The Cosmology Large Angular Scale Surveyor consists of four instruments performing a CMB polarization survey. Currently, the 40 GHz and first 90 GHz instruments are deployed and observing, with the second 90 GHz and a multichroic 150/220 GHz instrument to follow. The receiver is a central component of each instrument's design and functionality. This paper describes the CLASS receiver design, using the first 90 GHz receiver as a primary reference. Cryogenic cooling and filters maintain a cold, low-noise environment for the detectors. We have achieved receiver detector temperatures below 50mK in the 40 GHz instrument for 85% of the initial 1.5 years of operation, and observed in-band efficiency that is consistent with pre-deployment estimates. At 90 GHz, less than 26% of in-band power is lost to the filters and lenses in the receiver, allowing for high optical efficiency. We discuss the mounting scheme for the filters and lenses, the alignment of the cold optics and detectors, stray light control, and magnetic shielding.
We present here a study of the use of the SiAl alloy CE7 for the packaging of silicon devices at cryogenic temperatures. We report on the development of baseplates and feedhorn arrays for millimeter wave bolometric detectors for astrophysics. Existing interfaces to such detectors are typically made either of metals, which are easy to machine but mismatched to the thermal contraction profile of Si devices, or of silicon, which avoids the mismatch but is difficult to directly machine. CE7 exhibits properties of both Si and Al, which makes it uniquely well suited for this application.
We measure CE7 to a) superconduct below a critical transition temperature, Tc, ~1.2 K, b) have a thermal contraction profile much closer to Si than metals, which enables simple mating, and c) have a low thermal conductivity which can be improved by Au-plating. Our investigations also demonstrate that CE7 can be machined well enough to fabricate small structures, such as #0-80 threaded holes, to tight tolerances (~25 μm) in contrast with pure silicon and similar substrates. We have fabricated CE7 baseplates being deployed in the 93 GHz polarimetric focal planes used in the Cosmology Large Angular Scale Surveyor (CLASS).1 We also report on the development of smooth-walled feedhorn arrays made of CE7 that will be used in a focal plane of dichroic 150/220 GHz detectors for the CLASS High-Frequency camera.
The search for inflationary primordial gravitational waves and the measurement of the optical depth to reionization, both through their imprint on the large angular scale correlations in the polarization of the cosmic microwave background (CMB), has created the need for high sensitivity measurements of polarization across large fractions of the sky at millimeter wavelengths. These measurements are subject to instrumental and atmospheric 1=f noise, which has motivated the development of polarization modulators to facilitate the rejection of these large systematic effects.
Variable-delay polarization modulators (VPMs) are used in the Cosmology Large Angular Scale Surveyor (CLASS) telescopes as the first element in the optical chain to rapidly modulate the incoming polarization. VPMs consist of a linearly polarizing wire grid in front of a movable flat mirror. Varying the distance between the grid and the mirror produces a changing phase shift between polarization states parallel and perpendicular to the grid which modulates Stokes U (linear polarization at 45°) and Stokes V (circular polarization). The CLASS telescopes have VPMs as the first optical element from the sky; this simultaneously allows a lock-in style polarization measurement and the separation of sky polarization from any instrumental polarization further along in the optical path.
The CLASS VPM wire grids use 50 μm copper-plated tungsten wire with a 160μm spacing across a 60 cm clear aperture. The mirror is mounted on a flexure system with one degree of translational freedom, enabling the required mirror motion while maintaining excellent parallelism with respect to the wire grid. The wire grids and mirrors are held parallel to each other to better than 80 μm, and the wire grids have RMS flatness errors below 50 μm across the 60 cm aperture. The Q-band CLASS VPM was the first VPM to begin observing the CMB full time, starting in the Spring of 2016. The first W-band CLASS VPM was installed in the Spring of 2018.
The Cosmology Large Angular Scale Surveyor (CLASS) aims to detect and characterize the primordial Bmode 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√s.
We report on the design of a 240 GHz double-side-band receiver for the Submillimeter Array (SMA). The heart of this
receiver is a 3-junction series connected SIS mixer, which allows it to provide intermediate frequency (IF) output up to
more than 12 GHz. We have custom built a low noise Amplifier-Multiplier Chain for use as the receiver’s Local
Oscillator module, which is tunable from 210 to 270 GHz. The receiver has demonstrated low noise performance in
laboratory. 7 out of the 8 SMA antennas are now equipped with this receiver. The receiver has already participated in
Event Horizon Telescope observations in April 2016, working with the SMA-200 receiver to provide dual polarization
coverage for the EHT Hawaii Station. This receiver has enabled the SMA to provide 32 Gbit per second data stream to
the EHT observations. We are currently trying to improve the on-sky beam co-alignment of this receiver with respect to
other SMA receivers.
The Submillimeter Array (SMA) is an 8-element mm/sub-mm interferometer on the summit of Maunakea, Hawaii that is
operated jointly by the Smithsonian Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy
and Astrophysics (ASIAA). After nearly 13 years of operation, we are undertaking a major upgrade of the array's
cryogenics, receivers and other systems that will enhance the science capabilities of the array and replace components
reaching end-of-life. Here we describe the new receivers, containing dual-polarization, ultra-wideband SIS mixers
operating at 230 and 345 GHz, the new ultra-wideband IF signal transport and correlator system, and the enhanced
observing capabilities that will be enabled by this upgrade.
