High-Efficiency and Low-Noise Detectors for the Upgraded CLASS 90 GHz Focal Plane
Carolina Núñez, John W. Appel, Rahul Datta, Charles L. Bennett, Michael K. Brewer, Sarah Marie Bruno, Ricardo Bustos, David T. Chuss, Nick Costen, Jullianna Denes Couto, Sumit Dahal, Kevin L. Denis, Joseph R. Eimer, Thomas Essinger-Hileman, Jeffrey Iuliano, Yunyang Li, Tobias A. Marriage, Jennette Mateo, Matthew A. Petroff, Rui Shi, Karwan Rostem, Deniz A. N. Valle, Duncan Watts, Edward J. Wollack, Lingzhen Zeng
We present the in-lab and on-sky performance for the upgraded 90 GHz focal plane of the Cosmology Large Angular Scale Surveyor (CLASS), which had four of its seven detector wafers updated during the austral winter of 2022. The update aimed to improve the transition-edge-sensor (TES) stability and bias range and to realize the high optical efficiency of the sensor design. Modifications included revised circuit terminations, electrical contact between the TES superconductor and the normal metal providing the bulk of the bolometer’s heat capacity, and additional filtering on the TES bias lines. The upgrade was successful: 94% of detectors are stable down to 15% of the normal resistance, providing a wide overlapping range of bias voltages for all TESs on a wafer. The median telescope efficiency improved from 0.42+0.15−0.22 to 0.60+0.10−0.32 (68% quantiles). For the four upgraded wafers alone, median telescope efficiency increased to 0.65+0.06−0.06. Given our efficiency estimate for the receiver optics, this telescope efficiency implies a detector efficiency exceeding 0.90. The overall noise-equivalent temperature of the 90 GHz focal plane improved from 19 μKs√ to 11.3 μKs√.

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.

Calibration of TES bolometer arrays with application to CLASS
John W. Appel, Charles L. Bennett, Michael K. Brewer, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna D. Couto, Sumit Dahal, Rahul Datta, Kevin Denis, Joseph Eimer, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Yunyang Li, Tobias A. Marriage, Carolina Núñez, Keisuke Osumi, Ivan L. Padilla, Matthew A. Petroff, Karwan Rostem, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
Current and future cosmic microwave background (CMB) experiments fielding kilo-pixel arrays of transition-edge sensor (TES) bolometers require accurate and robust gain calibration methods. We simplify and refactor the standard TES model to directly relate the detector responsivity calibration and optical time constant to the measured TES current I and the applied bias current Ib. The calibration method developed for the Cosmology Large Angular Scale Surveyor (CLASS) TES bolometer arrays relies on current versus voltage (I–V) measurements acquired daily prior to CMB observations. By binning Q-band (40GHz) I–V measurements by optical loading, we find that the gain calibration median standard error within a bin is 0.3%. We test the accuracy of this “I–V bin” detector calibration method by using the Moon as a photometric standard. The ratio of measured Moon amplitudes between detector pairs sharing the same feedhorn indicates a TES calibration error of 0.5%. We also find that for the CLASS Q-band TES array, calibrating the response of individual detectors based solely on the applied TES bias current accurately corrects TES gain variations across time but introduces a bias in the TES calibration from data counts to power units. Since the TES current bias value is set and recorded before every observation, this calibration method can always be applied to raw TES data and is not subject to I–V data quality or processing errors.

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.

Four-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: On-sky Receiver Performance at 40, 90, 150, and 220 GHz Frequency Bands
Sumit Dahal, John W. Appel, Rahul Datta, Michael K. Brewer, Aamir Ali, Charles L. Bennett, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna D. Couto, Kevin L. Denis, Rolando Dünner, Joseph Eimer, Francisco Espinoza, Thomas Essinger-Hileman, Joseph E. Golec, Kathleen Harrington, Kyle Helson, Jeffrey Iuliano, John Karakla, Yunyang Li, Tobias A. Marriage, Jeffrey J. McMahon, Nathan J. Miller, Sasha Novack, Carolina Núñez, Keisuke Osumi, Ivan L. Padilla, Gonzalo A. Palma, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Gary Rhoades, Karwan Rostem, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
The Cosmology Large Angular Scale Surveyor (CLASS) observes the polarized cosmic microwave background (CMB) over the angular scales of 1∘≲θ≤ 90∘ with the aim of characterizing primordial gravitational waves and cosmic reionization. We report on the on-sky performance of the CLASS Q-band (40 GHz), W-band (90 GHz), and dichroic G-band (150/220 GHz) receivers that have been operational at the CLASS site in the Atacama desert since June 2016, May 2018, and September 2019, respectively. We show that the noise-equivalent power measured by the detectors matches the expected noise model based on on-sky optical loading and lab-measured detector parameters. Using Moon, Venus, and Jupiter observations, we obtain power-to-antenna-temperature calibrations and optical efficiencies for the telescopes. From the CMB survey data, we compute instantaneous array noise-equivalent-temperature sensitivities of 22, 19, 24, and 56 μKcmbs√ for the 40, 90, 150, and 220 GHz frequency bands, respectively. These noise temperatures refer to white noise amplitudes, which contribute to sky maps at all angular scales. Future papers will assess additional noise sources impacting larger angular scales.

