Conference Papers | CLASS | Johns Hopkins University

Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90 GHz band telescope

Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90 GHz band telescope
Rui Shi, Michael K. Brewer, Carol Yan Yan Chan, David T. Chuss, Jullianna Denes Couto, Joseph R. Eimer, John Karakla, Koji Shukawa, Deniz A. N. Valle, John W. Appel, Charles L. Bennett, Sumit Dahal, Thomas Essinger-Hileman, Tobias A. Marriage, Matthew A. Petroff, Karwan Rostem, Edward J. Wollack
Front-end polarization modulation enables improved polarization measurement stability by modulating the targeted signal above the low-frequency 1/f drifts associated with atmospheric and instrumental instabilities and diminishes the impact of instrumental polarization. In this work, we present the design and characterization of a new 60-cm diameter Reflective Half-Wave Plate (RHWP) polarization modulator for the 90 GHz band telescope of the Cosmology Large Angular Scale Surveyor (CLASS) project. The RHWP consists of an array of parallel wires (diameter 50 μm, 175 μm pitch) positioned 0.88 mm from an aluminum mirror. In lab tests, it was confirmed that the wire resonance frequency (fres) profile is consistent with the target, 139 Hz<fres<154 Hz in the optically active region (diameter smaller than 150 mm), preventing the wire vibration during operation and reducing the RHWP deformation under the wire tension. The mirror tilt relative to the rotating axis was controlled to be <15″, corresponding to an increase in beam width due to beam smearing of <0.6″, negligible compared to the beam’s full-width half-maximum of 36′. The median and 16/84th percentile of the wire–mirror separation residual was 0.048+0.013−0.014 mm in the optically active region, achieving a modulation efficiency ϵ=96.2−0.4+0.5% with an estimated bandpass of 34 GHz. The angular velocity of the RHWP was maintained to an accuracy of within 0.005% at the nominal rotation frequency (2.5 Hz). The RHWP has been successfully integrated into the CLASS 90 GHz telescope and started taking data in June 2024, replacing the previous modulator that has been in operation since June 2018.

On-Sky Performance of New 90 GHz Detectors for the Cosmology Large Angular Scale Surveyor (CLASS)

On-Sky Performance of New 90 GHz Detectors for the Cosmology Large Angular Scale Surveyor (CLASS)
Núñez, Carolina ; Appel, John W. ; Brewer, Michael K. ; Bruno, Sarah Marie ; Datta, Rahul ; Bennett, Charles L. ; Bustos, Ricardo ; Chuss, David T. ; Dahal, Sumit search by orcid ; Denis, Kevin L. ; Eimer, Joseph ; Essinger-Hileman, Thomas ; Helson, Kyle ; Marriage, Tobias ; Pérez, Carolina Morales ; Padilla, Ivan L. ; Petroff, Matthew A. ; Rostem, Karwan ; Watts, Duncan J. ; Wollack, Edward J. search by orcid ; Xu, Zhilei
The Cosmology Large Angular Scale Surveyor (CLASS) is a polarization-sensitive telescope array located at an altitude of 5,200 m in the Chilean Atacama Desert and designed to measure the polarized Cosmic Microwave Background (CMB) over large angular scales. The CLASS array is currently observing with three telescopes covering four frequency bands: one at 40 GHz (Q); one at 90 GHz (W1); and one dichroic system at 150/220 GHz (HF). During the austral winter of 2022, we upgraded the first 90 GHz telescope (W1) by replacing four of the seven focal plane modules. These new modules contain detector wafers with an updated design, aimed at improving the optical efficiency and detector stability. We present a description of the design changes and measurements of on-sky optical efficiencies derived from observations of Jupiter.

