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ARCADE Schematic
ARCADE consists of a set of 7 cryogenic radiometers maintained
at a temperature near 2.7 K in an open liquid helium bucket dewar.
The signal to each first-stage amplifier is switched at 75 Hz
between a corrugated conical horn antenna and a precision internal
reference load. Each antenna alternately views the sky or a
precision blackbody calibrator. The internal load acts as a
transfer standard to enable a precise determination of the
sky--calibrator temperature difference.
Fountain effect pumps
lift superfluid liquid helium from the bottom
of the 3500 liter bucket dewar to reservoirs located at the instrument
aperture, inside the
calibrator target,
and on each radiometer.
A proportional-integral-differential control loop maintains
each component at the desired temperature near 2.7 K. A set of
cold flares surrounds the aperture to shield the antennas from
emission from warm objects.
Boiloff helium vapor vents under the cold flares to the aperture,
forming a barrier between the cold optics and the atmosphere.
ARCADE maintains a high boiloff rate (several cubic meters of
helium gas per second) throughout the entire flight to prevent
the atmosphere from freezing onto the optics. Video footage
taken in flight confirms that atmospheric condensation is negligible.
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