Cosmic Microwave Background with a Long Duration
Balloon-borne Receiver
Thesisby
Brendan P. Crill
In Partial Fulllmentof theRequirements
fortheDegreeof
Doctor ofPhilosophy
CaliforniaInstituteofTechnology
Pasadena,California
2001
c
2001
Abstract
This thesis describes Boomerang; a balloon-borne telescope and receiver designed to
map the Cosmic Microwave Background (CMB) at a resolution of 10 0
from the Long
Duration Balloon (LDB) platform. The millimeter-wave receiver employs new technology
in bolometers, readout electronics, cold re-imaging optics, millimeter-wave lters, and
cryogenics to obtain high sensitivity to CMB anisotropy. Sixteen detectors observe in 4
spectral bands centered at 90, 150, 240 and 400 GHz. The wide frequency coverage, the
longightduration,theopticaldesignandtheobservingstrategyallprovidestrongrejection
of systematiceects. We reportthein-ightperformanceof theinstrumentduringashort
test ight from Palestine, Texas, that mapped 230 square degrees and during a 10.5 day
stratospheric balloon ight launched from McMurdo Station, Antarctica, that mapped
2000 square degreesof the sky. The Antarctic datayielded a measurement of theangular
powerspectrumof theCMB between 50<`< 600 which shows apeakat `
peak
=1976
(1 error). A maximum likelihood estimation of cosmological parameters within the cold
darkmatter(CDM)paradigmofstructureformationindicatesthattheuniverseisatwith
a precisionof 6%and thatthedensityofbaryonsin theuniversemaybeslightlyhigher
than previously thought. The combination of observations of large scale structure (LSS)
and the Boomerang power spectrum impliesthepresence of bothdark matter and dark
Acknowledgments
...let me ask you how the scholar who cultivates science to pass his life
agreeably diers from the drunkard who also seeks in life no more than
immediate enjoyment and nds it in wine? It is true that the scholar makes
a better choice of the source of his pleasures, since they are more intense and
more durable,butthat isall. Both ofthem,the drunkardand thescholarhave
thesame egotisticalaim, personal enjoyment.
... ifyouarereallythinkingofhelpinghumanity,ifthatiswhatyouaspireto
inyourstudies,youwillndyourselffacingaformidableobjection,for,insofar
asyou have anysenseof justice,you will immediatelyobserve that in
present-daysociety,scienceis onlya kindof luxurythatmakeslifemoreagreeable to a
few,and remainsabsolutelyinaccessibleto almost thewholeof humanity.
Peter Kropotkin,To the Young(1885)
Boomerang is an enormous project which has required huge amounts of labor from
quite a few people since it began in 1993. The author lists on each of the Boomerang
publications, no matter how long, can never properly credit all of the people who have
contributed to this project over the years. My thanks goes out to all of these people,
withoutwhomthisthesis wouldnot exist.
As of June 2000, here is the list in alphabetical order of everyone who has worked
on Boomerang: Peter Ade, Kashif Alvi, Joa Applegate, Elisabetta Aquilini, Ricardo
Artusa,AndyBeard,JeBeeman,RavinderBhatia,JamieBock,DickBond,JulianBorrill,
Andrea Boscaleri, KurtCampbell, John Cartwright, Sarah Church,Kim Coble, Matthieu
Contensou, Paolo deBernardis, Giancarlo de Gasperis, Marco dePetris, Grazia deTroia,
HectorDelCastillo,Mark Devlin,PhilFarese,Ken Ganga,Massimo Gervasi, Massimiliano
Giacometti,VicHaynes,CaraHenson,EricHivon,DaveHorseley,ViktorHristov,Armando
Iacoangli, Andrew Jae, Andrew Lange, Adrian Lee, Jon Leong, Sam Li, Tao Long,
Tom Montroy, Barth Nettereld, Dan Osgood, Chris Paine, Ricardo Paniagua, Enzo
Pascale, ByronPhilhour,Francesco Piacentini, Francesco Pongetti, SimonPrunet, Andrea
Raccanelli, Mike Radford, Paul Richards, Gianni Romeo, John Ruhl, Marcus Runyan,
FrancescoScarramuzzi,KacieShelton,GeorgeSmoot,EricTorbet,IvyTurkington,Valerio
Venturi,Nicola Vittorio, BetsyVillalpando, JohnnyWu, andLunming Yuen.
ThankstoStephenPeterzen,DannyBall,thelaunchcrewatNSBFandtheNSFsupport
team at McMurdoStation.
Additional thanks to Ravinder Bhatia, Samantha Edgington, Bill Jones, Eric Hivon,
Contents
Abstract iii
Acknowledgments iv
1 Introduction 1
1.1 Overviewof Boomerang . . . 4
2 Theory 6 2.1 Map Making . . . 6
2.2 AngularPowerSpectrum . . . 8
2.3 CosmologicalParameters . . . 10
3 Instrument Design 12 3.1 Telescope and Gondola. . . 12
3.2 Thermal Design. . . 13
3.3 Cryogenics. . . 14
3.4 Optics . . . 15
3.5 Bandpass Selection . . . 17
3.6 SingleMode Feed . . . 17
3.7 Multi-colorPhotometer . . . 20
3.8 Calibration Lamp. . . 21
3.9 Detectors . . . 21
3.10 Integrating Cavity . . . 24
3.11 Readout Electronics . . . 24
3.12 Rejection ofRFand MicrophonicPickup . . . 25
4 Instrument Characterization 27 4.1 OpticsTestbed . . . 27
4.4 OpticsBox . . . 29
4.5 Feed Testbed . . . 30
4.6 OpticalEÆciency ofFeed Structures . . . 31
4.7 Characterization ofNeutral DensityFilters . . . 32
4.8 IntegratedFocal Plane . . . 35
4.9 SpectralBandpass . . . 35
4.10 BlueLeaks . . . 36
4.11 LoadCurves. . . 37
4.12 Time Constant . . . 38
4.13 NoisePerformance . . . 41
4.14 BeamMaps . . . 44
4.15 LabCalibration . . . 45
5 1997 Test Flight 48 5.1 TestReceiver . . . 48
5.2 Observations . . . 48
5.3 PowerSpectrumEstimation . . . 49
5.4 Systematics Tests . . . 50
5.5 Measuring Curvature . . . 51
5.6 Discussion . . . 51
6 1998 Flight Performance 53 6.1 Electronics . . . 54
6.2 In-ight Calibrationof theSunSensor . . . 55
6.3 Attitude Reconstruction . . . 55
6.4 Thermal Performance . . . 57
6.5 Cryogenics. . . 58
6.6 BeamMap . . . 59
6.7 Deglitching . . . 64
6.8 TransferFunction. . . 65
6.9 Detector Noise . . . 67
7 Calibration 73
7.1 LabScaledto Flight . . . 73
7.2 Dipole . . . 74
7.3 Relative Calibration . . . 78
7.4 Sources . . . 79
7.5 Calibration Stability . . . 84
7.6 Sensitivity . . . 85
7.7 Discussion . . . 86
8 Power Spectrum and Cosmological Parameters 88 8.1 Maps. . . 88
8.2 PowerSpectrum . . . 91
8.3 CosmologicalParameterEstimation . . . 96
8.3.1 Choice of Parameters . . . 97
8.3.2 Results . . . 99
9 Other Science with Boomerang 103 9.1 Galactic Dust . . . 103
9.2 Sunyaev-Zel'dovichEect in Clusters. . . 103
9.3 SerendipitousCluster Searches . . . 104
9.4 High-redshiftGalaxies . . . 105
10Discussion 106 A Bolometer Data Summary 111 A.1 Time Constant . . . 111
A.2 Bolometer Impedence . . . 112
A.3 In-ight loadcurve . . . 113
A.4 LabLoadCurves . . . 114
A.5 Flight Noise . . . 115
A.6 Flight OpticalBackground. . . 116
A.7 In-Flight Responsivity . . . 117
B Calculation of Beam Oset Parameters 119
B.1 Rotation oftheGondola Boresight . . . 119
B.2 Calculationof BeamPosition . . . 120
B.3 Calculationof BeamOset Parameters. . . 121
C Roll Correction to Azimuth and Elevation 123
Chapter 1 Introduction
The2.7KCosmicMicrowaveBackground(CMB),discoveredin1964[55 ],isoneofthemost
powerfulobservational pillars supportingthe current picture of cosmology. The spectrum
of the CMB (measuredbyCOBE/FIRAS to bethe best t to a blackbodyspectrum ever
observedinnature[41 ])stronglyindicatesthattheuniverseevolvedfromahot,densestate.
The radiation originates from thetime when the matter in theuniverse transitioned from
an opaque plasma to transparent gas(the timeof recombination), at a redshiftof 1000,
about300,000 years afterthe BigBang.