Long-Timescale Stability in CMB Observations at Multiple Frequencies using Front-End Polarization Modulation

Long-Timescale Stability in CMB Observations at Multiple Frequencies using Front-End Polarization Modulation
Joseph Cleary, Rahul Datta, John W. Appel, Charles L. Bennett, David T. Chuss, Jullianna Denes Couto, Sumit Dahal, Francisco Espinoza, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Yunyang Li, Tobias A. Marriage, Carolina Nunez, Matthew A. Petroff, Rodrigo A. Reeves, Rui Shi, Duncan J. Watts, Edward J. Wollack, Zhilei Xu
The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array observing the Cosmic Microwave Background (CMB) at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the CMB polarization on the largest angular scales to constrain the inflationary tensor-to-scalar ratio and the optical depth due to reionization. To achieve the long time-scale stability necessary for this measurement from the ground, CLASS utilizes a front-end, variable-delay polarization modulator on each telescope. Here we report on the improvements in stability afforded by front-end modulation using data across all four CLASS frequencies. Across one month of modulated linear polarization data in 2021, CLASS achieved median knee frequencies of 9.1, 29.1, 20.4, and 36.4 mHz for the 40, 90, 150, and 220 GHz observing bands. The knee frequencies are approximately an order of magnitude lower than achieved via CLASS pair-differencing orthogonal detector pairs without modulation.

Construction of a Large Diameter Reflective Half-Wave Plate Modulator for Millimeter Wave Applications

Construction of a Large Diameter Reflective Half-Wave Plate Modulator for Millimeter Wave Applications
Joseph R. Eimer, Michael K. Brewer, David T. Chuss, John Karakla, Rui Shi, John W. Appel, Charles L. Bennett, Joseph Cleary, Sumit Dahal, Rahul Datta, Thomas Essinger-Hileman, Tobias A. Marriage, Carolina Núñez, Mattew A. Petroff, Duncan J. Watts, Edward J. Wollack, Zhilei Xu
Polarization modulation is a powerful technique to increase the stability of measurements by enabling the distinction of a polarized signal from dominant slow system drifts and unpolarized foregrounds. Furthermore, when placed as close to the sky as possible, modulation can reduce systematic errors from instrument polarization. In this work, we introduce the design and preliminary drive system laboratory performance of a new 60 cm diameter reflective half-wave plate (RHWP) polarization modulator. The wave plate consists of a wire array situated in front of a flat mirror. Using \mbox{50 μm} diameter wires with \mbox{175 μm} spacing, the wave plate will be suitable for operation in the millimeter wavelength range with flatness of the wires and parallelism to the mirror held to a small fraction of a wavelength. The presented design targets the 77–108 GHz range. Modulation is performed by a rotation of the wave plate with a custom rotary drive utilizing an actively controlled servo motor.

Design and characterization of new 90 GHz detectors for the Cosmology Large Angular Scale Surveyor (CLASS)

Design and characterization of new 90 GHz detectors for the Cosmology Large Angular Scale Surveyor (CLASS)
Carolina Núñez, John W. Appel, Sarah Marie Bruno, Rahul Datta, Aamir Ali, Charles L. Bennett, Sumit Dahal, Jullianna Denes Couto, Kevin L. Denis, Joseph Eimer, Francisco Espinoza, Tom Essinger-Hileman, Kyle Helson, Jeffrey Iuliano, Tobias A. Marriage, Carolina Morales Pérez, Deniz Augusto Nunes Valle, Matthew A. Petroff, Karwan Rostem, Rui Shi, Duncan J. Watts, Edward J. Wollack, Zhilei Xu
The Cosmology Large Angular Scale Surveyor (CLASS) is a polarization-sensitive telescope array located at an altitude of 5,200 m in the Chilean Atacama Desert. CLASS is designed to measure “E-mode” (even parity) and “B-mode” (odd parity) polarization patterns in the Cosmic Microwave Background (CMB) over large angular scales with the aim of improving our understanding of inflation, reionization, and dark matter. CLASS is currently observing with three telescopes covering four frequency bands: one at 40 GHz (Q); one at 90 GHz (W1); and one dichroic system at 150/220 GHz (G). In these proceedings, we discuss the updated design and in-lab characterization of new 90 GHz detectors. The new detectors include design changes to the transition-edge sensor (TES) bolometer architecture, which aim to improve stability and optical efficiency. We assembled and tested four new detector wafers, to replace four modules of the W1 focal plane. These detectors were installed into the W1 telescope, and will achieve first light in the austral winter of 2022. We present electrothermal parameters and bandpass measurements from in-lab dark and optical testing. From in-lab dark tests, we also measure a median NEP of 12.3 aWs√ across all four wafers about the CLASS signal band, which is below the expected photon NEP of 32 aWs√ from the field. We therefore expect the new detectors to be photon noise limited.

