Search

The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over 75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale ($1^\circ\lesssim��\leqslant 90^\circ$) CMB polarization to constrain the tensor-to-scalar ratio at the $r\sim0.01$ level and the optical depth to last scattering to the sample variance limit. This paper presents the optical characterization of the 40 GHz telescope during its first observation era, from 2016 September to 2018 February. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is $, 32 pages, 24 figures, published in ApJ

The Cosmology Large Angular Scale Surveyor (CLASS) aims to detect and characterize the primordial B-mode signal and make a sample-variance-limited measurement of the optical depth to reionization. CLASS is a ground-based, multi-frequency microwave polarimeter that surveys 70% of the microwave sky every day from the Atacama Desert. The focal plane detector arrays of all CLASS telescopes contain smooth-walled feedhorns that couple to transition-edge sensor (TES) bolometers through symmetric planar orthomode transducer (OMT) antennas. These low noise polarization-sensitive detector arrays are fabricated on mono-crystalline silicon wafers to maintain TES uniformity and optimize optical efficiency throughout the wafer. In this paper, we discuss the design and characterization of the first CLASS 93 GHz detector array. We measure the dark parameters, bandpass, and noise spectra of the detectors and report that the detectors are photon-noise limited. With current array yield of 82%, we estimate the total array noise-equivalent power (NEP) to be 2.1 aW$\sqrt[]{\mathrm{s}}$., Comment: 16 pages, 9 figures

The Atacama B-mode Search (ABS) is an experiment designed to measure cosmic microwave background (CMB) polarization at large angular scales ($\ell>40$). It operated from the ACT site at 5190~m elevation in northern Chile at 145 GHz with a net sensitivity (NEQ) of 41 $��$K$\sqrt{\rm s}$. It employed an ambient-temperature sapphire half-wave plate rotating at 2.55 Hz to modulate the incident polarization signal and reduce systematic effects. We report here on the analysis of data from a 2400 deg$^2$ patch of sky centered at declination $-42^\circ$ and right ascension $25^\circ$. We perform a blind analysis. After unblinding, we find agreement with the Planck TE and EE measurements on the same region of sky. We marginally detect polarized dust emission and give an upper limit on the tensor-to-scalar ratio of $r, 38 pages, 11 figures

The Atacama B-mode Search (ABS) instrument, which began observation in February of 2012, is a crossed-Dragone telescope located at an elevation of 5,100 m in the Atacama Desert in Chile. The primary scientific goal of ABS is to measure the B-mode polarization spectrum of the Cosmic Microwave Background from multipole moments of about $$\ell \approx $$ 50 to $$\ell \approx $$ 500 (angular scales from $${\sim }0.4^\circ $$ to $${\sim }4^\circ $$ ), a range that includes the primordial B-mode peak from inflationary gravitational waves. The ABS focal plane array consists of 240 pixels designed for observation at 145 GHz by the TRUCE collaboration. Each pixel has its own individual, single-moded feedhorn and contains two transition-edge sensor bolometers coupled to orthogonal polarizations that are read out using time domain multiplexing. We will report on the current status of ABS and discuss the time constants and optical efficiencies of the TRUCE detectors in the field.

We present an evaluation of systematic effects associated with a continuously-rotating, ambient-temperature half-wave plate (HWP) based on two seasons of data from the Atacama B-Mode Search (ABS) experiment located in the Atacama Desert of Chile. The ABS experiment is a microwave telescope sensitive at 145 GHz. Here we present our in-field evaluation of celestial (CMB plus galactic foreground) temperature-to-polarization leakage. We decompose the leakage into scalar, dipole, and quadrupole leakage terms. We report a scalar leakage of ~0.01%, consistent with model expectations and an order of magnitude smaller than other CMB experiments have reported. No significant dipole or quadrupole terms are detected; we constrain each to be, 11 pages, 8 figures; revision to submitted version, Fig. 5 and Eqs. (14) and (15) corrected; added Fig. 9 and description, text revisions for clarification, Fig. 5 revised for better calibration, corrected labeling errors and plotting bugs in Fig. 3, 4, and Eq. (14) and (15)