Mapping the background radiation provides a snapshot of the universe in its infancy.
Thetiny(a partin10 5
ofthebackground)uctuationsseeninthetemperaturecorrespond
to uctuations in matter density at the time of recombination. A measurement of the
angular power spectrum of the uctuations of the CMB can test inationary models of
structure formation. Furthermore, within the context of the class of cold dark matter
models, constraintscan beplacedon cosmological parameters.
Thestandardinationary,colddarkmatterclassofmodelspredictsstructureformation
asfollows. Theseedsofstructureareprovidedbyrandomquantumuctuationsstretchedto
cosmologicalsizeduringtheperiodofrapidinationaryexpansion. Theamplitudeofthese
\primordial"uctuationsisgenerallyexpectedtobenearlyscaleinvariant. Duetothesmall
amplitude ofthe uctuations, they evolve linearlyinthe plasma. Gravitational collapseis
resisted by photon pressure and the perturbations oscillate acoustically. Recombination
occursrelativelyrapidly,sothephaseoftheoscillationsisfrozenintotheCMBatthetime
of lastscattering.
Because of the oscillation, the amplitude of uctuations at particular scales will be
enhancedrelativetotheprimordialscale-invariantamplitude. Thiseectappearsasaseries
of \acoustic peaks" inthe angular power spectrum. The rst peak in order of decreasing
angular scale corresponds to the sound horizon at the surface of last scattering; in other
words,the distancethat soundcould travel betweentheBig Bang andlastscattering.
geometryoftheuniverse. Forexample,aatuniverseisexpectedtohave anacousticpeak
at a multipoleof `=200,or1 Æ
scales.
There aredegeneracies inthisrelation;twodierent sets of parameterscan give nearly
identical angular power spectra. For example, a large fractional density of baryons in
the early universe can decrease thesound speed of the plasma,making thesound horizon
smaller. This eect mimics a change in curvature. By further constraining the shape of
theangularpowerspectrum,these degeneraciescan belargelyovercome. Forexample,the
ratioof power intherst and second peaks constrainsthe baryon density of theuniverse.
Another means of overcoming degeneracies is by including data from other branches of
cosmology which constrain parameters inorthogonal ways; for example, measurements of
lightelementabundances,mapsoflargescalestructure,measurementsofHubble'sconstant,
and Supernovae (SN1A) measurementsofthe expansionofthe universeto high redshift.
Because of the tiny amplitude of the anisotropy in the CMB, it has taken decades of
technological development of receivers, telescopes, and observation strategies forprecision
mapping to be successful. The past decade has seen a rapid series of advances in
mea-surement of the angular power spectrum and thecosmology that itimplies. Temperature
anisotropy was rst discovered on large scales (>7 Æ
) by the DMR instrument aboard the
COBE satellitein 1990. Moreover, at largescales it wasfoundthat thespectrumis
scale-invariant,lending furthersupportto theidea of ination.
Subsequently, a host of ground-based and balloon-borne experiments detected
aniso-tropy at a variety of angular scales (see Figure 1.1 for a summary). Starting with the
Saskatoon experiment [50 ], the eld beganto move from the realm of making a detection
to making spectroscopic measurements of power over a range of angular scales.
Ground-based experiments have been limited by atmospheric noise, prompting moves to dry and
high altitudesites, such as the Atacama desert in Chile[48 ] and theSouth Pole [16 ] with
interferometers thathave successfullydefeated atmosphericnoise[64]. Balloon-based
mea-surements have been limited by detector technology and integration time. Flights above
North Americaare normally5-6hours long and allowonlya smallregionof the skyto be
measured.
In1999resultswerereleasedrstbytheTOCOexperiment[48 ]and thenfromthe1997
Figure 1.1:
Measurementsof theangularpowerspectrum oftheCosmicMicrowave Backgroundinthe
vicinityoftherstpeakpublishedbetween1995andNovember1999. Thesolidcurveshows
thebestt modelfrom thetest ight of Boomerang (see Chapter5 of thisthesis).
1999,whichexploitedadvancesinreceiveranddetectortechnologyandthelongintegration
time available from the Long Duration Balloon (LDB) platform to make a highly precise
measurement of the angular scale of the rst acoustic peak [4 ]. A parabolic t to the
data nds that the peak is located at a multipole of ` = 203 6 (1 error). Standard
cosmological analysisimpliesthat thedensityparameter =1.01 0.06. The universe is
atto 6%.
Combined withother cosmologicaldata, an even more interestingpicture emerges. For
the total density of the universe to be so close to the critical density, a huge fraction of
it must be made up of dark matter. Furthermore, results from SN1A measurements of
theexpansionof the universeindicatethat Einstein'scosmological constant isnon-zero,
cosmological measurementindetail.
1.1 Overview of Boomerang
Boomerang incorporatesfour unique designfeatures which allow a precise measurement
of theangularpowerspectrumof theCMB.
First, Boomerang is a quasitotal-powerradiometer. The temperature of one part of
theskyismeasuredrelativeto itssurroundingsbyslowlyscanning(scanperiod 1minute)
theentiretelescopeinazimuth. TheoutputfromeachdetectorisACcoupledtoanamplier
at low frequency(f = 0.016 Hz). The stable, virtually transparent atmosphere at balloon
oat altitude and the intrinsic stability of the bolometric detectors and readout amplier
chainmake itpossibleto maplargeareas of theskywithhigh sensitivity.
Second,Boomerang isdesignedtotakeadvantageofthelongintegrationtimepossible
from a balloon borne platform own over the Antarctic. During the austral summer, the
polar vortex winds provide a stable orbit for balloons at the top of the stratosphere at
an altitude of roughly 38 km. This observation platform launched 1200 km from thepole
providesight durationsof 7to 20 days,thusallowingmeasurementsto berepeatedmany
times in order to check for systematic eects. The relatively small fraction (<10%) of
the sky that is accessible from a balloon platform duringthe austral summerfortuitously
includesthepart ofthesky thatislowest inforegroundcontamination.
Third, the Boomerang receiver has a high instantaneous sensitivity, due to its
opti-mized low-background bolometers and high-bandwidth feeds operating at cryogenic
tem-peratures. The channelsat 90 GHz and 150 GHz are positionedin frequencyto optimally
avoidgalactic foregroundcontamination. Inaddition,combiningthese channelswiththose
at 240 GHz and 400 GHz allowspowerfuldetection andremovalof foregroundsignals.
Finally, the Boomerang receiver uses re-imaging optics which provide good image
qualityoveralargefocalplane. Thereceiversimultaneouslymeasures16bolometerchannels
in8pixelswithbeamsseparatedbyupto4 Æ
onthesky. Thewideformatofthefocalplane
andlargenumberofdetectorsallowedbythere-imagingopticsprovidestheabilitytosearch
forandremove sourcesofcorrelated noise,sinceobservationsofa specicregionofthesky
2), the design and testing of the Boomerang instrument (Chapters 3 and 4) and its
performance during its 10.5 day ight above Antarctica during 1998-1999 (Chapter 6).
Chapter 5 contains a short section on the performance of the instrument during the test
ight of 1997. The calibration of the instrument with observations of the CMB dipole
is described in Chapter 7. A descriptionof the cosmological analysis and implications is
included in Chapter 8. Finally, the prospects for further research with the Boomerang
data are discussed in Chapter 9. The thesis emphasizes my contributions to the
Boom-erangexperiment,whichrequiredalargeinternationalcollaborationtodevelop,eld,and
produce results. My contributions were the development and testing of the receiver, the
Chapter 2 Theory
ThetheoryofhowuctuationsintheCosmicMicrowaveBackgroundareproducedandhow
they depend on various cosmological parameters has been well developed [61] [29 ]. This
thesisfocusesonthemeasurementofthesecosmologicalparametersasaninstrumentaland
an analyticalchallenge.
ExtractingcosmologicalinformationfromobservationsoftheCMBinvolvesthreesteps.
The rst step is the construction of the maximum likelihood map from the time stream
data. Second,apowerspectrum iscomputed fromthe map. Third,the power spectrumis
comparedto theoreticalpowerspectrafromvariousmodeluniversesto determinethemost
likelycosmological parameters.
2.1 Map Making
Following[68 ], thebolometertimestream dataare modeled asfollows:
d
t =P
tp
p +n
t
(2.1)
where d is the timestream data array,P is the pointing array which maps the data onto
thesky,is thetemperature of thesky,nis thedetector noise, t is thetimeindex and p
is the pixel index. Because Boomerang is a quasi total-power experiment, the pointing
array P contains onlyone non-zero element per row; at any given time, only one pixel is
beingobserved.
