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| CAPMAP | |
|---|---|
| Name | CAPMAP |
| Full name | Cosmic Anisotropy Polarization MAPper |
| Field | Astrophysics |
| Location | Crawford Hill, New Jersey |
| Operation period | 2003–2008 |
| Telescopes | 7-meter antenna (NRAO) |
| Collaborators | Princeton University, University of Chicago, Johns Hopkins University, University of British Columbia |
CAPMAP was an astrophysical polarimeter experiment designed to measure the polarization of the cosmic microwave background (CMB) on degree and sub-degree angular scales. It targeted the faint linear polarization patterns imprinted on relic radiation from the early universe, aiming to constrain models of inflation, recombination, and the growth of cosmological perturbations. The instrument operated from a ground site and contributed to a sequence of CMB polarization measurements alongside contemporaneous efforts such as WMAP, BOOMERanG, and DASI.
CAPMAP was conceived as a targeted follow-up to earlier anisotropy and polarization experiments, exploiting advances in cryogenic receivers and polarization modulation. The project was led by teams at Princeton University and the University of Chicago with technical and observational support from the National Radio Astronomy Observatory (NRAO) and other institutions such as Johns Hopkins University and the University of British Columbia. It observed from the NRAO site at Crawford Hill using an existing 7-meter antenna originally associated with radio-astronomy and geodetic programs. CAPMAP focused on angular scales overlapping those probed by satellite missions like Planck and by ground experiments such as QUaD and POLARBEAR.
The CAPMAP instrument employed cryogenically cooled heterodyne receivers coupled to corrugated feeds and orthomode transducers to separate linear polarization states. The optical system was based on the 7-meter Cassegrain antenna at Crawford Hill, using a refractive lens and a quasi-optical system to illuminate the primary reflector with well-characterized beams. The receiver array included multiple frequency channels near 90 GHz and 40 GHz to help disentangle polarized foregrounds from the CMB signal; detector technology drew on designs used in experiments at Caltech and Jet Propulsion Laboratory. Polarization was measured via correlation receivers and phase-switching modulators similar to those developed for DASI and CBI (Cosmic Background Imager). The cryogenic chain used helium refrigerators and low-noise amplifier technologies pioneered at NASA Jet Propulsion Laboratory and the National Institute of Standards and Technology, enabling system temperatures competitive with contemporary bolometric arrays such as those used by ACT (Atacama Cosmology Telescope).
CAPMAP conducted observing campaigns from 2003 through the latter 2000s, performing deep integrations on a low-foreground patch of sky chosen for minimal contamination by polarized emission from the Milky Way. The scan strategy combined azimuthal chopping with slow elevation drifts to build redundancy akin to that used by WMAP and Planck, while enabling robust characterization of instrumental systematics. Calibration used celestial sources such as Jupiter, Orion Nebula, and bright quasars including 3C 273 and 3C 279, together with ground-based calibration loads and artificial polarized sources adapted from techniques developed at Harvard-Smithsonian Center for Astrophysics and MIT. Observing logs and detector timestreams were processed to flag radio-frequency interference (RFI) and to remove atmospheric fluctuations leveraging methods used by the South Pole Telescope and BICEP teams.
Data reduction pipelines transformed raw timestreams into maps of Stokes parameters I, Q, and U, adopting maximum-likelihood mapmaking techniques proven in analyses for DASI and BOOMERanG. The team estimated angular power spectra for E-mode and B-mode polarization, cross-correlating polarization maps with temperature anisotropy maps from WMAP to improve signal-to-noise. CAPMAP reported detections of E-mode power consistent with predictions from ΛCDM models constrained by WMAP and Planck, and placed upper limits on B-mode power at degree scales that constrained tensor-to-scalar ratios competitive with contemporaneous ground-based limits from QUaD and CBI. Systematic error budgets accounted for beam asymmetries, polarization angle uncertainty, and gain drifts using calibration strategies similar to those of POLARBEAR and SPTpol.
CAPMAP contributed to the empirical confirmation of predicted E-mode polarization and helped refine constraints on cosmological parameters related to scalar perturbations and reionization history measured by missions like WMAP and Planck. Its upper limits on B-modes informed the design sensitivity requirements of successor experiments such as BICEP2, Keck Array, and Simons Observatory. The instrument served as a training ground for students and early-career researchers who later joined teams for ACT (Atacama Cosmology Telescope), SPT (South Pole Telescope), and satellite proposals, fostering cross-pollination with groups at Caltech, Harvard University, and Columbia University. CAPMAP’s methodology for ground-based polarization calibration influenced observing strategies in follow-up arrays and contributed data used in joint analyses with other CMB datasets.
The CAPMAP collaboration consisted of academic groups including Princeton University, the University of Chicago, Johns Hopkins University, and the University of British Columbia, with instrumentation support from facilities such as the National Radio Astronomy Observatory and technical contributions from the Jet Propulsion Laboratory. Funding was provided through grants from agencies and programs such as the National Science Foundation and fellowships associated with participating institutions, supplemented by in-kind support from observatory partners. Collaborative links were maintained with teams behind DASI, QUaD, and WMAP to compare analysis pipelines and cross-validate calibration approaches.