ARCADE - Absolute Radiometer for Cosmology, Astrophysics,
and Diffuse Emission
ARCADE 2 (Absolute Radiometer for Cosmology,
Astrophysics, and Diffuse Emission) is a balloon-borne instrument which will
measure the radiometric temperature of the sky at six frequencies, ranging
from 3 to 90 GHz. The radiometric temperature at a given frequency is a
measure of the intensity of electromagnetic radiation at that frequency,
being the temperature that the radiating body would be if it were a perfect
blackbody emitter and emitting with the observed intensity at the frequency
in question.
The major science goals of the ARCADE
project are: 1) to achieve a measurement of the extent to which the
CMB deviates from a blackbody spectrum at long wavelengths, and 2)
determine the absolute temperature of microwave emission from our own
Galaxy
Deviations in the CMB away from a
blackbody can be caused by emission from ionized elections in the
universe, and from non-equilibrium conditions in the early universe.
Most recent CMB intensity measurements,
such as the famous WMAP, have been
differencing
measurements, which quantify very small differences from place to
place on the sky.These measurements are generally
statistics limited.In contrast, absolute
measurements, such as with ARCADE, are limited by systematics and
present an entirely different set of challenges.
In an absolute CMB temperature
measurement, it is important to get above most of the atmosphere, to
dramatically reduce the level of atmospheric microwave emission.
Photograph of 2005
ARCADE 2 launch, from NASA’s Columbia Scientific Balloon Facility in
Palestine, TX. The balloon and flight train dominate the image, and
the instrument hangs from the crane-like launch vehicle.
In order to achieve
lower uncertainties than previous measurements, ARCADE 2 has a fully
open-aperture cryogenic optics mounted at the top of a vary large (2.4 m
tall, 1.5 m wide) open bucket liquid helium dewar. Radiation from the sky
is compared by the radiometer to that from an external cryogenic
full-aperture blackbody calibrator of known temperature.
ARCADE 2 builds on these instrument concepts which were tested with a
previous smaller prototype, known as ARCADE 1, with observing channels at
10 and 30 GHz, that observed in 2003.
Schematic of
ARCADE 2 instrument, showing the big dewar, the suspension structure
above, the horns hanging inside to form the core
Schematic of
ARCADE 2 instrument, highlighting the radiometric and thermal features
Radiation from the sky
is received by corrugated horn antennas which hang from a flat, horizontal
aperture plate at the top of the dewar. On top of the aperture, a carousel
structure turns to expose the horns to either the sky, through a hole, or
to the external calibrator. The calibrator consists of nearly 300 sharp
cones of a microwave absorber cast over an aluminum core. It can be seen
in the photograph below.
The horn antennas have 11.6 deg beams, so that the only flux that matters
is that from the diffuse CMB and Galactic free-free and synchrotron
signals. Point sources like stars are utterly irrelevant.
Photograph of
the carousel, containing the hole for sky viewing and the external
calibrator, being lowered on top of the horn antenna apertures
Photograph of
instrument core being lowered into the dewar
The major
instrumental challenge is actually thermal: how to maintain all
radiometrically active components (horns, external calibrator,
radiometers) at near 4 Kelvin at the top of a very large dewar in the
presence of varying gas atmospheres and without condensation of
ambient gasses on the cold surfaces. Cooling power is supplied by
constantly pumping liquid helium from the bottom of the dewar to
needed places, and boioff helium gas is channeled to discourage
condensation. Temperatures are often maintained with resistance
heaters, and are read with ruthenium-oxide cryogenic resistance
thermometers.
Photograph
aperture plate from above, showing corrugated horn apertures. Visible
in the center are small tubes and bundles of wires which extend upward
to the carousel once it is placed on top. The tubes carry liquid
helium for cooling, while the wires carry signals from thermometers.
There are four wires for each for approximately 50 thermometers on the
external calibrator and the rest of the carousel.
Photograph of
the instrument upon landing, 2005. The electronics box and
magnetometers which are mounted on the dewar are visible.
ARCADE measures the incoming
radiation in each frequency channel, whether from the sky or the
external calibrator, with radiometers. A radiometer is a device which
measures the power of incident EM radiation in a given frequency range
from a given solid angle by converting it to a voltage. It is
important that radiation with a radiometric temperature as cold as
that of the CMB be amplified in a cold stage before any warm
components are encountered, or else the emission from the warm
components would dominate the actual signal. ARCADE uses Dicke
radiometers, which switch at 75 Hz between the source and an internal
load, and then demodulate the output. This subtracts out gain
fluctuations in the cold amplifier.
Schematic of
ARCADE radiometers and photograph of the cold stage (cryogenic
section) of the 8 GHz radiometer
Schematic of ARCADE radiometers
For an observing flight, the ARCADE
2 instrument is launched on a high altitude balloon. It takes around
three hours to reach the observing altitude of around 120,000 feet. At
that point, the lid is opened and sky data is taken for four or more
hours, limited by the supply of liquid helium and the drift of the
balloon out of range of telemetry. At the conclusion of observing, the
lid is closed, and the instrument, separated from the balloon, drops
to the ground on a parachute.
ARCADE 2 has observed twice, in July of 2005 and July of 2006.
Portion of the
sky viewed by ARCADE 2 in the 2005 flight, in Galactic coordinates, on
top of a map of Galactic emission anisotropy at 22.3 GHz from WMAP.
That viewed in the 2006 flight is similar. The center of the antenna
beams traced out the circle shown as the dewar rotated below the
balloon, with the circles drifting downward over time due to the
combined effect of the balloon moving west and the Earth rotating. The
map of Galactic emission shown is the combined 23 GHz Galactic
emission map from the WMAP first year data release.
