The E and B EXperiment (EBEX); Progress and Status

We report on the status of EBEX, a NASA-funded balloon-borne polarimeter designed to measure the B-mode polarization of the cosmic microwave background radiation. The EBEX receiver is designed to set a 2σ upper limit on an inflationary tensor-to-scalar ratio of 0.02. This limit assumes a 14-day flight, a scan pattern covering 420 square degrees of the sky, and foreground subtraction to levels below detector noise. The instrument employs a 1.5 meter Gregorian-type telescope and 1440 bolometric transition edge sensor detectors distributed over two focal planes. Polarization is measured using a rotating achromatic half wave plate (AHWP) and a fixed polarizing grid. The AHWP is continuously rotated using a superconducting magnetic bearing. Sky signals will be observed in three bands centered at 150, 250 and 410 GHz, providing strong leverage against the polarized dust foreground. Integration of the gondola and the receiver will occur in fall 2008. A short-duration test flight employing 384 detectors in one focal plane is planned for 2009.