The Cosmology Large Angular Scale Surveyor (CLASS) is a four telescope array designed to characterize relic primordial gravitational waves from in ation and the optical depth to reionization through a measurement of the polarized cosmic microwave background (CMB) on the largest angular scales. The frequencies of the four CLASS telescopes, one at 38 GHz, two at 93 GHz, and one dichroic system at 145/217 GHz, are chosen to avoid spectral regions of high atmospheric emission and span the minimum of the polarized Galactic foregrounds: synchrotron emission at lower frequencies and dust emission at higher frequencies. Low-noise transition edge sensor detectors and a rapid front-end polarization modulator provide a unique combination of high sensitivity, stability, and control of systematics. The CLASS site, at 5200 m in the Chilean Atacama desert, allows for daily mapping of up to 70% of the sky and enables the characterization of CMB polarization at the largest angular scales. Using this combination of a broad frequency range, large sky coverage, control over systematics, and high sensitivity, CLASS will observe the reionization and recombination peaks of the CMB E- and B-mode power spectra. CLASS will make a cosmic variance limited measurement of the optical depth to reionization and will measure or place upper limits on the tensor-to-scalar ratio, r, down to a level of 0.01 (95% C.L.).
The Cosmology Large Angular Scale Surveyor (CLASS) experiment aims to map the polarization of the Cosmic Microwave Background (CMB) at angular scales larger than a few degrees. Operating from Cerro Toco in the Atacama Desert of Chile, it will observe over 65% of the sky at 38, 93, 148, and 217 GHz. In this paper we discuss the design, construction, and characterization of the CLASS 38 GHz detector focal plane, the first ever Q-band bolometric polarimeter array.
The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment to measure the signature of a gravitationalwave background from inflation in the polarization of the cosmic microwave background (CMB). CLASS is a multi-frequency array of four telescopes operating from a high-altitude site in the Atacama Desert in Chile. CLASS will survey 70% of the sky in four frequency bands centered at 38, 93, 148, and 217 GHz, which are chosen to straddle the Galactic-foreground minimum while avoiding strong atmospheric emission lines. This broad frequency coverage ensures that CLASS can distinguish Galactic emission from the CMB. The sky fraction of the CLASS survey will allow the full shape of the primordial B-mode power spectrum to be characterized, including the signal from reionization at low ɺ. Its unique combination of large sky coverage, control of systematic errors, and high sensitivity will allow CLASS to measure or place upper limits on the tensor-to-scalar ratio at a level of r = 0:01 and make a cosmic-variance-limited measurement of the optical depth to the surface of last scattering, Ƭ .
The Greenland Telescope project will deploy and operate a 12m sub-millimeter telescope at the highest point of the Greenland i e sheet. The Greenland Telescope project is a joint venture between the Smithsonian As- trophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA). In this paper we discuss the concepts, specifications, and science goals of the instruments being developed for single-dish observations with the Greenland Telescope, and the coupling optics required to couple both them and the mm-VLBI receivers to antenna. The project will outfit the ALMA North America prototype antenna for Arctic operations and deploy it to Summit Station,1 a NSF operated Arctic station at 3,100m above MSL on the Greenland I e Sheet. This site is exceptionally dry, and promises to be an excellent site for sub-millimeter astronomical observations. The main science goal of the Greenland Telescope is to carry out millimeter VLBI observations alongside other telescopes in Europe and the Americas, with the aim of resolving the event horizon of the super-massive black hole at the enter of M87. The Greenland Telescope will also be outfitted for single-dish observations from the millimeter-wave to Tera-hertz bands. In this paper we will discuss the proposed instruments that are currently in development for the Greenland Telescope - 350 GHz and 650 GHz heterodyne array receivers; 1.4 THz HEB array receivers and a W-band bolometric spectrometer. SAO is leading the development of two heterodyne array instruments for the Greenland Telescope, a 48- pixel, 325-375 GHz SIS array receiver, and a 4 pixel, 1.4 THz HEB array receiver. A key science goal for these instruments is the mapping of ortho and para H2D+ in old protostellar ores, as well as general mapping of CO and other transitions in molecular louds. An 8-pixel prototype module for the 350 GHz array is currently being built for laboratory and operational testing on the Greenland Telescope. Arizona State University are developing a 650 GHz 256 pixel SIS array receiver based on the KAPPa SIS mixer array technology and ASIAA are developing 1.4 THz HEB single pixel and array receivers. The University of Cambridge and SAO are collaborating on the development of the CAMbridge Emission Line Surveyor (CAMELS), a W-band `on- hip' spectrometer instrument with a spectral resolution of R ~ 3000. CAMELS will consist of two pairs of horn antennas, feeding super conducting niobium nitride filter banks read by tantalum based Kinetic Inductance Detectors.