The CLASS 150/220 GHz Polarimeter Array: Design, Assembly, and Characterization
Sumit Dahal, Mandana Amiri, John W. Appel, Charles L. Bennett, Lance Corbett, Rahul Datta, Kevin Denis, Thomas Essinger-Hileman, Mark Halpern, Kyle Helson, Gene Hilton, Johannes Hubmayr, Benjamin Keller, Tobias Marriage, Carolina Nunez, Matthew Petroff, Carl Reintsema, Karwan Rostem, Kongpop U-Yen, Edward Wollack
Video Link
We report on the development of a polarization-sensitive dichroic (150/220 GHz) detector array for the Cosmology Large Angular Scale Surveyor (CLASS) delivered to the telescope site in June 2019. In concert with existing 40 and 90 GHz telescopes, the 150/220 GHz telescope will make observations of the cosmic microwave background over large angular scales aimed at measuring the primordial B-mode signal, the optical depth to reionization, and other fundamental physics and cosmology. The 150/220 GHz focal plane array consists of three detector modules with 1020 transition edge sensor (TES) bolometers in total. Each dual-polarization pixel on the focal plane contains four bolometers to measure the two linear polarization states at 150 and 220 GHz. Light is coupled through a planar orthomode transducer (OMT) fed by a smooth-walled feedhorn array made from an aluminum-silicon alloy (CE7). In this work, we discuss the design, assembly, and in-lab characterization of the 150/220 GHz detector array. The detectors are photon-noise limited, and we estimate the total array noise-equivalent power (NEP) to be 2.5 and 4 aWs√ for 150 and 220 GHz arrays, respectively.
arXiv: 1908.00480

On-sky performance of the CLASS Q-band telescope
Appel, John W.; Xu, Zhilei; Padilla, Ivan L.; Harrington, Kathleen; Pradenas Marquez, Bastián et al.
The Cosmology Large Angular Scale Surveyor (CLASS) is mapping the polarization of the Cosmic Microwave Background (CMB) at large angular scales (2<ℓ≲200) in search of a primordial gravitational wave B-mode signal down to a tensor-to-scalar ratio of r≈0.01. The same dataset will provide a near sample-variance-limited measurement of the optical depth to reionization. Between June 2016 and March 2018, CLASS completed the largest ground-based Q-band CMB survey to date, covering over 31 000~square-degrees (75% of the sky), with an instantaneous array noise-equivalent temperature (NET) sensitivity of 32 μKcmbs√. We demonstrate that the detector optical loading (1.6 pW) and noise-equivalent power (19 aWs√) match the expected noise model dominated by photon bunching noise. We derive a 13.1±0.3 K/pW calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of 0.48±0.04 and a 27 K system noise temperature. Finally, we report a Tau A flux density of 315±19 Jy at 38.8±1.0 GHz, consistent with the WMAP Tau A time-dependent spectral flux density model.
arXiv:1811.08287

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).