In these proceedings, we summarize our in-field evaluation of temperature-to-polarization leakage associated with the use of a continuously-rotating, ambient-temperature half-wave plate (HWP) on the Atacama B-Mode Search (ABS) experiment. Using two seasons of data, we demonstrate scalar leakage of ∼ 0.01%. This is consistent with model expectations and an order of magnitude better than any previously-reported leakage. We constrain higher-order dipole and quadrupole leakage terms to be < 0.06% (95% confidence). Without any mitigation from scan cross-linking or boresight rotation, this corresponds to an upper limit on systematic errors in the tensor-to-scalar ratio r ;S 0.01. The HWP significantly reduces temperature-to-polarization leakage systematic errors for ABS and shows the promise of fast polarization modulation with HWPs for future experiments. Full details can be found in Ref. 1.

Upcoming experiments aim to produce high fidelity polarization maps of the cosmic microwave background. To achieve the required sensitivity, we are developing monolithic, feedhorn-coupled transition edge sensor polarimeter arrays operating at 150 GHz. We describe this focal plane architecture and the current status of this technology, focusing on single-pixel polarimeters being deployed on the Atacama B-mode Search (ABS) and an 84-pixel demonstration feedhorn array backed by four 10-pixel polarimeter arrays. The feedhorn array exhibits symmetric beams, cross-polar response less than -23 dB and excellent uniformity across the array. Monolithic polarimeter arrays, including arrays of silicon feedhorns, will be used in the Atacama Cosmology Telescope Polarimeter (ACTPol) and the South Pole Telescope Polarimeter (SPTpol) and have been proposed for upcoming balloon-borne instruments.

A number of groups have produced TES bolometers and other devices using dielectric membranes for thermal isolation in which excess heat capacity is reported. We have fabricated a series of TES devices using large area membranes and narrow legs for thermal isolation to study the scaling of intrinsic parameters like the heat capacity of the membrane. The devices are arranged on a single chip to ensure similarity of devices parameters (i.e., film thicknesses) and processing history. Measurements (non-multiplexed) of the current-voltage characteristic and complex impedance are made using a well-characterized, highly uniform, multichannel SQUID setup. We have developed model for non-ideal transition edge sensor performance that enables extraction of heat capacities and thermal conductances identified in the models. We report on the magnitude of excess heat capacity in the dielectric membrane and thermal conductance of the metal-to-dielectric interface in each of the designs.

We investigated the possibility of using transformers to replace SQUIDs for the readout of microcalorimeters. This simple scheme has been used in the past for bolometers, however it was discarded for the use with TES microcalorimeters because of the inadequate performance. Our work shows that, with a few simple changes, the performance of transformers as current transducers, while still not comparable to that of SQUIDs, is sufficient to read out the signal from TES microcalorimeters without any degradation in speed or energy resolution. In contrast to SQUIDs, transformers do not dissipate any power and their working principle makes them natural candidates for frequency multiplexing. Their extension to several channels is therefore straightforward.

The Millimeter Bolometer Array Camera (MBAC) will be installed on the 6-meter Atacama Cosmology Telescope (ACT) in late 2007. For the first season of observations, MBAC will comprise a 145 GHz diffraction-limited, 1024-pixel, focal plane array of Transition Edge Sensor (TES) Bolometers. This will be the largest array of pop-up-detector bolometers ever fielded as well as one of the largest arrays of TES bolometers. We discuss the design specifications for the array and pre-assembly testing procedures for the cryogenic components. We present dark measurements of the TES bolometer properties of numerous 32-pixel columns that have been assembled into the first kilopixel array for ACT, as well as optical measurements made with our 256-pixel prototype array, including first light measurements on ACT.

