Dipartimento di Fisica - Universita' di Roma La Sapienza
B2K: The Focal Plane

The focal plane is new: Caltech/JPL has developed special polarization sensitive detectors (PSB) consisting of two orthogonal grid absorbers with separate thermistors, mounted in the same groove of a circular corrugated waveguide.


The two orthogonal absorbers of a 145 GHz PSB. The distance of the two is much less than one wavelength. The absorbers are made of metalized Si3N4. The top rectangle is the NTD Ge thermistor, able to sense temperature variations of the absorber of about 10^-9 degrees.

Corrugated feedhorns are used as focal plane collection optics for the 145 GHz PSBs.


Front view of the B2K focal plane. The principal axis of polarization for all detectors are shown as arrows on the feedhorn apertures. The top row has PSBs, the bottom one has two-band photometers.

The focal plane includes 8 pixels and 16 bolometers: the top row of the pixels array uses 4 PSBs (8 bolometers) at 145 GHz, with principal axis orientations of 45/135 deg, 0/90 deg, 22.5/102.5 deg, 67.5/157.5, deg for pixels W,X,Y,Z respectively.

The bottom row uses four 2-band photometers (245/345 GHz) consisting of conventional spider-web bolometers with wire grid polarizers in
front of the feedhorn. Each pixel is thus sensitive to a single polarization, with principal axis orientation of 45, 135, 90, 0 deg for pixels
W,X,Y,Z respectively.


Global view of the back-side of the focal plane array, with the bolometers and load resistors visible on top of the feedhorns. The feedhorns are tilted to follow the curvature of the focal plane.

All these photometers have cross-polarization lower than 5%. This has been confirmed for the full instrument by means of a custom full-beam/full-area polarized illuminator developed in Rome.


The calibration source uses a 77K blackbody and rotating polarizers in the focus of the spare primary mirror of BOOMERanG to produce a modulated beam with well known polarization properties.

The band-defining filters have been produced in Cardiff, and the focal plane has been integrated in UCSB. The assembled focal plane is mounted on top of the optics box in the "insert" cooled by the dewar at 1.6K during the flight.


The cold insert including the reimaging optics, filters, feed, bolometers and fridge.

The beams of the 16 channels have been measured using a tethered source in the far field of the telescope. We have used a small blimp to lift a spherical blackbody at an altitude of about 1 km.
We confirmed that our 145 GHz beam ~ 9' FWHM, while the 245/345 beams are ~6' FWHM.

blimp source

Releasing the blimp (orange) and the two blackbody sources black, the first one below the blimp, the smaller one still near the BOOMERanG crew), in the  foregroundof Mt. Erebus.

See also the other BOOMERanG web pages
University of Toronto