arXiv subjects

Bradley R. Johnson

Publications and source records attributed to Bradley R. Johnson.

At least 19 recordsLinked to original sources

arXiv2026

The Simons Observatory: Design and Initial Performance of the Detector Readout System for the Large-Aperture Telescope

We present the design, implementation, and initial performance of a highly multiplexed cryogenic superconducting detector readout system in the context of its deployment to the Simons Observatory large-aperture telescope (LAT). The Simons Observatory is a cosmic microwave background experiment located at 5,200 m in Chile's Atacama Desert. In addition to its 6 m LAT, whose broad range of science goals spans probing large-scale structure to searching for new Solar System objects, the observatory also currently includes three 0.42 m small-aperture telescopes, which search for signatures of primordial gravitational waves. Nearly 100,000 optically-coupled transition-edge sensor bolometers have been deployed across the observatory's four telescopes to enable these science goals. The readout systems for these telescopes use microwave frequency multiplexing to simultaneously read out 860 optically-coupled bolometers per transmission line. This is enabled by the large-scale fabrication of multiplexing chips which house radio-frequency superconducting quantum interference devices for each detector, and the development of room-temperature electronics to read out these bolometers. This has enabled the largest deployment of superconducting sensors for astronomical observations to date. This paper focuses on the design and implementation of the readout system for the 63,000 detectors in the LAT's cryogenic receiver. We present performance results from an early phase in the telescope's operation. We show that the readout system's performance enables the instrument to meet key benchmarks, including successful readout of 81.4% of its optically-coupled bolometers, which exceeds projections, and readout noise-equivalent current consistent with pre-deployment specifications.

astro-ph.IM↗

arXiv2026

The Simons Observatory: Rotation Performance of Cryogenic Half-Wave Plate Polarization Modulators

We present the on-site rotation performance of the first three cryogenic continuously rotating half-wave plate (HWP) polarization modulators for the Simons Observatory small aperture telescopes (SATs). The SATs operate at an altitude of 5200 m in the Atacama Desert in northern Chile, and measure the degree-scale cosmic microwave background polarization to search for primordial $B$-mode polarization. To this end, the SATs employ a 505 mm diameter 50 K cryogenic HWP polarization modulator to suppress atmospheric $1/f$ noise and to mitigate systematic uncertainties. We present methods for reconstructing the rotation of our polarization modulators, enabling detailed evaluations of their rotation angle accuracy, stability, displacements, and vibrations. We achieve the required rotation angle accuracy in more than 99.9% of observations, with a median noise level of $0.16\,μ\text{rad}\sqrt{\text{s}}$. We also achieve one-dimensional measurement of the rotor displacement with an accuracy of $0.06\,μ\text{m}\sqrt{\text{s}}$. Our results demonstrate the on-site rotation performance, rotation angle reconstruction method, and robustness of the polarization modulators. This is a crucial step towards achieving the SAT science goals.

astro-ph.IM↗

arXiv2026

The Simons Observatory: Commissioning of the Mid-Frequency Small Aperture Telescopes

The Simons Observatory (SO) is a cosmic microwave background (CMB) survey experiment observing from an altitude of 5,200 m in the Atacama Desert in Chile. The SO Small Aperture Telescopes (SATs) will cover six spectral bands between 30 and 280 GHz primarily to search for evidence of primordial gravitational waves in the large angular scale $B$-modes of the CMB polarization. The first two Mid-Frequency (MF) SATs, operating in the 90 and 150 GHz frequency bands, began commissioning observations in late 2023, and we present the results of those observations here. Each MF SAT operates with approximately 9,400 biased transition-edge sensors (TES) at 1 mm of precipitable water vapor (PWV), representing a typical 80% yield. Planet observations confirm the telescope optics performance, calibrate the instrument response, and measure the end-to-end optical efficiency of the SATs. The realized instrument efficiencies are within the range expected based on instrument model predictions. In nominal observing conditions, the noise equivalent temperatures from both telescopes combined are $2.4~\mathrm{μK_{CMB}\sqrt{s}}$ at 90 GHz and $3.2~\mathrm{μK_{CMB}\sqrt{s}}$ at 150 GHz, substantially outperforming the baseline instantaneous sensitivity targets for the observatory.

astro-ph.IM↗

arXiv2026

An Open Source RFSoC-based Readout Electronics System for Arrays of Kinetic Inductance Detectors and Superconducting Resonators