Cosmology Large Angular Scale Surveyor (CLASS) Focal Plane Development
Chuss, D. T. et al.
The Cosmology Large Angular Scale Surveyor (CLASS) will measure the polarization of the Cosmic Microwave Background to search for and characterize the polarized signature of inflation. CLASS will operate from the Atacama Desert and observe ~70% of the sky. A variable-delay polarization modulator provides modulation of the polarization at ~10 Hz to suppress the 1/ f noise of the atmosphere and enable the measurement of the large angular scale polarization modes. The measurement of the inflationary signal across angular scales that spans both the recombination and reionization features allows a test of the predicted shape of the polarized angular power spectra in addition to a measurement of the energy scale of inflation. CLASS is an array of telescopes covering frequencies of 38, 93, 148, and 217 GHz. These frequencies straddle the foreground minimum and thus allow the extraction of foregrounds from the primordial signal. Each focal plane contains feedhorn-coupled transition-edge sensors that simultaneously detect two orthogonal linear polarizations. The use of single-crystal silicon as the dielectric for the on-chip transmission lines enables both high efficiency and uniformity in fabrication. Integrated band definition has been implemented that both controls the bandpass of the single-mode transmission on the chip and prevents stray light from coupling to the detectors.
arXiv151104414C [astro-ph.CO] Accepted by the Journal of Low Temperature Physics (2015).

Fabrication of Feedhorn-Coupled Transition Edge Sensor Arrays for Measurement of the Cosmic Microwave Background Polarization
Kevin Denis et al.
Characterization of the minute cosmic microwave background polarization signature requires multi-frequency, high-throughput precision instrument systems. We have previously described the detector fabrication of a 40 GHz focal plane and now describe the fabrication of detector modules for measurement of the CMB at 90 GHz. The 90 GHz detectors are a scaled version of the 40 GHz architecture where, due to smaller size detectors, we have implemented a modular (wafer level) rather than the chip-level architecture. The new fabrication process utilizes the same design rules with the added challenge of increased wiring density to the 74 TES’s as well as a new wafer level hybridization procedure. The hexagonally shaped modules are tile-able, and as such can be used to form the large focal planes required for a space-based CMB polarimeter. The detectors described here will be deployed in two focal planes with seven modules each in the Johns Hopkins University led ground-based Cosmology Large Angular Scale Surveyor (CLASS) telescope.
arXiv151105036D [astro-ph.CO] Presented at Low Temperature Detectors Conference Grenoble France (2015).

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).

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).

Fabrication of a Silicon Backshort Assembly for Waveguide-Coupled Superconducting Detectors
Erik J. Crowe et al.
The Cosmology Large Angular Scale Surveyor (CLASS) is a ground-based instrument that will measure the polarization of the cosmic microwave background to search for evidence for gravitational waves from a posited epoch of inflation early in the Universe’s history. This measurement will require integration of superconducting transition-edge sensors with microwave waveguide inputs with excellent control of systematic errors, such as unwanted coupling to stray signals at frequencies outside of a precisely defined microwave band. To address these needs, we present work on the fabrication of micromachined silicon, producing conductive quarter-wave backshort assemblies for the CLASS 40 GHz focal plane. Each 40 GHz backshort assembly consists of three degeneratively doped silicon wafers. Two spacer wafers are micromachined with through-wafer vias to provide a 2.04-mm-long square waveguide delay section. The third wafer terminates the waveguide delay in a short. The three wafers are bonded at the wafer level by Au-Au thermal compression bonding then aligned and flip chip bonded to the CLASS detector at the chip level. The micromachining techniques used have been optimized to create high aspect ratio waveguides, silicon pillars, and relief trenches with the goal of providing improved out of band signal rejection. We will discuss the fabrication of integrated CLASS superconducting detector chips with the quarter-wave backshort assemblies.
IEEE Transactions on Applied Superconductivity, vol. 23 , Issue, 3 (2013).

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).

Properties of a variable-delay polarization modulator
Chuss, David T.; Wollack, Edward J. et al.
We investigate the polarization modulation properties of a variable-delay polarization modulator (VPM). The VPM modulates polarization via a variable separation between a polarizing grid and a parallel mirror. We find that in the limit where the wavelength is much larger than the diameter of the metal wires that comprise the grid, the phase delay derived from the geometric separation between the mirror and the grid is sufficient to characterize the device. However, outside of this range, additional parameters describing the polarizing grid geometry must be included to fully characterize the modulator response. In this paper, we report test results of a VPM at wavelengths of 350 microns and 3 mm. Electromagnetic simulations of wire grid polarizers were performed and are summarized using a simple circuit model that incorporates the loss and polarization properties of the device.
Applied Optics, vol. 51, Issue 2, p. 197 (2012).