Cosmology Large Angular Scale Surveyor (CLASS): Pointing Stability and Beam Measurements at 90, 150, and 220 GHz

Cosmology Large Angular Scale Surveyor (CLASS): Pointing Stability and Beam Measurements at 90, 150, and 220 GHz
Rahul Datta, Michael K. Brewer, Jullianna D. Couto, Joseph R. Eimer, Yunyang Li, Zhilei Xu, John W. Appel, Ricardo Bustos, David T. Chuss, Joseph Cleary, Sumit Dahal, Thomas Essinger-Hileman, Jeffrey Iuliano, Tobias A. Marriage, Carolina Núñez, Matthew A. Petroff, Karwan Rostem, Duncan J. Watts, Edward J. Wollack
The Cosmology Large Angular Scale Surveyor (CLASS) telescope array surveys 75% of the sky from the Atacama desert in Chile at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the largest-angular-scale CMB polarization with the aim of constraining the tensor-to-scalar ratio, measuring the optical depth to reionization to near the cosmic variance limit, and more. The CLASS Q-band (40 GHz), W-band (90 GHz), and dichroic high frequency (150/220 GHz) telescopes have been observing since June 2016, May 2018, and September 2019, respectively. On-sky optical characterization of the 40 GHz instrument has been published. Here, we present preliminary on-sky measurements of the beams at 90, 150, and 220 GHz, and pointing stability of the 90 and 150/220 GHz telescopes. The average 90, 150, and 220 GHz beams measured from dedicated observations of Jupiter have full width at half maximum (FWHM) of 0.615+/-0.019 deg, 0.378+/-0.005 deg, and 0.266+/-0.008 deg, respectively. Telescope pointing variations are within a few percent of the beam FWHM.

Control and systems software for the Cosmology Large Angular Scale Surveyor (CLASS)

Control and systems software for the Cosmology Large Angular Scale Surveyor (CLASS)
Matthew A. Petroff, John W. Appel, Charles L. Bennett, Michael K. Brewer, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Denes Couto, Sumit Dahal, Joseph R. Eimer, Thomas Essinger-Hileman, Pedro Fluxá Rojas, Kathleen Harrington, Jeffrey Iuliano, Tobias A. Marriage, Nathan J. Miller, Deniz Augusto Nunes Valle, Duncan J. Watts, Zhilei Xu
The Cosmology Large Angular Scale Surveyor (CLASS) is an array of polarization-sensitive millimeter wave telescopes that observes ~70% of the sky at frequency bands centered near 40GHz, 90GHz, 150GHz, and 220GHz from the Atacama desert of northern Chile. Here, we describe the architecture of the software used to control the telescopes, acquire data from the various instruments, schedule observations, monitor the status of the instruments and observations, create archival data packages, and transfer data packages to North America for analysis. The computer and network architecture of the CLASS observing site is also briefly discussed. This software and architecture has been in use since 2016, operating the telescopes day and night throughout the year, and has proven successful in fulfilling its design goals.

The Cosmology Large Angular Scale Surveyor Receiver Design

The Cosmology Large Angular Scale Surveyor Receiver Design
Jeffrey Iuliano, Joseph Eimer, Lucas Parker, Gary Rhoades et al.

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 50 mK 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.
Proceedings of the SPIE: Instrumentation and Methods for Astrophysics (2018).

