Instrument Details
The EBEX experiment will consist of a gondola suspended from a balloon with a payload of a telescope, readout electronics, and a pointing and telemetry system.
Gondola and Telemetry
The EBEX gondola contains an inner and outer structure. The outer frame of the gondola hangs from a pivot which is attached to the helium balloon that keeps the experiment aloft. The outer frame acts as a support for the National Science Balloon Facility (NSBF) equipment, the sunshields, some of the pointing sensor hardware and electronics, and the inner frame. The inner frame supports the telescope (cryostat and detectors), detector readout electronics, and pointing sensor hardware, and it rotates in elevation with respect to the outer frame.
Observing Area and Scan
We have identified a 350 square degree area in the southern part of the sky away from the disk of the galaxy which contains low amplitudes of emission from galactic dust and will be opposite the direction of the sun at observing time. An additional consideration in our choice of sky area is to attempt to overlap with the observing area of other CMB polarization experiments such as QUIET. The scan strategy is still in development, but we will scan back and forth in azimuth for 15 degrees with a variable elevation step on the order of 2'.
Pointing
Accurate pointing information for the telescope on the timescale of the detector readout is essential since position information on the sky is required for making maps and power spectra. Polarization data is collected by the telescope on the inner gondola, so the pointing strategy is to precision point the inner frame of the gondola, and then to index the position of the outer gondola relative to the inner gondola with a precision rotary encoder.
Optics
The CMB radiation will be collected and focused by two parabolic mirrors and directed into the cryostat window. Inside the cryostat, the radiation's polarization will be modulated by a rotating half wave plate, and it will be separated into orthogonal polarizations by a wire grid polarizer oriented at 45 degrees to the beam. The orthogonal polarizations of the radiation will be incident on two focal planes which contain band defining filters in front of an array of bolometric detectors.
Detector
The transition edge sensitive (TES) bolometers are packed into arrays and the signals from groups of 16 or 32 bolometers will be frequency-domain multiplexed before they are read out by SQUIDs. The multiplexing is essential to reduce the number of wires into the cryostat, the power consumption of the readout electronics from the solar panels, and the power dissipation required to prevent the electronics from overheating. The large number of detectors (>1000 in the science flight) will allow us to attain a thermal noise in the hundreds of nK.
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