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Register a new userBroadband anti-reflective coatings for cosmic microwave background experiments
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The desire for higher sensitivity has driven ground-based cosmic microwave background (CMB) experiments to employ ever larger focal planes, which in turn require larger reimaging optics. Practical limits to the maximum size of these optics motivates the development of quasi-optically-coupled (lenslet-coupled), multi-chroic detectors. These detectors can be sensitive across a broader bandwidth compared to waveguide-coupled detectors. However, the increase in bandwidth comes at a cost: the lenses (up to $sim$700 mm diameter) and lenslets ($sim$5 mm diameter, hemispherical lenses on the focal plane) used in these systems are made from high-refractive-index materials (such as silicon or amorphous aluminum oxide) that reflect nearly a third of the incident radiation. In order to maximize the faint CMB signal that reaches the detectors, the lenses and lenslets must be coated with an anti-reflective (AR) material. The AR coating must maximize radiation transmission in scientifically interesting bands and be cryogenically stable. Such a coating was developed for the third generation camera, SPT-3G, of the South Pole Telescope (SPT) experiment, but the materials and techniques used in the development are general to AR coatings for mm-wave optics. The three-layer polytetrafluoroethylene-based AR coating is broadband, inexpensive, and can be manufactured with simple tools. The coating is field tested; AR coated focal plane elements were deployed in the 2016-2017 austral summer and AR coated reimaging optics were deployed in 2017-2018.
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A technological milestone for experiments employing Transition Edge Sensor (TES) bolometers operating at sub-kelvin temperature is the deployment of detector arrays with 100s--1000s of bolometers. One key technology for such arrays is readout multiplexing: the ability to read out many sensors simultaneously on the same set of wires. This paper describes a frequency-domain multiplexed readout system which has been developed for and deployed on the APEX-SZ and South Pole Telescope millimeter wavelength receivers. In this system, the detector array is divided into modules of seven detectors, and each bolometer within the module is biased with a unique ~MHz sinusoidal carrier such that the individual bolometer signals are well separated in frequency space. The currents from all bolometers in a module are summed together and pre-amplified with Superconducting Quantum Interference Devices (SQUIDs) operating at 4 K. Room-temperature electronics demodulate the carriers to recover the bolometer signals, which are digitized separately and stored to disk. This readout system contributes little noise relative to the detectors themselves, is remarkably insensitive to unwanted microphonic excitations, and provides a technology pathway to multiplexing larger numbers of sensors.
ASTRI is a Flagship Project of the Italian Ministry of Education, University and Research, led by the Italian National Institute of Astrophysics, INAF. One of the main aims of the ASTRI Project is the design, construction and verification on-field of a dual mirror (2M) end-to-end prototype for the Small Size Telescope (SST) envisaged to become part of the Cherenkov Telescope Array. The ASTRI SST-2M prototype adopts the Schwarzschild-Couder design, and a camera based on SiPM (Silicon Photo Multiplier); it will be assembled at the INAF astronomical site of Serra La Nave on mount Etna (Catania, Italy) within mid 2014, and will start scientific validation phase soon after. The peculiarities of the optical design and of the SiPM bandpass pushed towards specifically optimized choices in terms of reflective coatings for both the primary and the secondary mirror. In particular, multi-layer dielectric coatings, capable of filtering out the large Night Sky Background contamination at wavelengths $lambda gtrsim 700$ nm have been developed and tested, as a solution for the primary mirrors. Due to the conformation of the ASTRI SST-2M camera, a reimaging system based on thin pyramidal light guides could be optionally integrated aiming to increase the fill factor. An anti-reflective coating optimized for a wide range of incident angles faraway from normality was specifically developed to enhance the UV-optical transparency of these elements. The issues, strategy, simulations and experimental results are thoroughly presented.
The large size of the time ordered data of cosmic microwave background experiments presents challenges for mission planning and data analysis. These issues are particularly significant for Antarctica- and space-based experiments, which depend on satellite links to transmit data. We explore the viability of reducing the time ordered data to few bit numbers to address these challenges. Unlike lossless compression, few bit digitisation introduces additional noise into the data. We present a set of one, two, and three bit digitisation schemes and measure the increase in noise in the cosmic microwave background temperature and polarisation power spectra. The digitisation noise is independent of angular scale and is well-described as a constant percentage of the original detector noise. Three bit digitisation increases the map noise level by < 2%, while reducing the data volume by a factor of ten relative to 32-bit floats. Extreme digitisation is a promising strategy for upcoming experiments.
The POLARBEAR-2 CosmicMicrowave Background (CMB) experiment aims to observe B-mode polarization with high sensitivity to explore gravitational lensing of CMB and inflationary gravitational waves. POLARBEAR-2 is an upgraded experiment based on POLARBEAR-1, which had first light in January 2012. For POLARBEAR-2, we will build a receiver that has 7,588 Transition Edge Sensor (TES) bolometers coupled to two-band (95 and 150 GHz) polarization-sensitive antennas. For the large arrays readout, we employ digital frequency-domain multiplexing and multiplex 32 bolometers through a single superconducting quantum interference device (SQUID). An 8-bolometer frequency-domain multiplexing readout has been deployed on POLARBEAR-1 experiment. Extending that architecture to 32 bolometers requires an increase in the bandwidth of the SQUID electronics to 3 MHz. To achieve this increase in bandwidth, we use Digital Active Nulling (DAN) on the digital frequency multiplexing platform. In this paper, we present requirements and improvements on parasitic inductance and resistance of cryogenic wiring and capacitors used for modulating bolometers. These components are problematic above 1 MHz. We also show that our system is able to bias a bolometer in its superconducting transition at 3 MHz.
The observation of cosmic microwave background (CMB) anisotropies is one of the key probes of physical cosmology. The weak nature of this signal has driven the construction of increasingly complex and sensitive experiments observing the sky at multiple frequencies with thousands of polarization sensitive detectors. Given the high sensitivity of such experiments, instrumental systematic effects can become the limiting factor towards the full scientific exploitation of their data. In this paper we present s4cmb (Systematics for CMB), a Python package designed to simulate raw data streams in time domain of modern CMB experiments based on bolometric technology, and to inject in these realistic instrumental systematics effects. The aim of the package is to help assessing the contamination due to instrumental systematic effects on real data, to guide the design of future instruments, as well as to increase the realism of simulated data sets required in the development of accurate data analysis methods.
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