The Cosmic Foreground Explorer (COFE): A balloon-borne microwave
  polarimeter to characterize polarized foregrounds
The Cosmic Foreground Explorer (COFE): A
balloon-borne microwave polarimeter to
characterize polarized foregrounds
Rodrigo Leonardi a,b, Brian Williams a, Marco Bersanelli c,
Ivan Ferreira b, Philip M. Lubin a, Peter R. Meinhold a,
Hugh O’Neill a, Nathan C.Stebor a, Fabrizio Villa d,
Thyrso Villela b, Carlos A. Wuensche b
aPhysics Department, University of California, Santa Barbara, CA 93106
bInstituto Nacional de Pesquisas Espaciais, Divisão de Astrof́ısica, Caixa Postal
515, 12227-010, São José dos Campos, SP, Brazil
cDipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133,
Milan, Italy
dINAF - IASF Bologna, Via P. Gobetti, 101, 40129, Bologna, Italy
Abstract
The COsmic Foreground Explorer (COFE) is a balloon-borne microwave polarime-
ter designed to measure the low-frequency and low-ℓ characteristics of dominant
diffuse polarized foregrounds. Short duration balloon flights from the Northern and
Southern Hemispheres will allow the telescope to cover up to 80% of the sky with
an expected sensitivity per pixel better than 100 µK/deg2 from 10 GHz to 20 GHz.
This is an important effort toward characterizing the polarized foregrounds for fu-
ture CMB experiments, in particular the ones that aim to detect primordial gravity
wave signatures in the CMB polarization angular power spectrum.
Key words: cosmology: observations, cosmic microwave background, polarization
foregrounds
1 Introduction
Measurement of polarization anisotropies in the Cosmic Microwave Back-
ground (CMB) is one of the great challenges in cosmology today. Very sensitive
measurements of these anisotropies, particularly at large angular scales, will
Preprint submitted to Elsevier Science 30 October 2018
http://arxiv.org/abs/0704.0810v2
provide unique constraints on the influence of gravitational waves on the pro-
duction of structure in the very early Universe and information on the epoch
of reionization.
Several experiments are running or in the planning stages, and long term devel-
opment for a future space mission attacking CMB polarization is underway. To
date, nearly all of the effort has been directed towards maximizing the number
of detectors in the focal plane to achieve the required sensitivity. Relatively
little work is going into sub-orbital efforts to constrain polarization fluctua-
tions at the largest angular scales, those most interesting for their impact on
understanding the inflationary epoch and ionization history of the universe.
This is primarily because of an unproven perception that very low multipoles
will not be accessible to any but space-based missions. Indeed, large scale
polarization has been searched for with ground based experiments over the
last 30 years. The COsmic Foreground Explorer (COFE) is a balloon-borne
instrument to measure the low frequency and low-ℓ characteristics of some
dominant polarized foregrounds. Good understanding of these foregrounds is
critical both for interpreting recent results, e.g. Spergel et al. (2006), and for
appropriately planning future CMB missions. The experiment also explores
low-ℓ limits to CMB polarization measurements at moderate frequencies from
non-space based platforms. We believe that balloon and ground-based mea-
surements to characterize in detail the polarized microwave sky are essential
to prepare a future space mission dedicated to CMB B-modes.
2 Science
The CMB radiation field is an observable that provides direct information
from the early Universe. The temperature and polarization characteristics of
this field impose constraints on cosmological scenarios relevant to understand
the origin and the structure of the Universe. Accurate measurements of the
CMB are vital to improve our understanding about geometry, mass-energy
composition, and reionization of the Universe. Ultimately, the CMB could
also provide indirect detection of a stochastic gravitational background and
information from the inflationary epoch itself. Having this big picture in mind,
several CMB experiments are now trying to constrain the tensor-to-scalar ratio
value and to detect the B-mode signature.
Among all practical limitations to primordial tensor amplitude detection, con-
tamination due diffuse microwave foreground polarized emission is certainly
the fundamental one. This emission presents spatial and frequency variations
that are not well known, and the residuals from foreground subtraction are
restricting our knowledge of CMB polarization. This is particularly true for fu-
ture B-mode experiments that will benefit if accurate determinations of spatial
and spectral characteristics of polarized foreground are made. For this reason,
multifrequency measurements of the polarized foregrounds in the microwaves
is now recognized as a key objective within the CMB community.