An estimator for theminimumvariance mapcan be foundbyminimizingthequantity
2
=(d P) y
N 1
(d P),whereN 1
istheinversetime-timecorrelationmatrixwhich
isdened ashnn y
i 1
:
~
=
P y
N 1
P
1
P y
N 1
d (2.2)
~ = P y P 1 P y d (2.3)
The term P y
d is a pixel array containing the sum of all measurements of a pixel and
P y
P
isthenumberofhitsperpixel. Thisversionoftheequationisequivalentto anaive
binning of the data in pixels. In the case of non-white noise, the map is still made by
binningthedata inpixels,except witha\whitening"lter, N 1
applied to thedata.
Thepracticalchallengesinvolvedinsolvingequation2.2areasfollows. First,thematrix
to be inverted is huge (n
pix n
pix
, typically 100,000 100,000 for Boomerang) and
involves considerable computational resources. Second, the noise correlation matrix must
be estimated fromthe timestream datad
t
itself because n
t
isnot a separately measurable
quantity. Third,there aregapsinthedata(duetocosmic rayhitsorother glitches) which
complicate estimationof thenoiseand lteringofthedata.
An iterative scheme was developed [58 ] to address these problems. For each iteration,
a noise time stream estimate is made by subtracting the signal estimate from the data
time stream. Next, a noise correlation matrix and a correction to the previous map is
constructed:
n (j)
= d P
~ (j) (2.4) N (j) 1 = hn (j) n (j)y i 1 (2.5) ~ (j+1) ~ (j) = P y WP 1 PN (j) 1 n (j) (2.6)
The initialmapused istheunlteredbinnedmap:
~ (0) = P y P 1 P y d (2.7)
Forcomputational speed,thelter W istaken to be thenumberofobservations perpixel.
Gapsinthedataareofparticularconcernwhenanylterisappliedtothedatabecause
each unagged sample is aected by the value of nearby agged data. To deal with this
issue, at each step in the iteration scheme, a new constrained realizationof the measured
A pixel-pixel noise correlation matrix is necessary for calculating the power spectrum
fromthemap. Thiscan beobtainedfrom thenoisetimestream thatisestimatedwiththis
iterative process: N pp 0 = P y N 1 tt 0 P 1 (2.8)
A map and a noisecorrelation matrixobtained inthisway can be used inthe subsequent
powerspectrumanalysis.
2.2 Angular Power Spectrum
The temperatureon theskyat each pixelp can beexpandedin sphericalharmonics:
p = X `m a `m B ` Y `m ( p ;' p ) (2.9) whereY `m
arethesphericalharmonicsandB
`
isthe\windowfunction"ofthebeam,orthe
patternof theexperimentalbeamin`space. The correlation betweensignalsis therefore:
S pp 0 = X `m X ` 0 m 0 ha `m a ` 0 m 0iB ` B ` 0Y `m ( p ;' p )Y ` 0 m 0( p 0;'
p
0) (2.10)
The uctuations are assumed to be isotropic, meaning that ha
`m a
` 0
m 0i =C
` Æ`` 0 Æmm 0 ,
sothesignal correlationsreduce to:
S pp 0 = X `
2`+1
4 C ` P ` ( pp 0 )B 2 ` (2.11) where P `
are the Legendre polynomials and
pp
0 is the angle between pixels p and p'.
The coeÆcientsC
`
completelycharacterizethe temperatureeldon the skyand providea
model-independentway to comparemeasurementswithmodels.
Because of incomplete sky coverage, C
`
cannot be determined independently for each
`. Therefore, the power spectrum is computed for `-space bins. The sky coverage also
determinesthesmallest`(the largest angularscale)at whichpowercan bemeasured.
The angularresolution oftheexperimentwhichmakestheobservations determinesthe
maximum `for which a power spectrum can becomputed. The beam shape of the
shape generally determines thecut-o insensitivity dueto the beamsize. For example, a
Gaussianbeamprole withFWHM
C
(inradians) yieldsa windowfunction:
B 2
` exp
`(`+1)
( p
8ln2 1 C ) 2 ! (2.12)
The value of `at which thesensitivityfallsto halfthemaximumsensitivityistherefore:
` c p 8 C (2.13)
For example, the Boomerang experiment had an angular resolution of 10 0
,therefore the
half-powervalueof `is 972.
The MADCAP software package [10 ] computes the power spectrum C
`
by using a
Newton-Raphsonmethod todetermine themaximumlikelihoodpowerspectrum. A
Gaus-sian probabilitydistribution forthemapgiven a particularpowerspectrumC isassumed:
P(mjC)=(2) Np=2 exp 1 2 (m T M 1
m+Tr[lnM])
(2.14)
where N
p
is the number of pixels, m is the experimental map, and M is the matrix of
average pixel-pixel correlations expected for thetheoretical power spectrum C. This leads
to the following expression for the log likelihood which must be maximized to determine
thebest-tpowerspectrum:
L(C)= 1 2 (m T M 1
m+Tr[lnM]) (2.15)
If this function were quadratic, the correction ÆC
0
to an initial guess C
0
that would
yield thepeak likelihoodwouldbe:
ÆC 0 = 0 @ " @ 2 L @C 2 # 1 @L @C 1 A C=C 0 (2.16)
L(C)isnotaperfectquadraticfunction,sothiscorrectionisnotstrictlycorrect.
There-fore, thecorrectionto C
`
is iterated untilÆC
n
2.3 Cosmological Parameters
Inthecontext ofcolddark matter (CDM)models, itispossibleto usethemeasured C
` to
accurately determine cosmological parameters such as the baryon content of the universe
(
b
), thetotalenergy densityoftheuniverse (),and thespectralshapeoftheprimordial
density uctuations(n
s ) [7 ].
Here, a maximum likelihood method is used to measure cosmological parameters. A
database of model power spectra are created assuming a particular range of parameters.
Thesearecompared to theexperimentalmeasurementsbyevaluatinga likelihoodfunction
for each model power spectrum which estimates the probability of this power spectrum
given themeasuredpower spectrum.
Given a set of parameters x with a prior probability distribution P(xjprior) and a
likelihood function of theparameters given theexperiment L(x), theBayes theorem gives
thetotal probabilitydistributionof theparameters:
P(x)/L(x)P(xjprior) (2.17)
If errorsinthecosmologicalparametersÆx=x x
0
aboutthemeanx
0
aresmall,then
L can be quadraticallyexpandedaboutthemaximum:
LL
m exp 0 @ 1 2 X ij F ij Æx i Æx j 1 A (2.18) where F ij
is theFisher informationmatrix, which is given by the derivatives of the CMB
powerspectrumwith respect to theparametersx:
F ij = X ` 1 (C ` ) 2 @C ` @x i @C ` @x j (2.19)
Oset-lognormalapproximationsto theFisherinformationmatrix[9 ]are usedto compute
thelikelihoodL(x)=P(C
B
jx) foreach combinationof parameters inthedatabase.
Some parametersarenotofinterestand can be marginalized. Marginalizedparameters
can includecosmological parameters whichare notof interest, orexperimental parameters
like-priors andintegrating over themarginalizedparameters x
m :
L=P(xjC
B )=
Z
P
prior
(x)L(x)dx
m
(2.20)
AglobalsearchformaximaofLisperformedfortheentiredatabase. Theaverageand1
points are determined by ndingthe location in the database at which the integral of L
Chapter 3 Instrument Design
ThischapterdescribesthedesignoftheBoomerang experiment. Theexperimenthasthe
following requirements: a high sensitivity to Cosmic Microwave Background uctuations
and the rejection of spurioussignals from the atmosphere, the ambient environment, and
astrophysicalsources.
In addition,all systemsintheBoomerang payloadmustoperateat balloon altitudes,
which include very low ambient pressure and extreme temperature swings (varying from
-50 Æ
C in the shade to +50 Æ
C in the Sun). The controlsystems in the payload must be
autonomous forthe7-14 days of along durationballoon ight.
The characterization ofthe instrument isdescribed inChapter4.
3.1 Telescope and Gondola
Thetelescopeisdesignedtosmoothlyscaninazimuthatxedelevation,andhasanattitude
control system similar to those described in [11 ], [12 ], and [13 ]. The azimuth pointing of
thetelescopeiscontrolledbytwotorquemotors(InlandQT6205d);thersttorquesalarge
ywheel, the other torques against the steelcables connecting the payloadto theballoon.
The two motors provideenough torqueto move thegondola inazimuthand to correct for
randomrotationoftheballoon. Theelevationofthetelescopecan becontrolledbytipping
the inner frame with a DC gear motor. An overview of the gondola frame is shown in
Figure3.1. Thecontrolhardwareconsistsoftworedundant386CPU's. Awatchdogcircuit
switchescontrolof thepointing fromone CPU ina fewmillisecondsincase of areboot.