The Cosmology Large Angular Scale Surveyor (CLASS) instrument will measure the polarization of the cosmic microwave background at 40, 90, and 150 GHz from Cerro Toco in the Atacama desert of northern Chile. In this paper, we describe the optical design of the 40 GHz telescope system. The telescope is a diffraction limited catadioptric design consisting of a front-end Variable-delay Polarization Modulator (VPM), two ambient temperature mirrors, two cryogenic dielectric lenses, thermal blocking filters, and an array of 36 smooth-wall scalar feedhorn antennas. The feed horns guide the signal to antenna-coupled transition-edge sensor (TES) bolometers. Polarization diplexing and bandpass definition are handled on the same microchip as the TES. The feed horn beams are truncated with 10 dB edge taper by a 4 K Lyot-stop to limit detector loading from stray light and control the edge illumination of the front-end VPM. The field-of-view is 19° x 14° with a resolution for each beam on the sky of 1.5° FWHM.
The cosmic microwave background (CMB) provides a powerful tool for testing modern cosmology. In particular, if inflation has occurred, the associated gravitational waves would have imprinted a specific polarized pattern on the CMB. Measurement of this faint polarized signature requires large arrays of polarization-sensitive, background- limited detectors, and an unprecedented control over systematic effects associated with instrument design. To this end, the ground-based Cosmology Large Angular Scale Surveyor (CLASS) employs large-format, feedhorn- coupled, background-limited Transition-Edge Sensor (TES) bolometer arrays operating at 40, 90, and 150 GHz bands. The detector architecture has several enabling technologies. An on-chip symmetric planar orthomode transducer (OMT) is employed that allows for highly symmetric beams and low cross-polarization over a wide bandwidth. Furthermore, the quarter-wave backshort of the OMT is integrated using an innovative indium bump bonding process at the chip level that ensures minimum loss, maximum repeatability and performance uniformity across an array. Care has been taken to reduce stray light and on-chip leakage. In this paper, we report on the architecture and performance of the first prototype detectors for the 40 GHz focal plane.
We present a smooth-walled feedhorn with cross polarization and reflected power lower than -30 dB across the
entire 30% bandwidth. A prototype feedhorn has been fabricated, and the wide-band, low-cross polarization
performance has been demonstrated. The feedhorn has a circular aperture and monotonically narrows towards
an input waveguide interface. This allows it to be manufactured by progressively milling the profile using a set
of custom tools. This is especially useful in applications where a large number of feeds are desired in a planar
array format. Such applications include astronomical cameras in millimeter waveband that require large arrays
of detectors for future increases in mapping speed and sensitivity. Specifically, large arrays of feedhorns are
well-matched to the problem of measuring the polarization of the cosmic microwave background to search for
the faint signature of inflation, as they provide good beam control, the requisite sensitivity, and compatibility
with low-noise bolometric detectors.
The Primordial Inflation Polarization Explorer (PIPER) is a balloon-borne instrument designed to search for
the faint signature of inflation in the polarized component of the cosmic microwave background (CMB). Each
flight will be configured for a single frequency, but in order to aid in the removal of the polarized foreground
signal due to Galactic dust, the filters will be changed between flights. In this way, the CMB polarization at a
total of four different frequencies (200, 270, 350, and 600 GHz) will be measured on large angular scales. PIPER
consists of a pair of cryogenic telescopes, one for measuring each of Stokes Q and U in the instrument frame.
Each telescope receives both linear orthogonal polarizations in two 32 × 40 element planar arrays that utilize
Transition-Edge Sensors (TES). The first element in each telescope is a variable-delay polarization modulator
(VPM) that fully modulates the linear Stokes parameter to which the telescope is sensitive. There are several
advantages to this architecture. First, by modulating at the front of the optics, instrumental polarization is
unmodulated and is therefore cleanly separated from source polarization. Second, by implementing this system
with the appropriate symmetry, systematic effects can be further mitigated. In the PIPER design, many of the
systematics are manifest in the unmeasured linear Stokes parameter for each telescope and thus can be separated
from the desired signal. Finally, the modulation cycle never mixes the Q and U linear Stokes parameters, and
thus residuals in the modulation do not twist the observed polarization vector. This is advantageous because
measuring the angle of linear polarization is critical for separating the inflationary signal from other polarized
One technique for mapping the polarization signature of the cosmic microwave background uses large, polarizing grids
in reflection. We present the system requirements, the fabrication, assembly, and alignment procedures, and the test
results for the polarizing grid component of a 50 cm clear aperture, Variable-delay Polarization Modulator (VPM). This
grid is being built and tested at the Goddard Space Flight Center as part of the Polarimeter for Observing Inflationary
Cosmology at the Reionization Epoch (POINCARE).
For the demonstration instrument, 64 μm diameter tungsten wires are being assembled into a 200 μm pitch, free-standing
wire grid with a 50 cm clear aperture, and an expected overall flatness better than 30 μm. A rectangular,
aluminum stretching frame holds the wires with sufficient tension to achieve a minimum resonant frequency of 185 Hz,
allowing VPM mirror translation frequencies of several Hz. A lightly loaded, flattening ring with a 50 cm inside
diameter rests against the wires and brings them into accurate planarity.