Aerogel scattering filters for cosmic microwave background observations

Aerogel scattering filters for cosmic microwave background observations
Thomas Essinger-Hileman et al.
We present the design and performance of broadband and tunable infrared-blocking filters for millimeter and sub-millimeter astronomy composed of small scattering particles embedded in an aerogel substrate. The ultra-low-density (< 100 mg/cm3) aerogel substrate provides an index of refraction as low as 1.05, removing the need for anti-reflection coatings and allowing for broadband operation from DC to above 1 THz. The size distribution of the scattering particles can be tuned to provide a variable cutoff frequency. Aerogel filters with embedded high-resistivity silicon powder are being produced at 40-cm diameter to enable large-aperture cryogenic receivers for cosmic microwave background polarimeters, which require large arrays of sub-Kelvin detectors in their search for the signature of an inflationary gravitational-wave background.
Proceedings of the SPIE: Instrumentation and Methods for Astrophysics (2018).

Design and characterization of the Cosmology Large Angular Scale Surveyor (CLASS) 93 GHz focal plane

Sumit Dahal et al.
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 aWs√.
Proceedings of the SPIE: Instrumentation and Methods for Astrophysics (2018).

Variable-delay Polarization Modulators for the CLASS Telescopes

Kathleen Harrington, Joseph Eimer et al.
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 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.
Proceedings of the SPIE: Instrumentation and Methods for Astrophysics (2018).

SiAl Alloy Feedhorn Arrays: Material Properties, Feedhorn Design, and Astrophysical Applications

SiAl Alloy Feedhorn Arrays: Material Properties, Feedhorn Design, and Astrophysical Applications
Aamir M. Ali, Thomas Essinger-Hileman, Tobias Marriage et al.
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 polarimeteric focal planes used in the Cosmology Large Angular Scale Surveyor (CLASS). 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.
Proceedings of the SPIE: Instrumentation and Methods for Astrophysics (2018).

The Cosmology Large Angular Scale Surveyor

The Cosmology Large Angular Scale Surveyor
Harrington, Kathleen; Marriage, Tobias et al.
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.).
Proceedings of the SPIE, Volume 9914, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII, pp. 99141K (2016).

Silicon-Based Antenna-Coupled Polarization-Sensitive Millimeter-Wave Bolometer Arrays for Cosmic Microwave Background Instruments

Silicon-Based Antenna-Coupled Polarization-Sensitive Millimeter-Wave Bolometer Arrays for Cosmic Microwave Background Instruments
Rostem, Karwan et al.
We describe feedhorn-coupled polarization-sensitive detector arrays that utilize monocrystalline silicon as the dielectric substrate material. Monocrystalline silicon has a low-loss tangent and repeatable dielectric constant, characteristics that are critical for realizing efficient and uniform superconducting microwave circuits. An additional advantage of this material is its low specific heat. In a detector pixel, two Transition-Edge Sensor (TES) bolometers are antenna-coupled to in-band radiation via a symmetric planar orthomode transducer (OMT). Each orthogonal linear polarization is coupled to a separate superconducting microstrip transmission line circuit. On-chip filtering is employed to both reject out-of-band radiation from the upper band edge to the gap frequency of the niobium superconductor, and to flexibly define the bandwidth for each TES to meet the requirements of the application. The microwave circuit is compatible with multi-chroic operation. Metalized silicon platelets are used to define the backshort for the waveguide probes. This micro-machined structure is also used to mitigate the coupling of out-of-band radiation to the microwave circuit. At 40 GHz, the detectors have a measured efficiency of ˜90%. In this paper, we describe the development of the 90 GHz detector arrays that will be demonstrated using the Cosmology Large Angular Scale Surveyor (CLASS) ground-based telescope.
Proceedings of the SPIE Astronomical Telescopes+ Instrumentation, Volume 99140, pp. 99140D-99140D-10 (2016).

CLASS: the cosmology large angular scale surveyor

CLASS: the cosmology large angular scale surveyor
Essinger-Hileman, Thomas et al.
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 .
Proceedings of the SPIE, Volume 9153, id. 91531I 23 pp. (2014).