We present a novel readout electronics system for large arrays of superconducting electromagnetic resonators, such as kinetic inductance detectors (KIDs), based on a Radio Frequency System on Chip (RFSoC) architecture. Each channel in the readout system is designed for frequency division multiplexing of up to 1024 high quality factor resonances placed on a single microwave transmission line at unique frequencies within a 512 MHz bandwidth. We describe the design of the digital and analog signal processing chains for the implementation of a two-channel 2048-resonator system on the Xilinx ZCU111 RFSoC evaluation board in combination with a custom intermediate frequency (IF) system to convert the RFSoC band to higher frequencies up to 4 GHz. We also detail an associated software interface that provides a range of tools commonly utilized for characterizing KID resonators and for operating them as part of a photometric millimeter-wave imager. We additionally provide noise characterizations of the individual readout components, along with the complete readout system in isolation and in operation to readout KID resonators.

physics.ins-det↗

arXiv2026

Investigating anomalous microwave emission near G107.2+5.20 in Ku-band with the Green Bank Telescope

Anomalous microwave emission (AME) is 30 GHz-peaking continuum emission thought to arise from spinning dust grains. Observations suggest that the local environment shapes the AME spectral energy distribution (SED), so building a spatially resolved sample of AME regions is a key step towards understanding its emission mechanism. Using the Green Bank Telescope Ku-band receiver, we obtained a ~1 arcmin resolution map of radius 1.25 deg centered on G107.2+5.20. Our first objective was to constrain the low-frequency side of the AME SED with 13 GHz data. Using matched-resolution aperture photometry, we measure the SED from 408 MHz-3 THz and fit two emission models: one including spinning dust, the other optically thick free-free emission. We find that the spinning dust model is superior, with an amplitude of $14.1\pm1.1$ Jy and a peak frequency of $27\pm2$ GHz. Our second objective was to spatially locate excess 13 GHz emission consistent with spinning dust. We compare our Ku-band map to multi-wavelength gas and dust tracers at ~4 arcmin resolution. We observe two sources of 13 GHz excess at 3$σ$ significance consistent with spinning dust emission, though potential contributions from optically thick free-free emission remain ambiguous. ``Region A'' (G106.95+5.19) is spatially coincident with peak dust radiance. ``Region B'' (G107.08+4.90) is spatially coincident with peak PAH abundance. If regions A and B are indeed sources of spinning dust emission, they are a resolved example of how AME-dust correlation varies with environment.

astro-ph.GA↗

arXiv2026

Axion search with telescope for radio astronomy (ASTRA): forecast for observations between 0.5 and 4~GHz

Axion dark matter (DM) is predicted to convert into radio waves in neutron star magnetospheres. We assess the detectability of this signal using a 5 m radio telescope to be installed at the Fan Mountain Observatory, operating in the UHF, L- and S-bands from 0.5 to 4~GHz. We demonstrate that such a telescope can search new parameter space for axion-like particles over a broad range from $2\,μ\text{eV}<m_a<17\,μ\text{eV}$ for axion-photon couplings $g_{aγγ} \gtrsim 2\times 10^{-12}\text{ GeV}^{-1}$ with a three year observing period assuming the standard halo model -- improving neutron star observations by more than an order of magnitude. The search is broadband and is thus complementary to other techniques in the same frequency range. We describe in detail our neutron star population model, noise model, and proposed observing strategy. Most constraining power comes from neutron stars at the Galactic centre, where the smooth DM halo is densest. If a DM spike exists at the Galactic centre, the search is sensitive in the QCD axion model band. UHF and L-band observations (0.5 to 2~GHz) represent the pathfinder phase of a wider program we call ``Axion Search with Telescope for Radio Astronomy'' (ASTRA). Future higher mass searches aimed at discovery potential for the post-inflation axion require further hardware development to cover S, C, X and Ku bands (2 to 18~GHz).

hep-ph↗

arXiv2026

A New Window into the Baryon Cycle at Cosmic Noon with Line Intensity Mapping: Forecasts for auto- and cross-correlations in [CII]-158$μ$m, HI 21 cm, CO$_{J+1\rightarrow J}$, and H$α$ galaxies

