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QUaD is a bolometer-based polarimeter optimized for CMB polarization measurements. The QUaD experiment comprises three main elements: the telescope, the mount and the receiver. The telescope was built by collaborators at the University of Cardiff, in Wales and the University of Chicago. The mount is located at the South Pole and was formerly occupied by the University of Chicago DASI experiment. The receiver was designed, built and tested here at Stanford, using detectors provided by the Jet Propulsion Laboratory.
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Receiver return to top
The heart of QUaD is a 31-element focal plane shown schematically below. There are 31 corrugated feed horns, each feeding a polarization-sensitive bolometer (PSB) pair manufactured at the Jet Propulsion Laboratory. A photograph of a PSB is shown below. Each device comprises a linear mesh of metalized silicon-nitride that absorbs only one direction of linear polarization. The temperature change in the bolometer is then detected by an NTD (neutron transmutation-doped) germanium thermistor (the dark rectangle at the edge of the web). A PSB pair measures records the intensity of the radiation in orthogonal linear polarizations as the telescope scans across the sky. These signals are then processed into a map of the polarization pattern on the sky.
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Below left is a photograph of the QUaD focal plane, which was designed and built at Stanford. Most of the focal plane is constructed from gold-plated aluminum, with the exception of the feed-horns which are gold-plated copper. The different temperature stages are isolated with vespel -- a low thermal conductivity plastic. When this picture was taken the feed horns were blanked off with aluminum caps to allow the dark properties of the bolometers to be measured. The detail below right shows three of the feeds equipped with the metal-mesh filters that define the frequency band.
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The Cryostat return to top
Bolometers must be operated at temperatures well below 1K to achieve good sensitivity and so the focal plane is operated within a cryostat that is then attached below the telescope primary mirror. Inside the cryostat, cold lenses couple the radiation from the telescope onto the focal plane, as shown in the schematic below-left. The photograph below-right shows the cryostat being testing in the Stanford lab.
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Telescope and Mount
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The diameter of the telescope determines the size of the smallest feature on the sky that QUaD can resolve. We use an on-axis Cassegrain design with an under-illuminated 2.6m primary mirror for an angular resolution of 4.6 arcmin at 150 GHz. This in turn determines the highest l value QUaD can measure in the CMB power spectrum. The picture below-left shows the 2.6m primary mirror arriving at the University of Wales, Cardiff. Below-right is a picture of the QUaD telescope inside the reflective ground shield. The purpose of the ground shield is to prevent black-body radiation from the ground from reaching the telescope.
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