At low frequencies, foregrounds include synchrotron, free-free, and possible
spinning dust emission. Synchrotron dominates the low frequency range of the
microwave sky. Its emission is caused by relativistic charged particles interact-
ing with the Galactic magnetic field and can be highly polarized. Synchrotron
measurements provide better understanding of the Galactic magnetic field
structure and the density of relativistic electrons across the Galaxy. Free-free
emission becomes more important in the microwave intermediate frequency
range, and it is due to electron-ion scattering. Free-free is expected to be un-
polarized but this might not be true at the edges of HII clouds. Electrical
dipole emission from spinning dust has also been suggested by recent obser-
vations at low microwave frequencies, e.g. Finkbeiner et al. (2004).
COFE is a balloon-borne microwave polarimeter to measure spatial and low-
frequency characteristics of diffuse polarized foregrounds. This is an important
effort toward characterizing the polarized foregrounds for future CMB experi-
ments, in particular the ones that aim to detect primordial gravitational wave
signatures in the CMB polarization angular power spectrum.
3 Instrumentation
3.1 Telescope
Amodified BEAST telescope design is the basis for the COFE optics (Childers et al.,
2005; Figueiredo et al., 2005; Meinhold, P. R. et al., 2005; Mej́ıa, J. et al., 2005;
O’Dwyer, I. J. et al., 2005). It consists of an off-axis Gregorian configuration
obeying the DragoneMizuguchi condition (Dragone, 1978; Mizuguchi et al.,
1978). The telescope is optimized for minimal cross-polarization contamina-
tion and maximum focal plane area. The primary reflector is a 2.2 m off-axis
parabolic reflector. The incoming radiation is reflected off of the primary re-
flector towards a polarization modulating wave plate then to the secondary
reflector. The 0.9 m ellipsoidal secondary reflects the incoming radiation to-
ward the array of scalar feed horns that couple the radiation to an array of
cryogenic low noise amplifiers. The telescope will be mounted in a gondola that
has been simplified from a standard balloon-borne design due to the very light
carbon fiber optical elements. A schematic of the optics is shown in Figure 1.
3.2 Polarization modulator
COFE will employ a low-loss reflective polarization modulator for measur-
ing both Q and U simultaneously. It consists of a linear polarizing wire grid
mounted in front of a reflecting plate. The wire grid decomposes the input wave
into components, parallel and perpendicular to the wires, reflecting the par-
allel component with low loss. The perpendicular component passes through
the wire grid and reflects off the back short, passes through the grid again
and recombines with the parallel component. The distance between the plate
and the grid introduces a phase shift between the two components, effectively
rotating the plane of polarization of the input wave. A schematic of the polar-
ization modulator is shown in Figure 2. Rotating the grid chops between the
two polarization states four times per revolution as shown in Figure 3.
Tests of this modulator were performed at 41.5 GHz, using a 70 cm telescope.
We measured beam patterns for the rotated polarization states and integrated
for extended periods on the sky in Santa Barbara, CA. We were able to deter-
mine a 1/f knee lower than 50 mHz and very stable long term offsets. We also
demodulated sky data to the two different states and calculated the correct
combined sensitivity, as seen in Figure 4.
The polarization modulator has a broad bandwidth. We achieved 22 dB isola-
tion at 20% bandwidth. The radiometric loss of the elements in the modulator
can easily be made very low (of order 0.11%) up to relatively high frequencies.
The system works for a very wide range of frequency bands.
3.3 Receiver
COFE will use InP MMIC 1 amplifiers integrated into simple total power re-
ceivers. All of the RF gain will be integrated into a small compact module
inside the vacuum chamber. The module will contain 3 to 4 amplifiers (∼ 75
dB of gain), band pass filter, cryogenic detector diode, and an audio ampli-
fier. The module avoids the need for cryo/vacuum waveguide feedthrus on the
dewar simplifying the overall design. The audio amplifiers will be within the
cryostat vacuum vessel for simplicity and noise reasons, but will be at ambient
temperatures. COFE has a modest number of feeds required, and no ortho-
mode transducers or hybrid tees, so the passive components are minimal. A
schematic of the receiver is shown in Figure 5.
1 Indium Phosphide Monolithic Microwave Integrated Circuit
3.4 Data acquisition/demodulation
Data acquisition will use the same technique we have been using in our test
system, namely synchronous sampling of analog integrators. We oversample
the data by a large factor and perform the demodulation of Q and U Stokes
parameters (and other modes for systematic error analysis) in software. This
yields the most information and allows a variety of post-flight tests includ-
ing null signal analysis and analysis of the DC or total power components
(contaminated with 1/f , but still useful for systematic tests).