Across the peak of cosmic star formation at $z\sim1-2$, inflow, processing, and feedback drive rapid changes in the spatial distribution and chemical composition of baryons in galaxies and surrounding reservoirs; this baryon cycle can be tomographically mapped by line intensity mapping (LIM) of atomic hydrogen, ionized carbon, and carbon monoxide. We present a simulation-based forecasting framework for detecting auto- and cross-power spectra between spectroscopic surveys of four such tracers at $z\sim0.5-1.7$ mapping the same deep field - TIM, EoRSpec/FYST, MeerKAT, & Euclid. We forward-model 3-D distributions for these tracers from magnetohydrodynamic simulations, directly capturing the two-halo, one-halo, and shot statistics without relying on analytical decompositions. We further detail a signal-to-noise formalism, tailored to LIM surveys with highly anisotropic geometries and Fourier-space coverage. We demonstrate that galaxy cross-correlations will be the dominant discovery channel for current-generation surveys. These instruments will detect the auto-spectra for CO and HI 21 cm and the CO $\times$ 21 cm cross-spectrum at modest S/N $\sim 1-10$, while placing upper limits on the [CII]-158$μ$m signals. [CII], CO, and HI LIM will be $\sim3-30\times$ ($0.5-1.5$ dex) more sensitive to cross-correlation with the Euclid survey, however, than their respective auto-correlations, constraining all three models of line emission at high significance (S/N $\sim 10-40$) within this decade. Finally, we formulate a staged instrumental trajectory with planned or reasonable improvements, including the as-proposed SKA-Mid. We forecast advancing the per-$k$-mode sensitivities of each auto-, galaxy-line, and line-line spectrum by several orders of magnitude, enabling new percent- and sub-percent level constraints on cosmology and the redshift evolution of star formation and the baryon cycle.

astro-ph.CO↗

arXiv2026

The Simons Observatory: Characterization of the 220/280 GHz TES Detector Modules

The Simons Observatory (SO) is a new suite of cosmic microwave background telescopes in the Chilean Atacama Desert with an extensive science program spanning cosmology, Galactic and extragalactic astrophysics, and particle physics. SO will survey the millimeter-wave sky over a wide range of angular scales using six spectral bands across three types of dichroic, polarization-sensitive transition-edge sensor (TES) detector modules: Low-Frequency (LF) modules with bandpasses centered near 30 and 40 GHz, Mid-Frequency (MF) modules near 90 and 150 GHz, and Ultra-High-Frequency (UHF) modules near 220 and 280 GHz. Twenty-five UHF detector modules, each containing 1720 optically-coupled TESs connected to microwave SQUID multiplexing readout, have now been produced. This work summarizes the pre-deployment characterization of these detector modules in laboratory cryostats. Across all UHF modules, we find an average operable TES yield of 83%, equating to over 36,000 devices tested. The distributions of (220, 280) GHz saturation powers have medians of (24, 26) pW, near the centers of their target ranges. For both bands, the median optical efficiency is 0.6, the median effective time constant is 0.4 ms, and the median dark noise-equivalent power (NEP) is ~40 aW/rtHz. The expected photon NEPs at (220, 280) GHz are (64, 99) aW/rtHz, indicating these detectors will achieve background-limited performance on the sky. Thirty-nine UHF and MF detector modules are currently operating in fielded SO instruments, which are transitioning from the commissioning stage to full science observations.

astro-ph.IM↗

arXiv2026

The Simons Observatory: On-sky performance of radio-transparent multi-layer insulation (RT-MLI) using Styroace-II Styrofoam

We present the on-sky performance of a Radio-Transparent Multi-Layer Insulation filter (RT-MLI) that uses Styroace-II styrofoam to reject ambient thermal radiation from entering a 0.42 m diameter aperture to a sub-100 mK bolometric detector array cooled by a dilution-refrigerator. We find that greater than 90% of the expected incident infra-red (IR) radiation is rejected, resulting in $<$12 W of measured transmitted power. Transmitted power in the detector passbands is consistent with a lower bound of 95%. We address filter design and placement, thermal loading, and mm-wave transmission.

astro-ph.IM↗

arXiv2025

The Simons Observatory: Detector Polarization Angle Calibration using Sparse Wire Grid with Initial Data Sets of the Small Aperture Telescope

Improved measurements of $B$-modes in the cosmic microwave background can be obtained through accurate calibration of the orientation of detector antennas as projected onto the sky. Miscalibration of the detector polarization angle leads to a leakage of $E$-modes into $B$-modes, which can bias the detection of the latter. To achieve a $σ(r)$ of 0.003, the Simons Observatory Small Aperture Telescopes are required to calibrate the global polarization angle on the sky with an accuracy ${\lesssim}0.1^\circ$. We demonstrate a fully remote-controllable calibration system using a ``sparse wire grid," which injects a rotatable linear polarized signal across the telescope's focal plane. This calibration system is installed and operational on a Small Aperture Telescope at its observing site at the Parque Astronómico in the Atacama desert in Chile. We developed a pipeline for the detector polarization angle calibration, and demonstrate it using initial data for 93~GHz and 145~GHz frequency bands. The observed distribution of detector polarization angles is in agreement with the instrument design. Statistical uncertainties for the relatively calibrated polarization angles are $0.02^\circ$ and $0.03^\circ$ at 93~GHz and 145~GHz, respectively. Systematic uncertainty was evaluated to be $0.08^\circ$ at the hardware development and fabrication stage. Their sum in quadrature is less than $0.1^\circ$.