Pointing informationisprovided bya dierentialGPSarray,a two-axis Sunsensor, an
encoderontheelevationaxis,andthreeorthogonalaxislaserrategyroscopes. Theazimuth
gyroscopeprovidesvelocityfeedbackforcontrollingthescanofthetelescope. Theabsolute
pointingdatafromtheSunsensorisusedtoresetthedriftinthegyroscopesandtoprovide
dataforpost-ight attitudereconstruction.
of Boomerang will include a star tracking camera with the proper lter to operate in
daytime.
Sun Shields
Ground Shield
Primary Mirror
Solar Panels
Flywheel
Cryostat
To Balloon
Figure 3.1:
An overview of theBoomerang gondola.
3.2 Thermal Design
Theextremeconditions oftheballoon-borneobservation platformrequiresome careinthe
thermaldesign of thepayload. Electronicsmust notbe allowed to get too hot ortoo cold
tofunction. Moreimportantly,thecryostat mustnotreachatemperatureatwhichO-rings
freeze, which could lead to a leak. The cryostat O-rings are made from Buna-N rubber,
which freeze at temperatures below-50 Æ
C.
The outside of the gondola is covered with 1 00
thick open-cell styrofoam panels (Dow
extrudedpolystyrene\bluefoam")with50mmylarbondedto25mthickaluminumfoil.
Withthemylarsidefacingoutwards,thecombination oflayersprovideshighreectivityin
theopticalbandandhighemissivityintheinfraredband,thusprotectingthepayloadfrom
structedforthepayload. Twocaseswereconsideredfortheinputpower{\cold"and\hot."
The cold case assumed that the payload is over water and receives 1044 W/m 2
radiation
fromtheSunanda9%albedoforthesurfaceoftheEarthfor177W/m 2
ofpower. Thehot
caseassumed thatthepayloadisabovefresh snowand receives1397 W/m 2
radiationfrom
the Sunand 213 W/m 2
from the Earth with an albedo of 95%. Table 3.2 shows the
pre-dictedrangeoftemperatures fora fewofthekeyexperimentalcomponents. Allelectronics
were expected to maintain reasonabletemperaturesduringthe ight,aswasthecryostat.
Item PredictedTemperature
Æ
C
Attitude ControlSystem -8 Æ
to 12 Æ
Data Storage System 17
Æ
to 27 Æ
Data AcquisitionSystem -7 Æ to 6 Æ Cryostat -29 Æ to 27 Æ
BolometerReadout Electronics -31 Æ
to 2 Æ
SolarArray 55
Æ
to 92 Æ
Table3.1:
Predictedequilibriumtemperaturesforelectronic componentsof thepayload.
3.3 Cryogenics
Thecryogenic systemforBoomerang keeps thedetectorsat an operatingtemperatureof
0.28KfortheentiretwoweeksofanLDBballoonight. Ithas60litersofvolumeat2Kto
contain the photometers, re-imagingoptics, baes, and cold preampliers. The system is
composedof a self-containedsorption-pumped 3
He refrigerator[37 ] and ahelium/nitrogen
main cryostat [38 ]. The 3
He fridge contains 40 liters STP of 3
He and runs at 0.280 K
with a load of 27 W. The main cryostat holds 65 liters of liquid nitrogen and 60 liters
of liquid helium. The tanks, toroidal in shape, are suspended by means of Kevlar cords.
Radiation from the 300 K ambient environment to the77 K tank is shieldedbya blanket
of superinsulation aluminizedmylar (30 layers). The 77 K tank contains copper braidto
maintain cryogenic thermal contact in the event of the nitrogen freezing. Radiation from
the77 Ktankto theheliumtankis shieldedbyan intermediate temperature shieldcooled
to <20 Kbythe 4
plagued with thermo-acoustic oscillations, making it necessary to pump the bath down
in a controlled manner over the course of 12 hours. Therefore, Boomerang is launched
withtheheliumbathalreadyatlowpressure. Externalplumbingwithelectricallyactuated
valves is used to pressurize the nitrogen bath during the ight and to maintain the low
pressureof theheliumbathduringascent. At oat altitudetheheliumvalve isopenedfor
theremainderof theight.
3.4 Optics
The Boomerang telescope consists of an ambient temperature (-20 Æ
C in the shade at
oat altitude) o-axis paraboloidalprimary mirror which feeds a pair of cold re-imaging
mirrorsinsidethecryostat. Theprimarymirroris1.3mindiameterand hasa45 Æ
o-axis
angle for a projected size of 1.3 1.2 m. The mirror can be tipped in elevation by +10 Æ
and 12 Æ
to cover elevation angles from 33 Æ
to 55 Æ
. Radiation from the sky is reected
bytheprimary mirrorand passes into thecryostat througha thin (50 m)polypropylene
window near the prime focus. The window is dividedin two circles sideby side, each 6.6
cm indiameter. Filters to reject high frequencyradiation and to reduce the thermalload
on the 2 K and 0.3 K stages of the cryostat are mounted on the 77 K and 2 K shieldsin
front of the cold mirrors. These are capacitive multilayer metal mesh low pass lters [72 ]
withcutosat540GHzand480GHzrespectively. Neutraldensitylterswithtransmission
of 1.5% used to reduce the loading on the detectors for ground testing are mounted on a
mechanism which can move the lters in and out of the beam near the prime focus. See
Figure 3.2foran overview of theoptics.
The o-axis ellipsoidalsecondary and tertiary mirrors are surroundedby blackbaes
and re-image the prime focus onto a detector focal plane with diraction-limited
perfor-mance at 1 mmovera 2 Æ
5 Æ
eldof view.
The re-imaging optics form an image of the primary mirror at the 10 cm diameter
tertiarymirror. Thesizeofthetertiarymirrorlimitstheilluminationoftheprimarymirror
to thecentral50% inareato reduce sidelobe response.
The secondaryand tertiarymirrors areellipsoidalreectorswitheective focallengths
Figure 3.2:
An overview of theBoomerang optics.
mirror is 10 cm in diameter, corresponding to an 85 cm diameter aperture on the 1.2 m
diameterprimary.
The Boomerang focal plane containsa combination of four single-frequency
channel-s fed by smooth-walled conical horns and four multicolor photometers fed by parabolic
concentrators (Winston horns). Although the image quality from the optics is diraction
limitedat 150GHzovera2 Æ
5 Æ
eld,allofthefeedopticsareplacedinsidetwocircles2 Æ
indiameter,separatedcenterto centerby3:5 Æ
. The focalplaneareaoutsidethesecirclesis
vignetted by blockinglters at the entrance to theoptics boxand on the77 K shield and
is unusable. Dueto thecurvature ofthe focalplane, thehorns are placedat the positions
of thebeam centroidsdetermined fromgeometric raytracing. Allofthe feedsareoriented
towards thecenter of thetertiarymirror.
A schematic of the relative positions and sizes of the beams on the sky is shown in
Figure3.3. Thefocalplane layoutwaschosensoasto repeatobservationsofthesame part
of the sky on many dierent time scales. The telescope scans in azimuth, and at 1 Æ
/sat
45 Æ
elevation,channelsonoppositesidesof thefocal planewillobservethesame sky2 s
apart. TherotationoftheskyaboveAntarcticaallowseach rowofdetectorsto observethe
admixture ofcarbonlampblackto assistintherejection ofstrayhigh-frequency light.
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20’
12’
30’
2.4
90 GHz
150 GHz
150/240/400 GHz
2.7
3.5
4.0
A1
A2
A
A
B
B
B1
B2
Figure 3.3:The Boomerang focal plane projected onto the sky. The labels \A," \B," \A1," \A2,"
\B1," and \B2" aredesignationsforeach ofthechannels.
3.5 Bandpass Selection
The Boomerang bands were chosen to provide a wide enough frequency coverage that
CMB uctuations can be unambiguously measured and foreground sources of signal can
be separated from contaminants. SeeFigure 3.4 for a comparison of the four bandpasses
selected withthe spectra of theforegroundcontaminants: synchrotronradiation[65 ],
free-free emission [46 ], and thermal emission from dust [21]. Despite the small atmospheric
optical depth at balloon oat altitude, strong atmosphericabsorption lines were avoided.
Bandscenteredat90GHz,150GHz,and240GHzwereselectedtomeasureprimarilyCMB
anisotropy, and a fourth band at 400 GHz was chosen to monitor thermal dust emission,
which wasexpected to be themostimportantcontaminant.