The cosmology large angular scale surveyor (CLASS): 38-GHz detector array of bolometric polarimeters

The cosmology large angular scale surveyor (CLASS): 38-GHz detector array of bolometric polarimeters
Appel, John W. et al.
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.
Proceedings of the SPIE, Volume 9153, id. 91531J 15 pp. (2014).

Scalable background-limited polarization-sensitive detectors for mm-wave applications

Scalable background-limited polarization-sensitive detectors for mm-wave applications
Rostem, Karwan et al.
We report on the status and development of polarization-sensitive detectors for millimeter-wave applications. The detectors are fabricated on single-crystal silicon, which functions as a low-loss dielectric substrate for the microwave circuitry as well as the supporting membrane for the Transition-Edge Sensor (TES) bolometers. The orthomode transducer (OMT) is realized as a symmetric structure and on-chip filters are employed to define the detection bandwidth. A hybridized integrated enclosure reduces the high-frequency THz mode set that can couple to the TES bolometers. An implementation of the detector architecture at Q-band achieves 90% efficiency in each polarization. The design is scalable in both frequency coverage, 30-300 GHz, and in number of detectors with uniform characteristics. Hence, the detectors are desirable for ground-based or space-borne instruments that require large arrays of efficient background-limited cryogenic detectors.
Proceedings of the SPIE, Volume 9153, id. 91530B 7 pp. (2014).

Phase-controlled polarization modulators

Phase-controlled polarization modulators
Chuss, David T.; Wollack, Edward J.; Novak, G.; Pisano, Giampaolo; Eimer, J. R.; Moseley, S. H.; Krejny, M.; U-Yen, K.
We report technology development of millimeter/submillimeter polarization modulators that operate by introducing a variable, controlled phase delay between two orthogonal polarization states. The variable-delay polarization modulator (VPM) operates via the introduction of a variable phase delay between two linear orthogonal polarization states, resulting in a variable mapping of a single linear polarization into a combination of that Stokes parameter and circular (Stokes V) polarization. Characterization of a prototype VPM is presented at 350 and 3000 microns. We also describe a modulator in which a variable phase delay is introduced between right- and left- circular polarization states. In this architecture, linear polarization is fully modulated. Each of these devices consists of a polarization diplexer parallel to and in front of a movable mirror. Modulation involves sub-wavelength translations of the mirror that change the magnitude of the phase delay.
Proceedings of the SPIE, Volume 8452, article id. 84521Y, 6 pp. (2012).

The Cosmology Large Angular Scale Surveyor (CLASS): 40 GHz optical design

The Cosmology Large Angular Scale Surveyor (CLASS): 40 GHz optical design
Eimer, Joseph R. et al.
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.
Proceedings of the SPIE 8452, 845220 (September 24, 2012); doi:10.1117/12.925464

Detector architecture of the cosmology large angular scale surveyor

Detector architecture of the cosmology large angular scale surveyor
Rostem, K. et al.
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.
Proceedings of the SPIE, Volume 8452, id. 84521N-84521N-7 (2012).

A Detector for Cosmic Microwave Background Polarimetry

A Detector for Cosmic Microwave Background Polarimetry
Chuss, D.; Cao, N.; Denis, K.; Hsieh, W.; Moseley, S.; Schneider, G.; Stevenson, T.; Travers, D.; K. U-Yen; Wollack, E.
We present preliminary design and development work on polarized detectors intended to enable Cosmic Microwave Background polarization measurements that will probe the first moments of the universe. The ultimate measurement will be challenging, requiring background-limited detectors and good control of systematic errors. A large collection of single-mode polarized detectors will eventually be required for the reliable detection of the weak polarized signature that is expected to result from gravitational waves produced by cosmic inflation. Toward this end, we are integrating the beam control of HE11 feedhorns with the sensitivity of transition-edge sensors. The coupling between these two devices is achieved via waveguide probe antennas and superconducting microstrip lines. This implementation allows band-pass filters to be incorporated on the detector chip. This focal plane prototype is an important step along the path to this detection, resulting in a capability that will enable various future high-performance instrument concepts.
Proceedings of the ASP Conference Series, Volume 449. San Francisco: Astronomical Society of the Pacific, 2012., p.81 (2011).