astro-ph.IM↗

arXiv2025

First detections of methanol maser lines from a rare transition family

We report the first observations in a rare family of class II methanol maser transitions in both CH$_3$OH and $^{13}$CH$_3$OH toward three southern high-mass star formation regions, along with the first maser detected in the $^{13}$CH$_3$OH line. The $8_2 \rightarrow 9_1 A^{-}$ methanol transition was observed in both CH$_3$OH and $^{13}$CH$_3$OH (at 28.9 GHz and 41.9 GHz respectively) toward three sources; G358.93-0.03, NGC6334I and G345.01+1.79, all of which are star formation regions with recent maser flaring events. We report the first maser detection of the 41.9 GHz line in $^{13}$CH$_3$OH toward G358.93-0.03 and the first confirmed maser detection of the 28.9 GHz line in CH$_3$OH toward NGC6334I. Additionally we report a maser detection of the 28.9 GHz line in CH$_3$OH toward G358.93-0.03, meaning that with our detection of the 41.9 GHz line, this is the first isotopic detection of these lines toward G358.93-0.03. The newly detected maser transitions are associated with the primary millimetre continuum sources (MM1) in both G358.93-0.03 and NGC6334I, within the varying positional uncertainties.

astro-ph.GA↗

arXiv2025

The Simons Observatory: Assessing the Impact of Dust Complexity on the Recovery of Primordial $B$-modes

We investigate how dust foreground complexity can affect measurements of the tensor-to-scalar ratio, $r$, in the context of the Simons Observatory, using a cross-spectrum component separation analysis. Employing a suite of simulations with realistic Galactic dust emission, we find that spatial variation in the dust frequency spectrum, parametrized by $β_d$, can bias the estimate for $r$ when modeled using a low-order moment expansion to capture this spatial variation. While this approach performs well across a broad range of dust complexity, the bias increases with more extreme spatial variation in dust frequency spectrum, reaching as high as $r\sim0.03$ for simulations with no primordial tensors and a spatial dispersion of $σ(β_d)\simeq0.3$ -- the most extreme case considered, yet still consistent with current observational constraints. This bias is driven by changes in the $\ell$-dependence of the dust power spectrum as a function of frequency that can mimic a primordial $B$-mode tensor signal. Although low-order moment expansions fail to capture the full effect when the spatial variations of $β_d$ become large and highly non-Gaussian, our results show that extended parametric methods can still recover unbiased estimates of $r$ under a wide range of dust complexities. We further find that the bias in $r$, at the highest degrees of dust complexity, is largely insensitive to the spatial structure of the dust amplitude and is instead dominated by spatial correlations between $β_d$ and dust amplitude, particularly at higher orders. If $β_d$ does spatially vary at the highest levels investigated here, we would expect to use more flexible foreground models to achieve an unbiased constraint on $r$ for the noise levels anticipated from the Simons Observatory.

astro-ph.CO↗

arXiv2025

The Simons Observatory: Validation of reconstructed power spectra from simulated filtered maps for the Small Aperture Telescope survey

We present a transfer function-based method to estimate angular power spectra from filtered maps for cosmic microwave background (CMB) surveys. This is especially relevant for experiments targeting the faint primordial gravitational wave signatures in CMB polarisation at large scales, such as the Simons Observatory (SO) small aperture telescopes. While timestreams can be filtered to mitigate the contamination from low-frequency noise, usual methods that calculate the mode coupling at individual multipoles can be challenging for experiments covering large sky areas or reaching few-arcminute resolution. The method we present here, although approximate, is more practical and faster for larger data volumes. We validate it through the use of simulated observations approximating the first year of SO data, going from half-wave plate-modulated timestreams to maps, and using simulations to estimate the mixing of polarisation modes induced by an example of time-domain filtering. We show its performance through an example null test and with an end-to-end pipeline that performs inference on cosmological parameters, including the tensor-to-scalar ratio $r$. The performance demonstration uses simulated observations at multiple frequency bands. We find that the method can recover unbiased parameters for our simulated noise levels.