Electromagnetic Design of Feedhorn-Coupled Transition-Edge Sensors for Cosmic Microwave Background Polarimetry
Chuss, D. T. et al.
Observations of the cosmic microwave background (CMB) provide a powerful tool for probing the evolution of the early universe. Specifically, precision measurement of the polarization of the CMB enables a direct test for cosmic inflation. A key technological element on the path to the measurement of this faint signal is the capability to produce large format arrays of background-limited detectors. We describe the electromagnetic design of feedhorn-coupled, TES-based sensors. Each linear orthogonal polarization from the feedhorn is coupled to a superconducting microstrip line via a symmetric planar orthomode transducer (OMT). The symmetric OMT design allows for highly-symmetric beams with low cross-polarization over a wide bandwidth. In addition, this architecture enables a single microstrip filter to define the passband for each polarization. Care has been taken in the design to eliminate stray coupling paths to the absorbers. These detectors will be fielded in the Cosmology Large Angular Scale Surveyor (CLASS).
Journal of Low Temperature Physics, Volume 167, Issue 5-6, pp. 923-928 (2012).

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).

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).

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.

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
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.

Two Year Cosmology Large Angular Scale Surveyor (CLASS) Observations: Long Timescale Stability Achieved with a Front-End Variable-delay Polarization Modulator at 40 GHz
Kathleen Harrington, Rahul Datta, Keisuke Osumi, Aamir Ali, John W. Appel, Charles L. Bennett, Michael K. Brewer, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Denes Couto, Sumit Dahal, Rolando Dünner, Joseph R. Eimer, Thomas Essinger-Hileman, Johannes Hubmayr, Francisco Raul Espinoza Inostroza, Jeffrey Iuliano, John Karakla, Yunyang Li, Tobias A. Marriage, Nathan J. Miller, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Bastian Pradenas Márquez, Rodrigo Reeves, Pedro Fluxá Rojas, Karwan Rostem, Deniz Augusto Nunes Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
The Cosmology Large Angular Scale Surveyor (CLASS) is a four-telescope array observing the largest angular scales (2≲ℓ≲200) of the cosmic microwave background (CMB) polarization. These scales encode information about reionization and inflation during the early universe. The instrument stability necessary to observe these angular scales from the ground is achieved through the use of a variable-delay polarization modulator (VPM) as the first optical element in each of the CLASS telescopes. Here we develop a demodulation scheme used to extract the polarization timestreams from the CLASS data and apply this method to selected data from the first two years of observations by the 40 GHz CLASS telescope. These timestreams are used to measure the 1/f noise and temperature-to-polarization (T→P) leakage present in the CLASS data. We find a median knee frequency for the pair-differenced demodulated linear polarization of 15.12 mHz and a T→P leakage of <3.8×10−4 (95\% confidence) across the focal plane. We examine the sources of 1/f noise present in the data and find the component of 1/f due to atmospheric precipitable water vapor (PWV) has an amplitude of 203±12μKRJs√ for 1 mm of PWV when evaluated at 10 mHz; accounting for ∼32% of the 1/f noise in the central pixels of the focal plane. The low level of T→P leakage and 1/f noise achieved through the use of a front-end polarization modulator enables the observation of the largest scales of the CMB polarization from the ground by the CLASS telescopes.

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).

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).

Vector reflectometry in a beam waveguide
Eimer, J. R. et al.
We present a one-port calibration technique for characterization of beam waveguide components with a vector network analyzer. This technique involves using a set of known delays to separate the responses of the instrument and the device under test. We demonstrate this technique by measuring the reflected performance of a millimeter-wave variable-delay polarization modulator.
Review of Scientific Instruments, Volume 82, Issue 8, pp. 086101-086101-3 (2011).

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 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).

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
Zhilei Xu, Michael K. Brewer, Pedro Fluxa, Yunyang Li, Keisuke Osumi, Bastian Pradenas, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Couto, Sumit Dahal, Rahul Datta, Kevin Denis, Rolando Dunner, Joseph Eimer, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Tobias A. Marriage, Nathan Miller, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Duncan J. Watts, Janet Weiland, Edward J. Wollack
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∘≲θ⩽90∘) CMB polarization to constrain the tensor-to-scalar ratio at the r∼0.01 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 September 2016 to February 2018. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is <0.023∘ (<1.6% of the beam’s full width at half maximum (FWHM)). We estimate beam parameters per detector and in aggregate, as in the CMB survey maps. The aggregate beam has a FWHM of 1.579∘±.001∘ and a solid angle of 838±6 μsr, consistent with physical optics simulations. The corresponding beam window function has sub-percent error per multipole at ℓ<200. An extended 90∘ beam map reveals no significant far sidelobes. The observed Moon polarization shows that the instrument polarization angles are consistent with the optical model and that the temperature-to-polarization leakage fraction is <10−4 (95% C.L.). We find that the Moon-based results are consistent with measurements of M42, RCW 38, and Tau A from CLASS’s CMB survey data. In particular, Tau A measurements establish degree-level precision for instrument polarization angles.