3.6 Single Mode Feed
Thefocalplanecontainsfoursinglefrequencychannels;twoat90GHzandtwoat150 GHz
(see Figure 3.3). The feeds follow the Planck HFI prototype design [15 ]. The
Figure 3.4:
The Boomerang bands compared with spectra of sources on the sky. The upper
panel shows a model of the atmosphere at balloon oat atmosphere (HITRAN model,
http://www.hitran.com). Thethicklineshows thespectrumof theCMB. Thelowerpanel
compares the spectrum of CMB anisotropy with various galactic foregrounds. The heavy
solid line shows the spectrum of a CMB anisotropy. The small-dashed line shows the
spectrum of free-free radiation, the large-dashed line shows the spectrum of synchrotron
radiation,andthedottedlineshowsthespectrumofthermaldustemission. Thelightsolid
line shows the sum of the three galactic components. In each panel, the hatched columns
show theselected Boomerang bands.
beam waist at the phase center of the horn, which in turn denes a relation between the
hornaperturediameterand thehorn lengthasfollows.
Fora smoothwalledconicalfeed,thebeamwaistattheapertureisw
a
=0:768a,where
aistheapertureradius. ForahornpropagatingasingleGaussianmode,thewaistdiameter
at thephase centeris
w 0 =w a s 1+ w 2 a L 2 (3.1)
whereListhelengthofthehorn. Thewaistsizeasafunctionofdistancezfromthephase
center isgiven by:
w(z)=w
0 s 1+ z w 2 0 2 (3.2)
The hornaperturesizeisthendeterminedbyrequiringadesirededgetaperat theLyot
stop. Let the Lyot stop have radius r
L
and distance z
L
from the phase center. The edge
taperis:
T =10log
10 exp 2r 2 L w 2 (z L ) ! (3.3)
Specifyingtheedge taperTyieldsa beam waistat theLyot stop:
w(z
L )=r
L s
8:686
T(dB)
(3.4)
Solvingforw
0
,arange ofaperture sizes(a) and hornlengths(L)are specied:
L= w 2 a s w a w 0 2 1 (3.5)
The solution with minimum length is selected, which gives diameters of 19.66 mm and
11.8 mmand are anglesof19.7 Æ
and 10.5 Æ
forthe 90GHz and 150 GHz structures.
At the throat of the entrance feed, a 3 length of waveguide with a radius r =0:35
denesthelowfrequencyedgeofthepassbandandsetsthethroughputofthefeedstructure
suchthatonlythelowestorderGaussianmodeispropagatedthroughtherestofthesystem.
Capacitivemetalmeshlters denetheupperedgeof theband[72][34 ]. Eachofthese
has high-frequency leaks at twice and three times the cuto frequency and must be used
in combination with another lter which blocks the leaks. In the 150 GHz feeds, lters
with cutos at 168 GHz and 198 GHz are used, and in the 90 GHz feeds, 99 GHz and
177 GHz lters are used. An f/4 re-expanding feed re-emits the light through the lters
and anidenticalre-concentrating feedconcentrates thelightinto thebolometercavity. An
alkali-halide/carbon layered polyethylene lter [74 ] blocks infrared and optical light ( >
1650 GHz)and is tunedinthicknessto maximizetransmissionat the centerof theband.
Identicallowdensitypolyethelylenehyperboliclensesareplacedat theapertureofeach
of thetwo face-to-face horns. The focallengthof thelenses is setto maximally match the
lowest order Gaussian mode propagating from each of the horns through the intervening
lters inthethinlensapproximation.
A re-entrant black bae heat sunk to the1.8 Kstage surrounds the gap between the
twohorns,preventingstraylightfromenteringtheopticalsystem. Theinnersurfaceofthe
bae ispaintedwithBockblack[6].
Integrating Cavity
Bolometer
Thin Lenses
0.3 K
Waveguide
168 GHz LP
Alkali-Halide Filter
198 GHz LP
1.8 K
Baffles
Figure 3.5:
Schematic ofa 150 GHz singlefrequency feedstructure.
3.7 Multi-color Photometer
The focal plane for the Antarctic LDB ight of Boomerang contains 4 three-color
regionof skythroughthesame columnof atmosphereinthree frequencybands.
Thephotometersarefedbyback-to-backparabolicconcentrators(Winstonhorns)
mount-ed on the2 Kstage. The inputhornis set at f/3.4 to maximallycoupleto the re-imaging
optics. The horn has an entrance aperture diameter of 1 cm producing a 12 0
beam on
the sky, corresponding to the diraction limit at 150 GHz. The horn has a short are in
the shape of a 45 Æ
section of a circle witha diameter of 1.3 cm to reduce sidelobes at low
frequency.
The throughput of the photometer is dened by the 1.45 mm diameter exit aperture
of the Winston horn to be 0.05 cm 2
sr. The re-expanding Winston horn has an aperture
which matcheslight pipe locatedon the0.3 Kstageacross a 0.5mmgap. At the entrance
to the lightpipe,there is a metalmeshlowpasslter witha cuto of 480 GHz to improve
theout-of-band rejection.
In the light pipe, two dichroic lters tipped 22.5 Æ
o-axis direct light at frequencies
greater than 270 GHz and 180 GHz through inductive multi-layer metal mesh bandpass
lters which pass 400 GHz and 240 GHz, respectively. Radiation at frequency less than
180 GHz is transmittedthroughthetwo dichroicsand througha 150 GHz bandpasslter.
Parabolichornsconcentratethelightintointegratingbolometercavitieswhereitisdetected.
3.8 Calibration Lamp
A 1 cm diameter hole located in the center of the tertiary mirror contains a handmade
micromesh bolometer with copper leads for use as a calibration lamp. An NTD-4 chip
provides highimpedanceat 2K. The electronicssendaxed current ontheorder ofa few
microampsthroughthebolometerfor1second every 13 minuteswhich providesan optical
sourceto beused asatransfer calibrator.
Thecalibrationlampwasusedtodeterminethedetectorbiasatwhichmaximumoptical
responsivityoccursand to trackdrifts inresponsivityduringtheight.
3.9 Detectors
Figure 3.6:
Schematic ofthe Boomerang multi-colorphotometer.
suchasabovetheEarth'spolarregions. Thebolometerabsorberconsistsofasiliconnitride
meshratherthana solid. A NeutronTransmutationDoped (NTD) germaniumthermistor
isused to measure thetemperatureriseof theabsorber.
Figure 3.7:
A bolometer used by Boomerang. The web absorber shown here is 4 mm in diameter.
The entireringis 3/4 00
indiameter.
0
n d s G
(GHz) (mm) (m) (pW/K) (ms)
90 2 5.6 400 60 24
150 6 3.4 425 60 15
240 4 4.0 150 180 10
400 4 4.0 150 360 6
Table3.2:
Boomerang bolometerdesignparameters,includingtheweb diameterd,thegridspacing
s, thetarget thermalconductance,G andtarget timeconstant,.
use semiconductor thermistors,so Johnsonnoise isalso a contributorto theoverall noise.
In this case the Noise Equivalent Power NEP = p
4k
B T
2
G , where G is the thermal
conductance,T isthebathtemperature, and 1:2forourchoice ofthermistormaterial.
Foragivenbackgroundload,Q,maximumsensitivityisachievedforGQ=T. Thedesired
time constant of a bolometer C=G for a given modulation scheme sets a limit on the
minimum thermal conductance that can be selected. The bolometers are optimized for a
10 K RJ spectrum background using the optimization scheme of [26 ], and the resulting
3.10 Integrating Cavity
Since an electriceldgoesto zero at thesurfaceof a conductor, anaive guess would place
the bolometer absorber at a distance =4 from the exit aperture of the reconcentrating
horn and at a distance =4 from a backshort inorder to maximize theelectric eld inthe
cavityattheabsorber. SimulationsofabolometercavityusingtheHFSSsoftware package
(Hewlett-Packard) showthat the=4distance isindeedcorrect [24 ].
3.11 Readout Electronics
The slow scan observation scheme requires stabilityof the detector and the readout
elec-tronics from the bolometerthermalcuto frequency( 10 Hz) down to the characteristic
scanfrequencyattensofmHz. Thedetectorreadoutschemeimplementsanelectronic
mod-ulation/demodulationtechniqueto providestabilityatlowfrequency,bymovingthesignal
bandwidthwellabove the 1/f knee of theJFET and the warm electronics. Modulation is
achieved by biasing the bolometerwith AC current, hence modulating its responsivity by
a known periodicfunction.
ThedetectorsareACvoltagebiasedwitha318Hzsinewave. Dual10Mloadresistors
provideanapproximatecurrentbias. ThesignalfromeachdetectorpassesthroughaJFET
source follower circuit on the 2 K stage. It provides gain of the bolometer signal power,
thus reducing the susceptibility to microphonics. The JFETs were selected forlow power
dissipationand arepackagedbyIRLabs (IR LabsTIA).
ThesignalfromthecoldJFETspassesthroughapreamplierstageandaBandPass
Fil-terstagepriorreachingthesynchronousdetector. Thesynchronousdemodulator(Lock-In)
multipliesthesignalsynchronouslyby+1/-1timesthebiasreference. A4-poleButterworth
low pass lter with cuto at 20 Hz removes the high-frequency terms of the product and
actsasananti-aliasinglterfortheDataAcquisitionSystem (DAS). AnAC-couplinghigh
pass lter with a cuto at 0.016 Hz and extra gain is applied to the signal to match the
signaldynamics to theDAS dynamicrange.