The Atacama B-mode Search (ABS), which began observations in February of 2012, is a crossed-Dragone telescope located at an elevation of 5190 m in the Atacama Desert in Chile. ABS is searching for the B-mode polarization spectrum of the cosmic microwave background (CMB) at large angular scales from multipole moments of ` ~ 50 ~ 500, a range that includes the primor- dial B-mode peak from inflationary gravity waves at ~ 100. The ABS focal plane consists of 240 pixels sensitive to 145 GHz, each containing two transition-edge sensor bolometers coupled to orthogonal polarizations with a planar ortho-mode transducer. An ambient-temperature con- tinuously rotating half-wave plate and 4 K optics make the ABS instrument unique. We discuss the characterization of the detector spectral responses with a Fourier transform spectrometer and demonstrate that the pointing model is adequate. We also present measurements of the beam from point sources and compare them with simulations.

We describe the instrument architecture of the Johns Hopkins University-led CLASS instrument, a groundbased cosmic microwave background (CMB) polarimeter that will measure the large-scale polarization of the CMB in several frequency bands to search for evidence of inflation.

We evaluate the modulation of Cosmic Microwave Background (CMB) polarization using a rapidly-rotating, half-wave plate (HWP) on the Atacama B-Mode Search (ABS). After demodulating the time-ordered-data (TOD), we find a significant reduction of atmospheric fluctuations. The demodulated TOD is stable on time scales of 500-1000 seconds, corresponding to frequencies of 1-2 mHz. This facilitates recovery of cosmological information at large angular scales, which are typically available only from balloon-borne or satellite experiments. This technique also achieves a sensitive measurement of celestial polarization without differencing the TOD of paired detectors sensitive to two orthogonal linear polarizations. This is the first demonstration of the ability to remove atmospheric contamination at these levels from a ground-based platform using a rapidly-rotating HWP.

We present a description of the data reduction and mapmaking pipeline used for the 2008 observing season of the Atacama Cosmology Telescope (ACT). The data presented here at 148 GHz represent 12% of the 90 TB collected by ACT from 2007 to 2010. In 2008 we observed for 136 days, producing a total of 1423 hours of data (11 TB for the 148 GHz band only), with a daily average of 10.5 hours of observation. From these, 1085 hours were devoted to a 850 deg^2 stripe (11.2 hours by 9.1 deg) centered on a declination of -52.7 deg, while 175 hours were devoted to a 280 deg^2 stripe (4.5 hours by 4.8 deg) centered at the celestial equator. We discuss sources of statistical and systematic noise, calibration, telescope pointing, and data selection. Out of 1260 survey hours and 1024 detectors per array, 816 hours and 593 effective detectors remain after data selection for this frequency band, yielding a 38% survey efficiency. The total sensitivity in 2008, determined from the noise level between 5 Hz and 20 Hz in the time-ordered data stream (TOD), is 32 micro-Kelvin sqrt{s} in CMB units. Atmospheric brightness fluctuations constitute the main contaminant in the data and dominate the detector noise covariance at low frequencies in the TOD. The maps were made by solving the least-squares problem using the Preconditioned Conjugate Gradient method, incorporating the details of the detector and noise correlations. Cross-correlation with WMAP sky maps, as well as analysis from simulations, reveal that our maps are unbiased at multipoles ell > 300. This paper accompanies the public release of the 148 GHz southern stripe maps from 2008. The techniques described here will be applied to future maps and data releases., 20 pages, 18 figures, 6 tables, an ACT Collaboration paper

We present cosmological parameters derived from the angular power spectrum of the cosmic microwave background (CMB) radiation observed at 148 GHz and 218 GHz over 296 deg^2 with the Atacama Cosmology Telescope (ACT) during its 2008 season. ACT measures fluctuations at scales 500, Comment: 20 pages, 13 figures. Submitted to ApJ. This paper is a companion to Hajian et al. (2010) and Das et al. (2010)

We present a new calibration method based on cross-correlations with WMAP and apply it to data from the Atacama Cosmology Telescope (ACT). ACT's observing strategy and map making procedure allows an unbiased reconstruction of the modes in the maps over a wide range of multipoles. By directly matching the ACT maps to WMAP observations in the multipole range of 400 < ell < 1000, we determine the absolute calibration with an uncertainty of 2% in temperature. The precise measurement of the calibration error directly impacts the uncertainties in the cosmological parameters estimated from the ACT power spectra. We also present a combined map based on ACT and WMAP data that has high signal-to-noise over a wide range of multipoles.

