Engineering topics

Choi, Steve K.

Publications and source records attributed to Choi, Steve K..

7 records

DOE OSTI2024

The Atacama Cosmology Telescope: R eionization kSZ trispectrum methodology and limits

Patchy reionization generates kinematic Sunyaev–Zel’dovich (kSZ) anisotropies in the cosmic microwave background (CMB). Large-scale velocity perturbations along the line of sight modulate the small-scale kSZ power spectrum, leading to a trispectrum (or four-point function) in the CMB that depends on the physics of reionization. We investigate the challenges in detecting this trispectrum and use tools developed for CMB lensing, such as realization-dependent bias subtraction and cross-correlation based estimators, to counter uncertainties in the instrumental noise and assumed CMB power spectrum. We also find that both lensing and extragalactic foregrounds can impart larger trispectrum contributions than the reionization kSZ signal. We present a range of mitigation methods for both of these sources of contamination, validated on microwave-sky simulations. We use ACT DR6 and Planck data to calculate an upper limit on the reionization kSZ trispectrum from a measurement dominated by foregrounds. The upper limit is about 50 times the signal predicted from recent simulations.

cosmic background radiation↗

NASA NTRS2017

The Atacama Cosmology Telescope: Two-Season ACTPol Spectra and Parameters

We present the temperature and polarization angular power spectra measuredby the Atacama Cosmology Telescope Polarimeter (ACTPol). We analyze night-time datacollected during 2013-14 using two detector arrays at 149 GHz, from 548 deg(exp. 2) of sky onthe celestial equator. We use these spectra, and the spectra measured with the MBAC camera on ACT from 2008-10, in combination with Planck and WMAP data to estimate cosmological parameters from the temperature, polarization, and temperature-polarization cross-correlations. We find the new ACTPol data to be consistent with the CDM model. The ACTPol temperature-polarization cross-spectrum now provides stronger constraints on multiple parameters than the ACTPol temperature spectrum, including the baryon density, the acoustic peak angular scale, and the derived Hubble constant. The new ACTPol dataprovide information on damping tail parameters. The joint uncertainty on the number of neutrino species and the primordial helium fraction is reduced by 20% when adding ACTPol to Planck temperature data alone.

Louis, Thibaut↗

NASA NTRS2016

Highly Uniform 150 mm Diameter Multichroic Polarimeter Array Deployed for CMB Detection

The Advanced Atacama Cosmology Telescope Polarimeter is an upgraded receiver for the Atacama Cosmology Telescope, which has begun making measurements of the small angular scale polarization anisotropies in the Cosmic Microwave Background using the first of four new multichroic superconducting detector arrays. Here, we review all details of the optimization and characterization of this first array, which features 2012 AlMn transition edge sensor bolometers operating at 150 and 230 GHz. We present critical temperatures, thermal conductivities,saturation powers, time constants, and sensitivities for the array. The results show high uniformity across the 150 mm wafer and good performance in the field.

Ho, Shuay-Pwu Patty↗

NASA NTRS2016

Far Sidelobe Effects from Panel Gaps of the Atacama Cosmology Telescope

The Atacama Cosmology Telescope is a 6 meter diameter CMB telescope located at 5200 meters in the Chilean desert. ACT has made arc-minute scale maps of the sky at 90 and 150 GHz which have led to precise measurements of the fine angular power spectrum of the CMB fluctuations in temperature and polarization. One of the goals of ACT is to search for the B-mode polarization signal from primordial gravity waves, and thus extending ACT's data analysis to larger angular scales. This goal introduces new challenges in the control of systematic effects, including better understanding of far sidelobe effects that might enter the power spectrum at degree angular scales. Here we study the effects of the gaps between panels of the ACT primary and secondary reflectors in the worst case scenario in which the gaps remain open. We produced numerical simulations of the optics using GRASP up to 8 degrees away from the main beam and simulated timestreams for observations with this beam using real pointing information from ACT data. Maps from these simulated timestreams showed leakage from the sidelobes, indicating that this effect must be taken into consideration at large angular scales.

The Atacama Cosmology Telescope↗

NASA NTRS2016

Mechanical Design and Development of TES Bolometer Detector Arrays for the Advanced ACTPol Experiment

The next generation Advanced ACTPol (AdvACT) experiment is currently underway and will consist of four Transition Edge Sensor (TES) bolometer arrays, with three operating together, totaling 5800 detectors on the sky. Building on experience gained with the ACTPol detector arrays, AdvACT will utilize various new technologies, including 150 mm detector wafers equipped with multichroic pixels, allowing for a more densely packed focal plane. Each set of detectors includes a feedhorn array of stacked silicon wafers which form a spline pro le leading to each pixel. This is then followed by a waveguide interface plate, detector wafer, back short cavity plate, and backshort cap. Each array is housed in a custom designed structure manufactured from high purity copper and then gold plated. In addition to the detector array assembly, the array package also encloses cryogenic readout electronics. We present the full mechanical design of the AdvACT high frequency (HF) detector array package along with a detailed look at the detector array stack assemblies. This experiment will also make use of extensive hardware and software previously developed for ACT, which will be modi ed to incorporate the new AdvACT instruments. Therefore, we discuss the integration of all AdvACT arrays with pre-existing ACTPol infrastructure.

Advanced ACTPol (AdvACT)↗

NASA NTRS2016

The Design and Characterization of Wideband Spline-profiled Feedhorns for Advanced Actpol

Advanced ACTPol (AdvACT) is an upgraded camera for the Atacama Cosmology Telescope (ACT) that will measure the cosmic microwave background in temperature and polarization over a wide range of angular scales and five frequency bands from 28-230 GHz. AdvACT will employ four arrays of feedhorn-coupled, polarization- sensitive multichroic detectors. To accommodate the higher pixel packing densities necessary to achieve Ad- vACTs sensitivity goals, we have developed and optimized wideband spline-profiled feedhorns for the AdvACT multichroic arrays that maximize coupling efficiency while carefully controlling polarization systematics. We present the design, fabrication, and testing of wideband spline-profiled feedhorns for the multichroic arrays of AdvACT.

Atacama Cosmology Telescope (ACT)↗

NASA NTRS2016

Characterization of AlMn TES Impedance, Noise, and Optical Efficiency in the First 150 mm Multichroic Array for Advanced ACTPol

The Advanced ACTPol (AdvACT) upgrade to the Atacama Cosmology Telescope features large arrays of multichroic pixels consisting of two orthogonal-polarization pairs of superconducting bolometers at two observing frequency bands. We present measurements of the detector properties and noise data in a subset of a fielded multichroic array of AlMn transition-edge sensor (TES) detectors. In this array, the distribution of critical temperature T(sub c) across detectors appears uniform at the percent level. The measured noise-equivalent power (NEP) distributions over approximately 1200 detectors are consistent with expectations. We find median NEPs of 4.0×10(exp −17) W/ √ Hz for low-band detectors and 6.2×10(exp −17) W/ √ Hz for high-band detectors under covered-window telescope test conditions with optical loading comparable to observing with precipitable water vapor approximately 0.5 mm. Lastly, we show the estimated detector optical efficiency, and demonstrate the ability to perform optical characterization over hundreds of detectors at once using a cryogenic blackbody source.

Crowley, Kevin T.↗