largest and most sensitive single dish tele-
scope for millimeter wavelengths currently operating worldwide. Its cabin tradi-
tionally hosts both heterodyne receivers, ideally suited for high resolution spec-
troscopy,andbroad-bandcontinuumphotometricinstruments,whichareusedto
detectextremelyfaintsources.The30mprimarydishresultsinadiffractionlim-
ited resolution of 17 and 10arcsec for the 150 and 260GHz bands respectively.
Thus, in order to fully sample the 6.5arcmin correct FoV, a photometric camera
forthistelescopeneedsarrayscontaining∼103 pixels,havinganintrinsicnoise
comparableto,orbelow,thephotonnoiseexpectedatthetelescopesite.Onlyre-
cently the developments in the field of cryogenic detectors have made arrays of
thistypepossible.
Coupling high sensitivities to their intrinsic suitability for frequency domain
multiplexed readout, KID are the ideal candidates for this kind of application.
This is why they have been the technology of choice when the project to build
the pathfinder instrument, NIKA, started in 2008. NIKA has been an extremely
successful instrument, being the first camera to conduct multiplexed on-sky ob-
servations using KID and leading to outstanding scientific results34. In its final
configuration, NIKA counted a total of 356 pixels split over the two bands. Its
successhasbeenakeystepfortheapprovaloftheNIKA2projectasthecamera
ofchoicefortheIRAMtelescope.
2 TheNIKA2instrument
Whileinheritingalotofknow-howfromtheexperiencegainedbyNIKA,NIKA2
is a completely new instrument, whose development has implied changes at all
levels, from the detector arrays themselves all the way up to the optical chain of
the30mtelescope.
3
2.1 Optics
TomaximizethedimensionsofthecorrectFoV,alltheopticalelementsfollowing
the telescope’s primary mirror (M1) and subreflector (M2) have been modified.
The M3 and M4 mirrors, which are common to all the instruments of the cabin,
havebeenreplaced(thelatterbeingusedtoselectwhetherheterodyneorthecon-
tinuum camera shall be used for observations). Two new dedicated mirrors (M5
and M6) have also been made, to send the light to the cryostat window. Inside
the cryostat, two additional cold (∼80K) mirrors (M7 and M8) are present, fol-
lowedbyHDPElensesonthecoldeststages(1Kandbelow).Thebandsplitting
is achieved by means of a dichroic installed at the 0.1K stage. The light in the
260GHzbandisthenfurthersplitbyawire-gridpolarizer,thatseparatesitshor-
izontal and vertical components. This enables us to make polarization sensitive
observations with a very low cross-pol level. When carrying out this kind of ob-
servations,arotatinghalf-waveplateisplacedinfrontofthecryostatwindowto
modulatethepolarizedlight.
Allopticalelementshavealreadybeeninstalled,andthecoldopticshavebeen
extensivelyusedforlabtests.Theonlyexceptionistherotatinghalf-waveplate,
which is in the final fabrication steps and will be tested on-site during the first
technicalrun.
2.2 Cryostat
The NIKA2 cryostat is precooled to ∼5K by two Pulse Tubes (PT) working in
parallel.Adilutionrefrigeratoristhenusedtoreachthebaseworkingtemperature
ofabout150mK,whichisachievedafter5days.Thesystem,whichiscompletely
cryogenfreeandcanbefullyremotecontrolled,canthenbekeptcoldindefinitely.
ThePThaveremotemotors,whichareconnectedtothecryostatbodyusingflexi-
bletubesandrubberdampers.Thissuppressesthevibrationstheyinduce,thatcan
otherwise add a strong 1/f noise contribution to the detectors noise. To protect
Fig.1 SchematicrepresentationoftheNIKA2cryostat,withtheopticalpathtothethreearrays
inevidence.Thecryostatis2.3mlongandweighsmorethan1ton.
4
thedetectorsalsofromtheeffectsofexternalmagneticfields,weaddedhighper-
meabilitymaterialsateachstage:oneMumetalcylinderat300K,aCryopermone
at4K,alayerofMetglastapeat100Kand50K,andasuperconductingAluminum
screenat200mK.
2.3 Electronics
The multiplexed readout of KID is achieved using dedicated electronics boards
called NIKELv15. These board can excite and readout up to 400 pixels over a
500MHzbandwidth,andapplyaDirectDownConversionmethodontheacquired
outputsignaltodeterminethevariationsofamplitudeandphaseofeachtone.11
boards were available during the first technical run, out of the 20 needed for the
final,completeconfiguration.
Forthereadout,themodulationtechniquedevelopedforNIKA1willbeused6.
Thisallowsustoevaluatethevariationoftheresonantfrequency f withhighpre-
0
cision,leadingtoverygoodphotometricaccuracy.Theexpectederrorontheab-
solutecalibrationofourdetectorsisbelow10%,asalreadyachievedwithNIKA1.