The Atacama Cosmology Telescope was designed to measure small-scale anisotropies in the Cosmic Microwave Background and detect galaxy clusters through the Sunyaev-Zel'dovich effect. The instrument is located on Cerro Taco in the Atacama Desert, at an altitude of 5190 meters. A six-met.er off-axis Gregorian telescope feeds a new type of cryogenic receiver, the Millimeter Bolometer Array Camera. The receiver features three WOO-element arrays of transition-edge sensor bolometers for observations at 148 GHz, 218 GHz, and 277 GHz. Each detector array is fed by free space mm-wave optics. Each frequency band has a field of view of approximately 22' x 26'. The telescope was commissioned in 2007 and has completed its third year of operations. We discuss the major components of the telescope, camera, and related systems, and summarize the instrument performance.

The next generation of Cosmic Microwave Background (CMB) experiments probing for signals of inflation and small angular scale polarization anisotropies require higher sensitivity and better control of systematics. We are developing monolithic arrays of orthomode transducer (OMT) coupled transition edge sensor (TES) polarimeters designed for operation at 150 GHz to address these requirements. OMT coupling allows for simultaneous and independent detection of two orthogonal linear polarization states incident on a single pixel. We present measurements of optical efficiencies η op of single pixels with on-chip band-defining filters, with η op = 57±4 stat±9 sys %. We also provide evidence for an out-of-band blue leak and address possible sources as well as mitigation techniques. Additionally, we discuss methods for increasing efficiency being implemented in the next generation of pixels, currently in fabrication. Still under development, these pixels are produced as monolithic polarimeter arrays and are slated for use in the Atacama Cosmology Telescope Polarization (ACTpol) and South Pole Telescope Polarization (SPTpol) experiments, while single-pixel polarimeters are to be deployed in the Atacama B-mode Search (ABS) experiment.

The Atacama B‐mode Search (ABS) experiment is a 145 GHz polarimeter designed to measure the B‐mode polarization of the Cosmic Microwave Background (CMB) at large angular scales. The ABS instrument will ship to the Atacama Desert of Chile fully tested and ready to observe in 2010. ABS will image large‐angular‐scale CMB polarization anisotropies onto a focal plane of 240 feedhorn‐coupled, transition‐edge sensor (TES) polarimeters, using a cryogenic crossed‐Dragone design. The ABS detectors, which are fabricated at NIST, use orthomode transducers to couple orthogonal polarizations of incoming radiation onto separate TES bolometers. The incoming radiation is modulated by an ambient‐temperature half‐wave plate in front of the vacuum window at an aperture stop. Preliminary detector characterization indicates that the ABS detectors can achieve a sensitivity of 300 μKs in the field. This paper describes the ABS optical design and detector readout scheme, including feedhorn design and performance, magnetic shielding, focal plane architecture, and cryogenic electronics.

The next generation of cosmic microwave background (CMB) polarization experiments targeting the signatures of inflation will require unprecedented sensitivities in addition to careful control of systematics. With existing detector technologies approaching the photon noise limit, improvements in system sensitivities must come from ever‐larger focal plane arrays of millimeter‐wave detectors. We report on the design and performance of microfabricated planar orthomode transducer (OMT) coupled TES polarimeters and silicon micromachined platelet feedhorns optimized for scaling to large monolithic arrays. Future versions of these detectors are targeted for deployment in a number of upcoming CMB experiments, including ABS, SPTpol, and ACTpol.

We present data characterizing the optical properties of feedhorn‐coupled TES polarimeters useful for future CMB measurements. In this detector architecture, TES bolometers are coupled to radiation through superconducting microstrip to a planar ortho‐mode transducer inserted into waveguide. Filters in the microstrip define the pass bands for the detectors. We will present measurements of the co‐polar optical efficiency, the cross‐polar isolation and the detector bandpass and compare these results to expectations from simulations.

