QUBIC III: Laboratory characterization

Abstract

We report on an extensive test campaign of a prototype version of theQUBIC (Q & U Bolometric Interferometer for Cosmology) instrument,carried out at Astroparticle Physics and Cosmology (APC) inParis. Exploiting the novel concept called bolometric interferometry,QUBIC is designed to measure the CMB polarization at 150 and 220 GHzfrom a high altitude site at Alto Chorillo, Argentina. The prototypemodel called QUBIC Technological Demonstrator (QUBIC-TD) operates in asingle frequency band (150 GHz) and with a reduced number ofbaselines, but it contains all the elements of the QUBIC instrument inits final configuration. The test campaign included measurements ofthe synthesized beam and of the polarization performance, as well as averification of the interference fringe pattern. A modulated,frequency-tunable millimetre-wave source was placed in the telescopefar-field and was used to simulate a point source. The QUBIC-TD fieldof view was scanned across the source to produce beam maps. Ourmeasurements confirm the frequency-dependent behaviour of the beamprofile, which gives QUBIC the possibility to do spectral imaging. Themeasured polarization performance indicates a cross-polarizationleakage less than 0.6%. We also successfully tested the polarizationmodulation system, which is provided by a rotating half waveplate. We demonstrate the full mapmaking pipeline using datafrom this measurement campaign, effectively giving an end-to-endcheckout of the entire QUBIC system, including all hardwaresubsystems, their interfaces, and the software to operate the wholesystem and run the analysis. Our results confirm the viability ofbolometric interferometry for measurements of the CMB polarization.