Variable-delay Polarization Modulators (VPMs) for Far-infrared through Millimeter Astronomy

Variable-delay Polarization Modulators (VPMs) for Far-infrared through Millimeter Astronomy
Chuss, D. T.; Voellmer, G.; Wollack, E. J.; Moseley, S. H.; Novak, G.; Krejny, M.; Walker, C. K.; Kulesa, C.; D’Aubigny, C. Y. D.; Golish, D.; Loewenstein, R. F.; Bennett, C. L.; Zeng, L.; Eimer, J.
We describe a novel means of polarization modulation with potential applications for the far-infrared through millimeter. A Variable-delay Polarization Modulator (VPM) modulates polarization via introduction of a variable phase between two fixed linear orthogonal polarizations. Our implementation does so by employing a grid-mirror pair in which the mirror is translated, thereby changing the relative phase between the two polarizations and systematically modulating the polarization state. For the Hertz/VPM project, we have implemented a pair of VPMs in a submillimeter polarimeter at the University of Arizonas SMTO. Promising initial results have shown the instrumental polarization in this system to be below 1%. In this presentation, we describe this polarimeter, its initial results, and prospects for VPMs including use in measuring the polarization of the cosmic microwave background to search for the polarized signature of cosmic inflation.
Proceedings of the ASP Conference Series, Volume 449. San Francisco: Astronomical Society of the Pacific, 2012., p.9 (2011).

A wide-band smooth-walled feedhorn with low cross polarization for millimeter astronomy

A wide-band smooth-walled feedhorn with low cross polarization for millimeter astronomy
Zeng, Lingzhen; Bennett, Charles L.; Chuss, David T.; Wollack, Edward J.
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.
Proceedings of the SPIE, Volume 7741, pp. 774129-774129-9 (2010).

Fabrication of an Antenna-Coupled Bolometer for Cosmic Microwave Background Polarimetry

Fabrication of an Antenna-Coupled Bolometer for Cosmic Microwave Background Polarimetry
Denis, K. L.; Cao, N. T.; Chuss, D. T.; Eimer, J.; Hinderks, J. R.; Hsieh, W.-T.; Moseley, S. H.; Stevenson, T. R.; Talley, D. J.; U.-Yen, K.; Wollack, E. J.
We describe the development of a detector for precise measurements of the cosmic microwave background polarization. The detector employs a waveguide to couple light between a pair of Mo/Au superconducting transition edge sensors (TES) and a feedhorn. Incorporation of an on-chip ortho-mode transducer (OMT) results in high isolation. The OMT is micromachined and bonded to the microstrip and TES circuits in a low temperature wafer bonding process. The wafer bonding process incorporates a buried superconducting niobium layer with a single crystal silicon layer which serves as the leg isolated TES membrane and as the microstrip dielectric. We describe the micromachining and wafer bonding process and report measurement results of the microwave circuitry operating in the 29-45 GHz band along with Johnson noise measurements of the TES membrane structures and development of Mo/Au TES operating under 100 mK.
Proceedings of the Thirteenth International Workshop on Low Temperature Detectors-LTD13. AIP Conference Proceedings, Volume 1185, pp. 371-374 (2009).

A large free-standing wire grid for microwave variable-delay polarization modulation

A large free-standing wire grid for microwave variable-delay polarization modulation
Voellmer, G. M.; Bennett, C.; Chuss, D. T.; Eimer, J.; Hui, H.; Moseley, S. H.; Novak, G.; Wollack, E. J.; Zeng, L.
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.
Proceedings of the SPIE, Volume 7014, pp. 70142A-70142A-12 (2008).