Including thecoldJFETs, theentirereadoutelectronicscontribute lessthan
10nV
rms =
p
Hz to the signal noise in thefrequency range of 0.016 Hz to 20 Hz. The
Cold JFET Pair
Load Resistor
Bolometer
Load Resistor
Cryostat
4 K Faraday Cage
0.3 K Faraday Cage
Bias
JFET Power
Room Temparture
Faraday Cage
Lock-in
4 pole LPF
AC HPF
Signal Out
RF filters: Spectrum Pi filters
RF filters: Eccosorb feedthrough
RF filters: 20 pF feedthrough
Figure 3.8:
A schematic ofthe Boomerang readoutelectronics.
3.12 Rejection of RF and Microphonic Pickup
BolometricdetectorsaresusceptibletospuriousheatingfromRFdissipatedinthe
JFET source followers. There are several microwave transmitters for satellite
communi-cations on board theBoomerang payloadwhich arepotentially sources of RF pickupin
thebolometers. The frequencies ofconcern arean ARGOStransmitter at 400 MHz and a
TDRSS2.3GHz transmitter.
Toguardagainstdegradationofthedetectorperformance,thebolometercavitydenes
a Faraday cage into which the only entrance is through the feed horn. Interfering RF is
kept out of this entrance path by the waveguide cuto in each feed. The wires exit the
cavity through surface mount 20 pF feed-through capacitors to ground mounted on the
wires exiting the Faraday cage which provide protection from RFI close to the detector.
Allbolometerleadsfromthe JFETpreampstage to thewarm ampliersrunthroughcast
eccosorb lters. The cast eccosorblters consist of striplinecablespotted incast eccosorb
(EV Roberts CR-124) and have signicant attenuation above a few GHz. These provide
theonlyexitfrom a secondFaradaycage surroundingthe2 Kstage.
A third Faraday cage surrounds the bolometer, cryostat electronics and wiring. The
ampliedsignalsarefed throughSpectrum lters(Spectrum 1212-0502) to theDAS.
The low suspended mass of the micromesh bolometers naturally provides a reduction
in microphonicresponse (measured resonant frequency >1 kHz); however, care was taken
to securely tie down the high impedance wiring connecting the bolometers to the JFET
follower stage. The wiring on the 0.3 K stage consists of 27 AWG shielded twisted pair
which was strapped down with Teon tape and nylon cord. Between the 0.3 K and 2 K
stages, cablesmade withlow thermalconductivity50 mmanganinwire were strappedto
the vespel posts. Inside the JFET box, all high impedance wires were potted in silicone
Chapter 4 Instrument Characterization
Thischapter describesthetestingof thereceivercomponentsdescribed inChapter 3,both
asseparate components andasan integrated receiver. AppendixA containsasummary of
bolometerparameters,measuredbothpre-ight and in-ight.
4.1 Optics Testbed
Atestbedwassetuptotestvariousopticalcomponents,includingthein-bandtransmission
oftheblockinglters,thetransmissionof theopticsbox,and thebeampatternofthefeed
horns. The testbedconsisted of a dewar witha 300 mK bolometerfed bya Winstonhorn
and auniversalmount on theoutsideof thedewarwindowto whichanyhornto betested
could be mounted. Thebolometerwasrunintwo dierentltercongurations;90 GHzto
testthe90GHzsinglemodefeed,and150GHztotestthe150GHzsinglemodefeedandthe
multi-colorphotometerfeedat150 GHz. Inbothcases,theabove bandleakwasmeasured
witha thickgrilllterto beless than5%. The testbed achieveda NoiseEquivalent Power
(NEP) of 10 15
W/ p
Hzwith abolometerwitha thermalroll-oat 5 Hz.
4.2 Blocking Filters
The in-bandtransmission of the blockinglters was measured at two dierence angles of
incidence using the optics testbed and a chopped 77 K blackbody. The band-integrated
transmissionofthelters weremeasuredat f/2 bymountingthem intheirproperposition
in the optics box, and at f/3.3 by placing the lters directly between the feed and the
source. Inboththe150 GHz and90GHz cases,theltershadhigherin-bandtransmission
at higher f/ number. This is dueto the fact that there are more o-axis rays in a low f/
number beam and the eective thickness seen by these rays is dierent than that of the
on-axisrays. SeeTable4.2fora summary oftheresults.
The predictedin-bandtransmissionat 150 GHz of thecombination of ltersis
Filter Type 150 GHz 150 GHz 90 GHz 90 GHz
f/2 f/3.3 f/2 f/3.3
480 GHz LP metalmesh 0.95 0.96 0.94 0.96
540 GHZLP metalmesh 0.98 0.98 0.95 0.94
1650 GHZLP alkali-halide 0.94 0.95 0.81 0.85
total 0.88 0.69
Table4.1:
The in-bandtransmissionof thethree blockinglters measuredintheopticstestbed.
designed for maximum transmission at 150 GHz, which is near minimum transmissionat
90 GHz. As seen in Table 4.2 it is readily apparent that including the alkali-halidelter
takes away a large amount of optical eÆciency. Due to these tests, it was decided to use
smaller lters and place them within each feed structure, so that the thickness could be
optimizedforeach frequency.
4.3 Beam Maps of Horns
The main lobe response of the feed horns was mapped using two methods. First, a
two-dimensional map was made using a 200 Æ
C source viewed through a 19 mm aperture at
a distance of 15 cm from the feed mounted on the front of the optics testbed cryostat.
Second, a one-dimensional slice of the beam pattern was made using a 900 Æ
C blackbody
source viewed through a 25 mm aperture at a distance of 28 cm from the feed. The 2D
maps were made to a noise-limited sensitivity of around -15 dB from the peak, whilethe
1Dmaps achieved a sensitivityof -20dBof thepeaksignal.
Twocorrectionsweremadeto thedata. First,aview factorofthecosineoftheo-axis
angle wasremoved. A correction was then made forthe angular sizeof the source. Allof
the beams were well t by a Gaussian as expected for a horn which propagates a single
mode. These correctionsreducedthemeasured beamFWHM by7-8%. SeeTable4.3fora
summary of thecorrected beam sizeand the edge taperat the tertiary mirror. Figure 4.1
shows the beam maps of theentrance feed. Vertical linesindicatethe locationof theedge
ofthetertiary mirror. The measurededgetaperat thetertiaryagreeswellwiththedesign
Figure 4.1:
Beammaps ofthetwo 150 GHz entrance horns. Theverticallines indicatethe edgeofthe
tertiary mirror. The designedgetaperis -5dB.
Horn FWHM FWHM edgetaperon tertiary designedgetaper
(degrees) (cmon tertiary) (dBfrom peak) (dBfrom peak)
90 GHz 16.5 9.8 -4.5 -5
150GHz sm 11.0 6.4 -6.4 -5
150 GHz mm 11.2 6.5 -6.1 -5
Table4.2:
The measuredbeam sizeandtertiary edgetaper ofthree feedhorns.
4.4 Optics Box
oftheopticsbox. Thissignalwascomparedtothesignalfrom thesourceobserved without
theoptics boxintervening.
The transmissionof theopticsboxwasalsomeasuredwiththeblockingltersinplace.
SeeTable4.4fora summary.
Position 90 GHz 150 GHz 90 GHz +lters 150 GHz +lters
90A 0.68 0.65 0.47 0.57
150A 0.65 0.66 0.45 0.58
90B 0.68 0.66 0.47 0.59
150B 0.64 0.67 0.44 0.58
A1 0.77 0.68
A2 0.76 0.67
B1 0.76 0.67
B2 0.79 0.70
Table4.3:
Themeasuredtransmissionoftheopticsboxwithfeedsmountedintheeightpositions,with
and without the blocking lters installed. The rst four rows of the 150 GHz column are
usingdatafrom thesingle mode feedand the lastfourareusingdata fromthe multimode
feed.
The multimodefeedtransmitsbetterthroughtheopticsboxthanthesinglemodefeed,
despite the fact that their beam patterns are virtually identical. This is due to the fact
that thesingle mode feeds view thetertiary at a largero-axis angle and therefore view a
smaller eective mirrorareathanthe multimodefeeds.
4.5 Feed Testbed
The full feed structures were tested in a dewar with a 300 mK bolometer. The dewar
wascongured inthree dierent ways;withthemulti-colorphotometer, the90 GHz single
mode feedand the150 GHzsingle mode feed. The front-end blockinglters were identical
to those usedin thenalBoomerang receiver. Thebolometerused intheoptics testbed
was usedinthefeedtestbed.