3 Detectorsdesignandperformance
The NIKA2 pixels are based on Hilbert type LEKID7,8. Thanks to the fractal
shapeoftheirinductor,suchpixelscanefficientlyabsorbbothpolarizations.The
detectors are fabricated using thin (18–25nm) Aluminum films, in order to in-
crease the kinetic inductance fraction of the superconductor and make the KID
more responsive. The thin Al film also increases the normal state surface resis-
tivity,makingiteasiertomatchthefreespaceimpedanceanddirectlyabsorbthe
radiationintheinductor.
The focal plane has a diameter of ∼80mm, and each array is fabricated on
a single 4(cid:48)(cid:48) HR Silicon wafer. More details on the fabrication process can be
foundelsewhereintheseproceedings9,10.Tomaintainthetelescopeangularres-
olution, the focal plane sampling must be ≤1Fλ. We have tested different ap-
proaches,withapitchbetweenpixelsvaryingfrom0.7to1Fλ,whichcorresponds
to 2.3–2.8mm at 150GHz and 1.6–2mm at 260GHz, for a total pixels count of
600–1000and1200–2000respectively.AlthougheachNIKELboardcanreadout
up to 400 pixels, for practical reasons the actual multiplexing factor is kept at a
lowervalueof150–250,sothat4to8readoutlinesareneededperarray.
Wehaveinvestigatedtwodifferentfeedlinegeometries:CoplanarWaveguide
(CPW) and Microstrip (MS). The CPW solution is more widespread and has al-
ready been tested in NIKA1. The main advantage of such an approach is that a
back-illuminationconfigurationcanbeadopted,usingthesubstrateasanimpedance
matchingAnti-Reflection(AR)layer.Yet,theonsetofspuriouspropagationmodes
can lead to the presence of standing waves along the feedline which, in turn, re-
sult in a large scatter of the coupling of the different resonators to it. This effect
can be mitigated by implementing a series of bonding across the line. A simpler
and more effective solution is that of recurring to a MS feedline, whose geom-
etry is such that no spurious modes exist. The coupling of the detectors to the
line and hence their performance are thus more uniform and predictable. On the
5
Fig.2 Reconstructedfocalplanegeometryofthe2mmCPWarray.Eachcirclecorrespondsto
onepixel,andthedifferentcolorsareassociatedtothedifferentreadoutfeedlines.Weexcluded
fromthisplotpixelshavingatoolargenoiseortooellipticbeams.Evenexcludingsuchpixels,
theyieldobtainedisabove80%.
otherhand,thebacksideofthewaferisinthiscasemetallized,inordertoactas
agroundplane,andthepixelsmustbeforcefullyfront-illuminated.UsingthinAl
films and choosing the appropriate substrate thickness it is nonetheless possible
to have >40% absorption efficiency over a ∼20% bandwidth, a value which is
goodenoughfortheaimsoftheNIKA2experiment.
Tocharacterizetheperformanceofourarraysunderconditionssimilartothose
found at the telescope we use a sky simulator. This consists of a copper flange,
coveredwithalayerofcarbon-loadedStycasttomakeitalmostcompletelyblack
at millimetric wavelengths. This simulator is cooled using a single stage PT to
∼30K.Itsopticalemissionisthussimilartothatoftheatmospherewhich,under
typical observing condition, corresponds to a grey body with ∼10% emissivity
andatemperatureof∼270K.Ametallicsphereof4mmdiametercanbemoved
infrontoftheskysimulator,thusmimickingthepassageofaplanetcomparable
toUranus.Thisallowsustoreconstructanimageofthefocalplaneandevaluate
theresponsivityofeachpixel.
Thebaselinearraysforthefirsttechnicalrun(October2015)havealreadybeen
chosen.Forthe150GHzband,a1040pixelsarraywithCPWfeedlineandasam-
plingof0.7Fλ willbeused.Thebackofthewaferhasbeendicedtoimproveits
effectivenessasanARlayer.The260GHzbandwillbebasedontwoMSarrays,
each containing 1200 pixels, sampling the focal plane at 1Fλ. The three arrays
havebeenfullycharacterizedusingtheskysimulator.Theresultsaresummarized
intable1.
Theperpixelperformanceobtainedinthelabareinlinewiththosefoundfor
theNIKAdetectorswhencarryingthesamekindoftests.ForNIKA,wedemon-
6
Fig.3 Noisespectraofthepixelsofonethebaseline150GHzarrayfeedlines,obtainedunder
anopticalloadrepresentativeoftheoneexpectedattheIRAM30msite.Usingthesamepixels,
wemeasuredashiftoftheresonantfrequencyofabout3kHzforachangeof3.6Kinthesky
simulatortemperature.CombiningthistwodataitispossibletoevaluatetheSignal-to-Noise
Ratio(SNR)ofeachpixel.Thenoiseatlow f ismostlycorrelatedacrossallthepixels,andcan
thusbeefficientlyremovedusinganappropriatedecorrelationtechniqueduringdataanalysis.
stratedanon-skysensitiviyonpoint-likesourcesof10and25mJy·s1/2at150and
260GHzrespectivelyingoodobservingconditions.Wethereforeexpecttoobtain
similar values for the NIKA2 arrays. Thus, considering the larger FoV, NIKA2
willgrantafactor∼10increaseinmappingspeedwithrespecttoitspredecessor,
makingitoneofthemosteffectivecamerasworldwideatthesewavelengths.