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.

Cosmology Large Angular Scale Surveyor (CLASS): 90 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
Rahul Datta, Michael K. Brewer, Jullianna Denes Couto, Joseph Eimer, Yunyang Li, Zhilei Xu, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, David T. Chuss, Joseph Cleary, Sumit Dahal, Francisco Espinoza, Thomas Essinger-Hileman, Pedro Fluxá, Kathleen Harrington, Kyle Helson, Jeffrey Iuliano, John Karakla, Tobias A. Marriage, Sasha Novack, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Rui Shi, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Lingzhen Zeng
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 CMB polarization to constrain the tensor-to-scalar ratio and the optical depth to last scattering. This paper presents the optical characterization of the 90GHz telescope, which has been observing since July 2018. Observations of the Moon establish the pointing while dedicated observations of Jupiter are used for beam calibration. The standard deviations of the pointing error in azimuth, elevation, and boresight angle are 1.3, 2.1, and 2.0 arcminutes, respectively, over the first 3 years of observations. This corresponds to a pointing uncertainty ~7% of the beam’s full width at half maximum (FWHM). The effective azimuthally-symmetrized 1D beam estimated at 90 GHz from per detector intensity beam maps has a FWHM of 0.614+/-0.003 deg and a solid angle of 136.3+/-0.6(stats.)+/-1.1(sys.) usr integrated to a radius of 4 deg. The corresponding beam window function drops to b_ell^2 = 0.92, 0.70, 0.14 at ell = 30, 100, 300, respectively, with relative uncertainties < 2% for ell < 200. Far-sidelobes are studied using detector-centered intensity maps of the Moon and measured to be at a level of 10^-3 or below relative to the peak. The polarization angle of Tau A estimated from preliminary survey maps is 149.6+/-0.2(stats.) deg in equatorial coordinates consistent with prior measurements. Instrumental temperature-to-polarization (T-to-P) leakage is measured at a 95% confidence upper limit of (1.7+/-0.1) x 10^-3 in single detector demodulated data using observations of Jupiter and the Moon. Using pair-differenced demodulated data, a 95% confidence upper limit of 3.6 x 10^-4 is obtained on the T-to-P leakage.

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).

A 3D-printed broadband millimeter wave absorber  
Petroff, Matthew; Appel, John; Rostem, Karwan et al.
We present the design, manufacturing technique, and characterization of a 3D-printed broadband graded index millimeter wave absorber. The absorber is additively manufactured using a fused filament fabrication (FFF) 3D printer out of a carbon-loaded high impact polystyrene (HIPS) filament and is designed using a space-filling curve to optimize manufacturability using said process. The absorber’s reflectivity is measured from 63 GHz to 115 GHz and from 140 GHz to 215 GHz and is compared to electromagnetic simulations. The intended application is for terminating stray light in Cosmic Microwave Background (CMB) telescopes, and the absorber has been shown to survive cryogenic thermal cycling.
arXiv:1808.00820

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 (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

A Low Cross-Polarization Smooth-Walled Horn With Improved Bandwidth
Zeng, Lingzhen et al.
Corrugated feed horns offer excellent beam symmetry, main beam efficiency, and cross-polar response over wide bandwidths, but can be challenging to fabricate. An easier-to-manufacture smooth-walled feed is explored that approximates these properties over a finite bandwidth. The design, optimization and measurement of a monotonically-profiled, smooth-walled scalar feedhorn with a diffraction-limited ~14° FWHM beam is presented. The feed was demonstrated to have low cross polarization (< -30 dB) across the frequency range 33-45 GHz (30% fractional bandwidth). A power reflection below -28 dB was measured across the band.
IEEE Transactions on Antennas and Propagation, vol. 58, Issue 4, pp. 1383-1387 (2010).

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).

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.