3.5 Ground-based B-machine prototype
A prototype polarimeter for a B-mode project, named B-machine, is being
deployed at the WMRS 2 Barcroft facility, CA (118◦14′ W longitude, 37◦35′ N
latitude, 3800 m altitude). The WMRS facility is an excellent site for mi-
crowave observation because of a cold microwave zenith temperature, low
precipitable water vapor, and a high percentage of clear days (Marvil et al.,
2006). Many of the components that will be used by the B-machine prototype
are useful for COFE as well. For example, the prototype will allow systematic
checks of the polarization modulator, and COFE scan strategy. The B-machine
prototype will be able to yield some basic higher multipole results on the fore-
grounds as well as the polarization signature and establish a data analysis
pipeline.
The prototype possesses telescope and detector technology identical to COFE.
It has 2 Ka-band and 6 Q-band channels centered at 31 and 41.5 GHz with
FWHM resolution of 28′ and 20′ respectively. The receiver has been previ-
ously used in anisotropy measurements (Childers et al., 2005). The telescope
runs at constant elevation while continuously scanning the sky in azimuth. A
photograph of B-machine prototype is shown in Figure 6.
4 Performance
For any sub-orbital CMB experiment, minimizing atmospheric contamination
is important. For the COFE bands, total atmospheric emission at our target
altitude of 35 km is less than 1 mK. Common broad band bolometric at-
mospheric antenna temperature contributions at balloon altitudes are several
hundred mK or more. Since the effective CMB antenna temperature drops
2 White Mountain Research Station
with frequency, our effective atmospheric signal is approximately 1000 times
less than for a bolometric balloon-borne system. Hence low-ℓ information from
a balloon-borne system is very clean by comparison. Figure 7 shows the atmo-
sphere and predicted foreground emission over a range of frequencies interest-
ing for CMB work (the foreground prediction is calculated from Bennett et al.
(2003)).
4.1 Receiver bands and expected receiver sensitivity
Receiver sensitivity can be estimated according to the radiometer equation
σT = K
Tsys + Tsky
∆ν · τ
, (1)
where σT is the root-mean-square noise, Tsys is the system noise temperature,
Tsky is the sky antenna temperature, ∆ν is the bandwidth, τ is the integration
time, and K is the sensitivity constant of the receiver.
For COFE and B-machine prototype the sensitivity constant of each receiver is
K = π
. The signal is sine wave modulated, reducing the sensitivity by a factor
as compared with a standard Dicke receiver, with an addition factor of
from the the standard definition for Q and U in the Rayleigh-Jeans regime
of the CMB spectrum. Table 1 shows our estimation of the sensitivity of each
receiver.
Table 1 – Instrument parameters.
COFE B-machine
Central frequency (GHz) 10 15 20 31 41.5
FWHM beam (arcmin) 83 55 42 28 20
Tsys (K) 8 10 12 25 27
Tsky (K) at target altitude
3 2.5 2.4 2.3 6.4 13.0
Bandwidth (GHz) 4 4 5 10 7
Number of receivers 3 6 10 2 6
Sensitivity per receiver (µK
s) 261 308 318 493 751
Aggregate sensitivity (µK
s) 151 126 100 348 307
3 For COFE and B-machine (ground based), we compute expected Tsky antenna
temperature at target altitude of 35 km and 3.8 km, respectively.
By increasing the number of receivers, future ground-based or balloon-borne
experiments can significantly improve aggregate sensitivity. For instance, 30
detectors could reach 61 µK
s and 107 µK
s at 30 and 40 GHz, respectively.
4.2 Scan strategy, sky coverage and expected map sensitivity
COFE uses a simple scan strategy to cover the largest available sky area in
each flight. The telescope will be pointed nominally 45◦ from the horizon to
minimize ground and balloon pickup, and the gondola will rotate constantly
at approximately 1/2 rpm. Data acquisition sample rate will be synchronized
with the polarization rotator (at ∼ 30 Hz). For instance, using this strategy,
a 24 hour flight from Fort Sumner, NM, allows to cover 59% of the sky area
with a median aggregate pixel sensitivity of 92 µK/deg2, 77 µK/deg2, and 61
µK/deg2 at 10 GHz, 15 GHz, and 20 GHz respectively. COFE will acquire
data from nearly all of the sky (∼ 93%). This will be achieved in a set of 12
and/or 24 hour flights from the Northern and Southern Hemispheres. Figure
8 provides estimates for sensitivity per square degree pixel over the whole sky
for our flight plans. Figure 9 illustrates the expected sky coverage.