The effective bandpass of the different channels is determined by the prod-
uct of the transmissivity of the optical filters and of the absorption efficiency of
the corresponding pixels. We measure it using a Martin-Pupplet Interferometer
(MPI),withwhichwecandeterminetheshapeoftheabsorptionspectrumwitha
resolution of about 3GHz. Although the absolute value of the total absorption is
difficulttoknowprecisely,asitdependsuponthedetailsoftheopticsandofthe
Fig.4 AbsorptionspectraoftwosamplepixelsoftheNIKA2arrays:inbluethepixelofthe
150GHzarray,andinorangethatofoneofthe260GHzarrays.Theabsorptionineachbandhas
beennormalizedto1.The150GHzCPWarrayhasaflatterabsorptionbandthankstotheuseof
thesubstrateasanARlayer.
7
Array Noise@1Hz Responsivity SNR@1Hz
(Hz/Hz0.5) (kHz/K) (mK/Hz0.5)
150GHz 1–2 0.8 ∼1.5
260GHzH 2–3 2 ∼1.2
260GHzV 2–3 2 ∼1.2
Table 1 Overview of the performance of the baseline arrays of NIKA2. The reported values
accountforabout80%ofthetotalpixels,theremaining20%beingeitherdeadpixelsorpixels
thathaveabnormallyhighnoiselevels.
MPI components, its shape is all that is needed to perform accurate photometric
measurements.Theabsorptionspectraoftwoofthebaselinearraysareshownin
Fig.4.Thechoiceofthesubstratethicknessandtheuseofλ/4backshortsallows
ustofinetunethepositionoftheabsoprtionband,whichischoseninordertotake
advantage of the atmospheric windows available, while at the same time avoid-
ingthecontaminationduetotheemissionlinesofmolecules,inparticularwater
vapourandozone.
Until the end of 2016, NIKA2 will be devoted to technical runs for its com-
missioning. During this time, new arrays will be fabricated and tested. If major
performance improvements were to be obtained, access will be granted to the
cameraforthereplacementofthecurrentbaselinearrays.
4 Conclusions
NIKA2isthenextgeneration(2015-2025)residentphotometricinstrumentofthe
IRAM30mtelescope.Overthelastthreeyears,wehavefabricatedalltheneeded
components.Thisinclude,inparticular,adrycryostatwithadilutionrefrigerator
stage,thathasalreadybeencooleddownmultipletimestoabasetemperatureof
150mK.TomaximizetheavailableFoV,wehavemadeacompletelynewoptical
design and have replaced all the components of the optical chain that follow the
secondarymirror.ThecorrectFoVisnow6.5arcminindiameter.
In order to fully sample such a large FoV, we have increased the pixel count
by a factor ∼10 with respect to NIKA, all while keeping similar per pixel per-
formance,astheyalreadyapproachedthephotonnoiselimitoftheSierraNevada
site. NIKA2 will thus lead to a tenfold increase of the mapping speed, and will
also allow for the measurement of the linear polarization of light in the 260GHz
band.
With all the subsystems built and validated, NIKA2 has been installed at the
telescope during a dedicated technical run that took place in October 2015. This
will be followed by a one-year commissioning period, after which NIKA2 will
bemadeavailabletotheexternalastronomers.Theuniquefeaturesofthiscamera
willthenopenthewaytotheexplorationofnewandexcitingscientificcases.
Acknowledgements ThisworkhasbeenpartiallyfundedbytheANRunderthecontractNIKA2,
andhasbeenpartiallysupportedbytheLabExFOCUSANR-11-LABX-0013.Wewouldliketo
thanktheIRAMstafffortheirexcellentsupportduringthecampaign.Thisworkhasbeenpar-
tiallyfundedbytheFoundationNanoscienceGrenoble,theANRunderthecontracts”MKIDS”
and”NIKA”.ThisworkhasbeenpartiallysupportedbytheLabExFOCUSANR-11-LABX-
0013.ThisworkhasbenefitedfromthesupportoftheEuropeanResearchCouncilAdvanced
8
GrantORISTARSundertheEuropeanUnion’sSeventhFrameworkProgramme(GrantAgree-
mentno.291294).TheNIKA2cryostatandthereadoutelectronicshavebeendesignedandas-
sembledbytheCryogenicsandElectronicsgroupsinGrenoble.Inparticular,weacknowledge
thekeycontributionsofP.E.Wolf,A.Gerardini,G.Donnier-Valentin,H.Rodenas,O.Exshaw,
G.Simiand,C.Vescovi,E.PerbetandS.Roudier.R.A.wouldliketothanktheENIGMASS
FrenchLabExforfundingthiswork.
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