A77Kthermalsourcewasoftenusedasameasurementsource;thisconsistedofeccosorb
soaked in a bath of LN
2
. A styrofoam bucket containing a piece of eccosorb largeenough
4.6 Optical EÆciency of Feed Structures
Optical eÆciency of each of the feed structures was measured by comparison of two I-V
curvesviewingloadsattwodierenttemperatures(77Kand300K).SeeFigure4.2fortwo
load curvesmeasured withthe 150 GHz single mode feed. At equal bolometerresistances
(equivalent to equalbolometertemperatures), the total powerdissipated inthe bolometer
isequal: R 77 = R 300 (4.1) T 77 = T 300 (4.2) P 77 e +Q 77 opt G = P 300 e +Q 300 opt G (4.3)
wherethesuperscripts77and300refertothethermalloadtemperature,Risthebolometer
resistance, G is the thermal conductance, T is the bolometer temperature, Q
opt is the
incidentopticalpower,andP
e
istheelectricalpowerdissipatedinthebolometer. Sincethe
thermalconductivity is a functionof temperature, it is equalin thetwo cases. Therefore,
thedierence inelectricalpower(P
e
)between thetwo cases is equivalent to thedierence
inincidentoptical power(Q):
P 77 e +Q 77 opt = P 300 e +Q 300 opt (4.4) Q 77 opt Q 300 opt = P 300 e P 77 e (4.5)
The assumption that equal bolometer resistance impliesequal bolometer temperature
breaks down in the case where the \electric eld eect" contributes to the bolometer
re-sistance. In this regime,the bolometer resistance becomesa function of voltage as wellas
temperature:
R=R
0 exp 0 @ s T BV T m+1 1 A (4.6)
where , R
0
rsttermintheexponentialcontributes. Ifthesecondtermcontributes,however, anequal
resistance does not imply an equal temperature. A test for this eect is to measure the
dierenceinelectricalpoweratseveraldierentbiasvoltagesandtocheckthatthedierence
inpowerisequal.
Thepredicteddierenceinopticalpowercanbedeterminedbyintegrationofthe
black-bodycurve:
Q=A
Z
B
(300K)t
d
Z
B
(77K)t
d
(4.7)
where A is the throughput of the feed, B
is the blackbody function, and t
is the
bandpass function of the lters. The measured dierence in power is compared to this
predictedincidentpowerto determinean optical eÆciency.
With thismethod, the 150 GHz and 90 GHz feed structures are foundto have optical
eÆciencies of 42% and 36%, respectively, while the multi-color photometer channels are
found to have eÆciencies of 27% and 23% at 150 GHz and 240 GHz, respectively. The
opticaleÆciency at 400 GHz wasnotmeasureduntilthephotometers were integratedinto
theightreceiver.
Combiningthemeasurementsof thefeedstructureoptical eÆciency withthe
measure-mentsof thetransmissionoftheopticsboxprovidesapredictionoftheopticaleÆciencyof
eachBoomerang channel. Forthesinglemodechannels,24%isexpectedatboth90 GHz
and 150 GHz. In the multi-color photometer, 18% and 15% eÆciencies are expected at
150 GHz and 240 GHz,respectively.
4.7 Characterization of Neutral Density Filters
The neutraldensity lters(NDF) usedinBoomerang consist ofa lightlymetallizedthin
(25m)lmof polyethylene mountedinan aluminumring. Thering is\wedged"soasto
tip the lm at a small angle relative to the principal optical axis to avoid the creation of
reective cavitiesin theoptics.
Measurement of the transmission of theNDF usingthe Boomerang bolometric
pho-tometersisdiÆcultbecausetheopticalpowerdierencebetweenthelteredand unltered
Figure 4.2:
Comparisonof load curvesof the 150 GHz single mode feed withthe bolometerviewing a
77Kload(bluecurve)and a300Kload(redcurve)throughtheneutraldensitylter. The
upper panel shows current versus voltage. The lower panel shows electrical power versus
resistance. The dierence in electricalpower is foundto be 1.5 pW. The predicted power
dierence is 224 pW times the neutral density lter's transmission of 1.6%, yielding an
optical eÆciency measurement of 42%. The dierence inpoweris independent of the bias
voltage, showingthat theelectriceld eect isnot contributing.
opticalpowerincidentonthebolometerisafactorof50to100dierentbetweentheltered
and unlteredcases. To bypassthenon-linearityproblems,the bolometer wasbiasedwith
enoughelectricpowerthatthechangeinopticalpowerwastiny(1%oftheelectricpower)
incomparison,therebykeepingthebolometeratthe sameresponsivityinboththeltered
erang multi-color photometer. On the rst day, the bolometers viewed the chopped
300 K/77 Ksourcedirectly. Onthesecond day,thebolometers viewed thesource through
the NDF, which was mounted on the 2 K stage in the feed testbed. The signals were
measuredat a rangeof bolometer biaspowers andcompared betweendays.
The background optical load on the 150 GHz bolometer is estimated to be 150 pW
without the NDF in the beam and 2 pW with the NDF in the beam. The bias on the
detector was increasedso that the electrical power dissipated in the detector is at least a
factor of 100 greaterthantheoptical powerdierence.
Figure 4.3shows themeasuredtransmissionasafunctionofbolometerbiaspower. The
transmissionateachofthefrequencies150,240,and400GHzweremeasured. Atlargebias,
the resultsbecome independentof bias,showing that non-linearitiesinthe detectors were
overcome. Thisparticularltershowsatransmissionof1.5%. The scatterinthemeasured
transmission show that the results are good to 20%. Additionally, the transmission of
theNDF is seento beindependentof electromagneticfrequency.
Figure 4.3:
Measured transmissionof theneutral density lters asa functionof bolometer bias power
The transmission of the two NDF's that were measured in this manner was found to
increasefromtheir300 Ktransmissionwhen cooled to2K. Thetypicalincreasewasabout
10%,whichisconsistentwithanincreasedconductivityofthemetallmatlowtemperature.
4.8 Integrated Focal Plane
Thefocalplaneconsistsoftwoeachofthe90GHzand150 GHzsinglemodefeedsandfour
multi-color photometers, for a total of 16 optical bolometers. Two dark bolometers and
a 5 M resistor were installed on the focal plane as diagnostic channels. One bolometer
(Dark A)wasinthesame bolometermount astheother detectors to providea monitorof
thermaluctuationsinthecoldstage. Thesecond(DarkB)ismounteddirectlyonthefocal
plane. A groundedJFETwasalso readoutto monitorproblemswiththe JFETstage. As
a monitorforproblems withthebias generator and the warm electronics, thebias voltage
foreach readoutboxwasalso amplied(Bias1-4) and readoutbytheDAS.
The detectors wereread outby fourelectronicsboxes. TheACbias generatorsin each
electronicsboxwereslavedto a masterbias generator. Witha slaved bias,signal crosstalk
between boxes occurs at zero frequency. The rst box read out the single mode channels
(90A, 90B, 150A, and 150B) and the grounded JFET channel (GNDFET). The second
box read out themulti-moded150 GHz channels (150A1, 150A2, 150B1, and 150B2) and
one dark channel(Dark A). Thethirdbox readoutthe240 GHz channels(240A1, 240A2,
240B1, and240B2)andtheotherdarkchannel(DarkB).Thenalelectronicsboxreadout
the400 GHz channels(400A1, 400A2, 400B1, 400B2) and theresistor(Load Res).
See Figure3.3for aschematic of whereeach beamappears onthe sky.
4.9 Spectral Bandpass
The spectralbandpass oftheinstrument wasmeasuredbeforeight usingaFourier
trans-formspectrometer(FTS).Interferogramsaremeasuredforeachchanneland Fourier
trans-formed to measure the spectral response. The nal spectra are corrected for the 250 m
thickpolypropylenebeamsplitterandthespectrumofthe77Kthermalsource. Abandpass
Figure 4.4:
Relative spectral response of the four Boomerang channels. The peak response in each
bandhasbeennormalized to unity. The signal seenbelow-35dB isthought to be rectied
noise.
4.10 Blue Leaks
Blockingeach bandwith a high pass lter (thick grilllter) with a cut-o frequency just
above the band allows a search for spectral leaks at high frequency. Since this test is
sensitiveto integrated leaks,amuchhighersignalto noisemeasurementcan bemadethan
with theFTS. The signalsproducedbya chopped LN
2
source viewed withand withouta
thick grill high pass lter in the beam were compared. Table 4.10 lists the results. The
thick grill lter reduces the throughput of the beam, so a conservative factor of two was
included to correctforthiseect.
A high frequency leak in the bandpass can potentially make a channel more sensitive
to galactic dust emissionthan to CMB anisotropy. Therefore, itis usefulto determinethe
signaldueto a sourcewitha dust spectrum.