We present simulations and discuss measurements of a planar orthomode transducer (OMT) being developed for use with bolometric detectors in observations of the polarization of the cosmic microwave background (CMB). This OMT couples radiation from a circular waveguide onto microstrip where it is filtered and then detected. A corrugated feed horn defines the optical beam. Simulations suggest this OMT achieves a coupling efficiency of greater than 96% with cross‐polarization below 1% over 30% bandwidth.

We present results from modeling the electrothermal performance of feedhorn‐coupled transition edge sensor (TES) polarimeters under development for use in cosmic microwave background (CMB) polarization experiments. Each polarimeter couples radiation from a corrugated feedhorn through a planar orthomode transducer, which transmits power from orthogonal polarization modes to two TES bolometers. We model our TES with two‐ and three‐block thermal architectures. We fit the complex impedance data at multiple points in the TES transition. From the fits, we predict the noise spectra. We present comparisons of these predictions to the data for two TESes on a prototype polarimeter.

We are developing feedhorn‐coupled TES polarimeters to measure the polarization anisotropies of the cosmic microwave background (CMB) radiation. These devices will be deployed in arrays of hundreds to thousands and will be measured using multiplexed SQUID readout electronics. Since multiplexed devices share common circuitry, a high degree of uniformity is required in the electrothermal properties of the TES bolometers and readout circuits in order to operate all channels simultaneously with high sensitivity. Our cryogenic test bed can probe dozens of devices simultaneously, thus providing useful detector statistics on relatively short time scales. We describe the TES bolometer design and present dark (no optical loading) measurements of the electrical and thermal properties and uniformity of prototype bolometers across two 3‐inch diameter production wafers, including (standard deviation in parenthesis): TES transition temperature (∼1%), normal resistance (∼10%), thermal conductance (≲10%), time constant (∼20%), shunt resistance (≲5%), and noise properties.

The Millimeter Bolometer Array Camera (MBAC) was commissioned in the fall of 2007 on the new 6-meterAtacama Cosmology Telescope (ACT). The MBAC on the ACT will map the temperature anisotropies of theCosmic Microwave Background (CMB) with arc-minute resolution. For this first observing season, the MBACcontained a diffraction-limited, 32 by 32 element, focal plane array of Transition Edge Sensor (TES) bolometersfor observations at 145 GHz. This array was coupled to the telescope with a series of cold, refractive, reimagingoptics. To meet the performance specifications, the MBAC employs four stages of cooling using closed-cycle3He/4He sorption fridge systems in combination with pulse tube coolers. In this paper we present the design ofthe instrument and discuss its performance during the first observing season. Finally, we report on the statusof the MBAC for the 2008 observing season, when the instrument will be upgraded to a total of three separate1024-element arrays at 145 GHz, 220 GHz and 280 GHz.

The Atacama Cosmology Telescope (ACT) aims to measure the Cosmic Microwave Background (CMB) temperature anisotropies on arcminute scales. The primary receiver for ACT is the Millimeter Bolometer Array Camera (MBAC). The MBAC is comprised of three 32×32 transition edge sensor (TES) bolometer arrays, each observing the sky with an independent set of band-defining filters. The MBAC arrays will be the largest pop-up detector arrays fielded, and among the largest TES arrays built. Prior to its assembly into an array and installation into the MBAC, a column of 32 bolometers is tested at ~ 0.4 K in a quick-turn-around dip probe. In this paper we describe the properties of the ACT bolometers as revealed by data from those tests, emphasizing a characterization that accounts for both the complex impedance and the noise as a function of frequency.

The Atacama Cosmology Telescope (ACT) is designed to measure temperature anisotropies of the cosmic microwave background (CMB) at arcminute resolution. It is the first CMB experiment to employ a 32×32 close-packed array of free-space-coupled transition-edge superconducting bolometers. We describe the organization of the telescope systems and software for autonomous, scheduled operations. When paired with real-time data streaming and display, we are able to operate the telescope at the remote site in the Chilean Altiplano via the Internet from North America. The telescope had a data rate of 70 GB/day in the 2007 season, and the 2008 upgrade to three arrays will bring this to 210 GB/day.