Cryoconnect Reaching to the cold...
Cryoconnect Applications Support
QUEST is a bolometer-based polarimeter optimized for CMB polarization measurements. The QUEST experiment comprises three main elements: the telescope, the mount and the receiver. The telescope optics are being built by our collaborators at the University of Cardiff, in Wales. The mount is being supplied by the University of Chicago. Much of the receiver is being designed, built and tested here, at Stanford.
QUEST needs a telescope to collect CMB photons and focus them on the detectors. The QUEST telescope is an on-axis Cassegrain design with a 2.6m primary mirror (shown below as it arrived from manufacture). The diameter of the primary determines the size of the smallest feature on the sky that QUEST can resolve (4 arcmin at 150 GHz). This in turn determines the highest l value QUEST can measure on the power spectrum.
(primary and secondary mirrors) and
the cold receiver optics.
Cardiff after being manufactured.
Collaborators at the University of Chicago are supplying QUEST with a mount for the telescope. This mount is located at the South Pole in Antarctica and was used on their successful DASI experiment.
See http://astro.uchicago.edu/dasi/ for details on the DASI experiment as well as lots of cool photos of the South Pole.
QUEST mounted on the DASI mount (shown in the inset).
The QUEST receiver operates at cryogenic temperatures and is contained in a cryostat that attaches below the primary mirror. Inside the cryostat, cold lenses couple the radiation from the telescope onto the focal plane. A half-waveplate rotates the plane of polarization of incident radiation before it hits the detectors. By changing the rotation angle of the waveplate (with a cryogenic stepper motor), the expected polarization angle of the output signal changes, providing a powerful check for systematic errors.
The heart of the receiver is the focal plane, which is shown enlarged below. It is being entirely built and tested at Stanford. The focal plane holds the corrugated microwave feedhorns at the focus created by the telescope. The feedhorns efficiently couple the microwaves onto the polarization sensitive bolometers (PSBs) located on the back of the focal plane plate. The PSBs measure the intensity of the radiation in orthogonal linear polarizations and output proportional electrical signals that are recorded as the telescope scans across the sky. These signals are processed into a map of the polarization pattern on the sky.
In order to achieve the required sensitivity, the PSBs must be cooled to 0.250 K, and thus must be thermally isolated from the rest of the instrument. The vespel hexapod shown in the figure below provides this isolation as well as rigid mechanical support. The corrugated feed horns and the PSB detectors exist and are shown in the inset below.
