ACTPol

ACTPol
Name	ACTPol
Caption	Polarization receiver on the Atacama Cosmology Telescope
Organization	Princeton University; Columbia University; Cornell University; University of Pennsylvania
Location	Atacama Desert, Chile
Altitude	5190 m
Established	2013
Wavelength	microwave

ACTPol

The Atacama Cosmology Telescope Polarization receiver was a high-resolution microwave receiver installed on the Atacama Cosmology Telescope in the Atacama Desert to measure the polarization of the Cosmic Microwave Background and related astrophysical signals. It operated in coordination with contemporaneous instruments such as Planck (spacecraft), BICEP2, POLARBEAR, and SPTpol, producing maps that informed studies of cosmology, structure formation, and neutrino physics. The receiver enabled cross-correlation studies with surveys from the Sloan Digital Sky Survey, the Herschel Space Observatory, and the Dark Energy Survey.

Overview

ACTPol was a polarization-sensitive upgrade to the Atacama Cosmology Telescope program, following initial intensity-focused campaigns and preceding the Advanced ACTPol expansion. It targeted angular scales complementary to Planck (spacecraft) and higher-resolution instruments like South Pole Telescope. The instrument emphasized sensitivity to polarization modes (E-modes and B-modes) to constrain parameters of the Lambda-CDM model, probe inflation (cosmology), and detect secondary anisotropies from the Sunyaev–Zel'dovich effect in clusters identified by the ROSAT and XMM-Newton catalogs.

Instrumentation and Design

The receiver used arrays of transition-edge sensor (TES) bolometers fabricated through collaborations involving National Institute of Standards and Technology, NASA Goddard Space Flight Center, and university microfabrication facilities at Cornell University and Columbia University. Optical coupling employed cryogenic silicon lenses and anti-reflection coatings developed with teams at Princeton University and University of Pennsylvania. Frequency bands centered near 90 GHz and 150 GHz were chosen to minimize foreground contamination from Galactic synchrotron and thermal dust emission observed by WMAP and Planck (spacecraft). The cryogenic system relied on pulse-tube coolers and dilution refrigeration techniques similar to those used in Planck (spacecraft) and BICEP Array. Polarimetric modulation and detector orientation strategies were designed in consultation with experts from University of Toronto and Fermi National Accelerator Laboratory to mitigate systematic errors related to beam asymmetry and instrumental polarization.

Observations and Survey Strategy

ACTPol executed deep-field and wide-field surveys from the high, dry site near Cerro Toco and ALMA facilities, observing sky patches overlapping with the Sloan Digital Sky Survey, the Wide-field Infrared Survey Explorer, and ground-based optical surveys including Pan-STARRS. Repeated observations over seasons enabled time-domain studies and cross-calibration with Planck (spacecraft) for absolute calibration and mapmaking. The survey strategy balanced depth—targeting small angular scales for measurements of the damping tail—and breadth—mapping large contiguous regions to study lensing-induced B-modes and cross-correlations with Herschel Space Observatory far-infrared catalogs and SPTpol fields.

Data Processing and Analysis

Data reduction pipelines incorporated techniques from collaborators at Princeton University, University of Pennsylvania, and Cornell University for time-ordered data filtering, pointing reconstruction, and beam characterization. Mapmaking exploited maximum-likelihood methods adapted from analyses used by Planck (spacecraft) and the South Pole Telescope teams to produce Stokes I, Q, and U maps. Component separation used cross-correlation with external templates from Herschel Space Observatory and WMAP to identify thermal dust and synchrotron contributions, while foreground marginalization followed methods developed in the Planck Collaboration papers. Lensing reconstruction employed quadratic estimators and iterative techniques similar to those used by POLARBEAR and SPTpol to extract gravitational lensing potentials and to construct delensed B-mode constraints. Systematic error budgets were assessed against standards from NASA and national laboratory calibration protocols.

Scientific Results

ACTPol produced high-significance measurements of the polarized E-mode power spectrum across multipoles that complemented Planck (spacecraft) results, improving constraints on the scalar spectral index and optical depth parameters central to the Lambda-CDM model. The instrument contributed to detections of lensing B-modes and provided measurements of the lensing power spectrum that tightened limits on the sum of neutrino masses and on models of dark energy via cross-correlations with galaxy samples from the Sloan Digital Sky Survey and the Dark Energy Survey. ACTPol surveys identified and characterized galaxy clusters through the Sunyaev–Zel'dovich effect, enabling cosmological constraints on cluster abundance and calibration of mass-observable relations using follow-up from Chandra X-ray Observatory and Subaru Telescope. Measurements also advanced understanding of Galactic foregrounds by comparing polarization properties with maps from Planck (spacecraft) and the Galactic Arecibo L-band Feed Array Survey.

Collaborations and Funding

The ACTPol project was a multi-institutional collaboration including research groups at Princeton University, Columbia University, Cornell University, University of Pennsylvania, University of Michigan, University of Toronto, and international partners. Funding and support came from agencies and organizations such as the National Science Foundation (United States), NASA, and institutional grants from participating universities and national laboratories like Fermi National Accelerator Laboratory and Lawrence Berkeley National Laboratory. Operational coordination with Chilean authorities and site management involved liaison with regional scientific infrastructure near ALMA and compliance with national site-use policies.

Category:Scientific instruments