The B-machine prototype focuses on higher multipoles but uses a similar scan-
ning strategy from the ground. For a conservative 60 day observing campaign
at WMRS, we expect to cover 56% of the sky with an median aggregate sensi-
tivity of 27 µK/deg2, and 23 µK/deg2 at 31 GHz and 41.5 GHz, respectively.
5 Conclusion
Over the next few years we will field a balloon-borne telescope to map more
than 90% of the sky. Both polarization anisotropy and polarized foregrounds
will be measured over several bands. This is an important effort toward char-
acterizing the polarized foregrounds for future CMB experiments.
In addition to foreground detection, COFE will better characterize the po-
larization modulation capability for measuring Q and U simultaneously. As
discussed earlier, a large scale ground-based campaign will capitalize on the
technology that has been developed by COFE and B-machine prototype.
It is clear that our current understanding of the polarization foregrounds limits
our ability to make accurate observations of the B-mode signature. COFE will
lessen the effect that incomplete models of foregrounds will have on future
experiments.
6 Acknowledgments
We acknowledge support from the National Aeronautics and Space Admin-
istration (NASA), and the California Space Institute (CalSpace). T.V. and
C.A.W. acknowledge CNPq Grants 305219/2004-9 and 307433/2004-8, respec-
tively. Some of the results have been derived using the HEALPix 4 (Górski et al.,
2005) package.
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Fig. 1. Optical schematic for COFE and B-machine prototype telescopes, an off-axis
Gregorian configuration optimized for minimal cross-polarization contamination. A
2.2 m parabolic reflector primary, a 0.9 m ellipsoidal secondary, and a 0.3 m rotator
grid are shown.
Fig. 2. Schematic of the polarization modulator. The input wave is decomposed
into its two linear polarization states, parallel and perpendicular to the wires (rep-
resented by dots just above the conducting reflector). The perpendicular component
is phase shifted from the extra path length. When added back to the parallel com-
ponent, the plane of polarization of the input wave is rotated.
Fig. 3. Sample signal from a polarized thermal source. A single revolution of the
modulator is shown, along with the reference signal to be used for demodulation.
Commutating using this signal yields Q, for instance, while demodulating with a
reference phase shifted by π/4 gives U .
Fig. 4. Sample data from our room temperature radiometer viewing the sky at 41.5
GHz. The undemodulated PSD displays the 1/f knee of the HEMT radiometer of
10 Hz and a white noise of 5.4 mK
s. The demodulated data have no visible 1/f
and a white noise level consistent with expectation.
Fig. 5. Radiometer layout for COFE.
Fig. 6. Picture of prototype telescope to be deployed at WMRS.
Fig. 7. Atmosphere, CMB, and predicted foreground emission from 5 to 300 GHz.
COFE bands run from 10 to 20 GHz. The zenith atmosphere emission is shown at
3.8 and 35 km. The atmospheric emission and lines are mainly due to H2O, O2,
and O3. For the target altitude of 35 km, we expect well under 1 mK total emission
from the atmosphere. Foreground spectral index β for free-free, synchrotron, and
dust were assumed, respectively, as −2.15, −2.7, and 2.2.
Fig. 8. Integrated histogram of anticipated aggregate sensitivity per 1 deg2 pixel
assuming a 24 hour flight from the Northern Hemisphere (Fort Sumner, NM) and
a 24 hour flight from the Southern Hemisphere (Alice Springs, Australia). For each
COFE band, we plot the fraction of the entire sky measured with better than a
given aggregate sensitivity. The change of the curves slope is due to the fact that
35% of the sky can be observed from both hemispheres using COFE scan strategy.
Fig. 9. Sky coverage for COFE assuming a 24 hour flight from the Northern Hemi-
sphere (Fort Sumner, NM) and a 24 hour flight from the Southern Hemisphere (Alice
Springs, Australia). The region observed contains nearly the entire sky (93%). The
darker strip shows the overlap between the two observations. For illustration pur-
poses, we show the diffuse Galactic structure obtained adding synchrotron, free-free
and dust maps at 23 GHz (Bennett et al., 2003).
	Introduction
	Science
	Instrumentation
	Telescope
	Polarization modulator
	Receiver
	Data acquisition/demodulation
	Ground-based B-machine prototype
	Performance
	Receiver bands and expected receiver sensitivity
	Scan strategy, sky coverage and expected map sensitivity
	Conclusion
	Acknowledgments