R RJ = R B 300 B 77 t l eak ()d R (B 300 B 77 )t band ()d (4.8)
The desired quantityis theratioofin-band to out-of-bandpowerfrom adust source:
R dust = R B dust t l eak ()d R B dust t band ()d (4.9)
Without detailed spectroscopy, assumptions must be made aboutthe spectral
charac-teristics of the leak and aboutthe spectrum of dust. We assume that the spectral leak is
atinresponse from thecuto of thehighpass lterto the cuto ofthe alkali-halidelter
(1650GHz). The amplitudeof theleakistherefore:
T l eak = R RJ R band B 300 B 77 t band ()d R 1650 GHz 0 ( B 300 B 77 )t band ()d (4.10)
For a dust spectrum, we choose a single-component model with a frequency-varying
emissivity:
B
dust =
B( ;T
dust
) (4.11)
We choose favoredvalues ofthe emissivityspectral indexand the dusttemperature,
= 1.7 and T
dust
= 20 K [21 ]. The estimated in-band to out-of-band ratios are shown in
Table 4.10. While thenumbersappear highforthelowest frequencychannels,they are no
callfor alarm. Roughlya quarter of thesignal dueto dust in the90 GHz channels comes
fromtheout-of-bandleakratherthanfromtheband. Asshownlater,theabsoluteresponse
of the90 GHz channelsto dust isvery weak.
4.11 Load Curves
Currentversusvoltage curvesweremeasuredforeachofthedetectorsunder\ight"loading
conditionswhere a300 Kblackload wasviewed throughthe1.5% neutraldensitylters.
The loadcurvesweretaken automatically;each detector wasbiasedwitha DC current
which wasrampedbetween0A andtwice thecurrentat whichpeakvoltage wasobtained.
0
Thickgrillltercuto out-of-bandpower out-of-band power
(GHz) (GHz) (R-J) (dust)
90 150 1.7% 23%
150 sm 230 0.5% 3.1%
150 mm 230 0.9% 5.7%
240 300 0.8% 2.6%
400 540 0.5% 0.8%
Table4.4:
The integratedpowerfrom above-band leaksin each channelfora sourcewitha
Rayleigh-Jeansspectrumandwithadustspectrum. Theresultsshownaretheaverageoverchannels
at eachfrequency. Because ofdierentlteringschemes, the150GHz channelsaredivided
into single-mode (sm)and multi-mode(mm) here.
The thermistormaterial(NTD Germanium)givesa resistanceasa functionof
temper-ature asfollows:
R=R
0 exp
0
@ s
T 1
A
(4.12)
TheparametersandR
0
aresetbythematerialpropertiesandgeometryofthethermistor.
In thiscase R
0
=300 and= 30 K.
Figures 4.5 through 4.8 show the load curves of all of the detectors measured on the
ground. The thermal conductivity, G, is measured from the slope of temperature versus
electrical power (Table 4.11). The electrical responsivity is the derivative of the voltage
withrespectto electrical power.
The biasatwhichpeakresponsivityoccursisthebiasusedinight. To verifythatthis
biasvalueprovides peakresponsivity,responsetothecalibrationlampwasmaximizedasa
functionof bias.
4.12 Time Constant
The transfer function of a bolometer can be modeled as a low pass lter with transfer
function g = 1
1+i!
. The measured amplitude of the signal and the modulation frequency
can bet to thefunctionjgj= 1
p
1+! 2
2
where thetimeconstant isa freeparameter.
Figure 4.5:
Load curvesof 90 GHz and 150 GHz detectors measured pre-launch. The lower left panel
shows therawvoltage versuscurrent. Theupperleftpanel showsresistance plottedversus
electricalpowerdissipatedinthedetector. Theupperrightpanelshowstemperatureplotted
versus bias power. The slope of thiscurve givesthe thermal conductivityG (summarized
inTable4.11). The lowerrightpanel shows electricalresponsivityversus biascurrent.
rapidlyaspossible. Theprimaryexperimentalgoalof Boomerang isthemeasurement of
therst acoustic peakwhichappears at anangularscaleof 1 Æ
. Thissignal mustappear
in the bolometer time stream at a frequencywellabove the 1/f knee of the detector, but
Channel target G G
(pW/K) (pW/K)
90A 60 81
90B 60 82
150A 60 81
150B 60 88
150A1 60 88
150A2 60 104
150B1 60 90
150B2 60 73
240A1 180 181
240A2 180 180
240B1 180 195
240B2 180 202
400A1 360 519
400A2 360 412
400B1 360 422
400B2 360 424
DarkA 360 377
Dark B 60 28
Table4.5:
Thermalconductivityofeach detectormeasuredintheloadcurves,comparedto thetarget
conductivity.
knee(whichappearsat around5 mHz). ForatypicalBoomerang scanspeedof 1 Æ
/s,the
rst acoustic peak appears at 0.4Hz. The target bolometer time constants(see Figure
3.9) whichare all lessthan 25 ms (corresponding to a cut-o at 6.3 Hz)allow a decade of
bandwidthabove the1/f knee.
The time constant of the detectors in Boomerang were measured by measuring the
responseto a modulatedthermalsource. The modulation frequencywasvariedindiscreet
steps and a signal wasrecorded foreach. The signal as a function of frequency was t to
the above transfer function. It was found that in order to get a good t to the transfer
function, itis necessaryto have dataat a frequencywellbelowthetime constant cuto.
The rst column of Figure6.8 shows the bolometer timeconstants measured with this
Figure 4.6:
Loadcurvesof 150 GHzdetectors measured on theground. ChannelDarkA isa 400 GHz
detector and has a much higher thermal conductivity than the other bolometers in this
gure.
4.13 Noise Performance
Beforecoolingtheinstrument,noisespectra weremeasured(Table4.13). Asectionoftime
stream datawas recordedusingtheDAS and Fouriertransformed insoftware.
At room temperature, the impedance of the bolometers is 1 k, so bolometer noise
Figure 4.7:
Loadcurvesof 240 GHz detectors measuredonthe ground.
whitespectrumwithJFETnoisevaryingbetween6and10nV/ p
Hz. Thenoiseofthebias
monitor channels was 4-5nV/ p
Hz, so between 4-9nV/ p
Hz of the noise in the detectors
is due to JFET noise. The resistor channelwas measured to have noise at the levelof 30
nV/ p
Hz.
With the detectors at 0.280 K and viewing a 300 K source through the neutral
den-sity lters, noise spectra were taken again. Most channels have some excess noise at low
Figure 4.8:
Loadcurvesof 400 GHz detectors measuredonthe ground.
Channelsshownwithupperlimitsinthe1/fknee columnwerenotseento haveanyexcess
noiseat frequencies down to thelowestmeasured.
Severalchannelsalsohadexcess whitenoise: 240B2, 400A1,and DarkB.Swappingthe
JFETs, the bolometers, and the readoutelectronics of these channels with good channels
hadnoeectonthenoiseperformance. Therefore,thesourceofthisexcessnoiseisthought
tobeinthecoldwiring. Duetotimeconsiderations,thesechannelswerenotrepairedbefore
Channel warm noise cold white noise cold 1/fknee
nV/ p
Hz nV/
p
Hz Hz
90A 8 12 0.01
90B 8 12 <0.01
150A 8 13 <0.01
150B 7 13 0.1
150A1 7 13 0.05
150A2 7 14 0.02
150B1 6 13 0.1
150B2 7 12 <0.01
240A1 6 15 0.03
240A2 6 15 0.03
240B1 9 15 0.03
240B2 7 50 0.1
400A1 6 30 0.1
400A2 6 17 0.05
400B1 10 18 0.1
400B2 10 20 0.1
DarkA 6 11 0.2
DarkB 6 35 0.1
LoadRes 30 15 <0.01
GND FET 8 9 <0.01
Bias1 4 4 0.05
Bias2 4 4 0.05
Bias3 5 4 0.05
Bias4 5 4 0.1
Table4.6:
Noise characteristics of each Boomerang channel measured in the lab, at room
temper-ature and at 0.28 K. The white noise level is measured before and after cooling. The 1/f
knee is the frequencyat which the noiselevel reaches twice the white noisebaseline level.
In some channels,no 1/f spectrum was detected; an upperlimit in frequency is listedfor
these channels. Tests showthattheexcess whitenoiseseeninchannels240B2, 400A1, and
DarkBis dueto thecoldwiring.
4.14 Beam Maps
Beforelaunch,thebeam shapewasmeasuredbymappingan eccosorbballsuspendedbya
tethered balloon. Theballconsisted of foameccosorb (EV Roberts LS-30) inside asphere
constructed of chicken wire. A small ball(6 00
diameter) was used to map the mainlobe
and a large ball (18 00
diameter) was used to measure the sidelobe response (see