astro-ph.CO↗

arXiv2025

The Simons Observatory: Large Diameter and Large Load-Capacity Superconducting Magnetic Bearing for a Millimeter-Wave Polarization Modulator

We present the design methodology and characterization of a superconducting magnetic bearing (SMB) system for the polarization modulator in the SAT-LF, one of the small aperture telescopes (SATs) in the Simons Observatory (SO) that is sensitive at 30/40 GHz frequency bands. SO is a ground-based cosmic microwave background (CMB) polarization experiment, with the SATs specifically aiming to search for primordial parity-odd polarization anisotropies at degree scales. Each SAT is equipped with a cryogenic, continuously rotating half-wave plate (HWP) as a polarization modulator to mitigate atmospheric $1/f$ noise and instrumental systematics. The HWP system employs an SMB, consisting of a ring-shaped magnet and superconductor, to achieve a 550 mm clear aperture and stable 2 Hz rotation at a temperature of around 50 K. One challenge for the HWP system in the SAT-LF is the large 35 kg load on the SMB due to the thicker HWP than in previous telescopes. Since the SMB stiffness is critical for maintaining the alignment of the HWP in the telescope, we developed a method to quantitatively predict the stiffness using finite element simulations with the so-called H-formulation. We evaluated the stiffness for various geometries of the magnet and superconductor, thereby optimizing their dimensions. The prediction is in excellent agreement with experimental measurements of the fabricated SMB, demonstrating a $\sim$5\% accuracy. We also demonstrated that the SMB achieves sufficiently low friction-induced heat dissipation, measured at 0.26 W when rotating at 2 Hz. The design methodology and the implementation of the SMB demonstrated here not only provides an enabling technology for the SO SAT-LF, but also is a crucial stepping stone for future CMB experiments that make use of HWP polarization modulators.

astro-ph.IM↗

arXiv2025

A Millimeter-Wave Photometric Camera for Long-Range Imaging Through Optical Obscurants Using Kinetic Inductance Detectors

Passive imaging through optical obscurants is a promising application for mm-wave sensing. We have thus developed the Superconducting Kinetic Inductance Passive Radiometer (SKIPR), a 150 GHz polarization-sensitive photometric camera optimized for terrestrial imaging using a focal plane array with 3,840 kinetic inductance detectors (KIDs). We present a full description of the instrument design, with a particular emphasis on the cryogenic system based on a Gifford-McMahon cryocooler with a two-stage Adiabatic Demagnetization Refrigerator and a dedicated 1.59 m crossed Dragone telescope with an altitude/azimuth mount. We include a detailed lab-based characterization of the KIDs, which results in a determination of their superconducting resonator parameters and optical properties. We also present in situ measurements from the telescope, including point-spread functions and noise characterization. In sum, we find that SKIPR performs as expected, providing diffraction-limited imaging with detector noise performance set by the random arrivals of photons from the ambient background. There is minimal variation in detector characteristics over the full SKIPR focal plane array, and the overall detector yield is 92 per cent.

astro-ph.IM↗

arXiv2025

Simons Observatory: Characterization of the Large Aperture Telescope Receiver

The Simons Observatory (SO) is a ground-based cosmic microwave background (CMB) survey experiment that currently consists of three 0.42m small-aperture telescopes (SATs) and one 6m large-aperture telescope (LAT), located at an elevation of 5200m in the Atacama Desert in Chile. At the LAT's focal plane, SO will install >62,000 transition-edge sensor detectors across 13 optics tubes (OTs) within the Large Aperture Telescope Receiver (LATR), the largest cryogenic camera ever built to observe the CMB. Here we report on the validation of the LATR in the laboratory and the subsequent dark testing and validation within the LAT. We show that the LATR meets cryogenic, optical, and detector specifications required for high-sensitivity measurements of the CMB. At the time of writing, the LATR is installed in the LAT with six OTs (corresponding to >31,000 detectors), and the LAT mirrors and remaining seven OTs are undergoing development.

astro-ph.IM↗

arXiv2024

The Simons Observatory: Design, Optimization, and Performance of Low Frequency Detectors

The Simons Observatory (SO) is a cosmic microwave background (CMB) experiment located in the Atacama Desert in Chile that will make precise temperature and polarization measurements over six spectral bands ranging from 27 to 285 GHz. Three small aperture telescopes (SATs) and one large aperture telescope (LAT) will house $\sim$60,000 detectors and cover angular scales between one arcminute and tens of degrees. We present the performance of the dichroic, low-frequency (LF) lenslet-coupled sinuous antenna transition-edge sensor (TES) bolometer arrays with bands centered at 27 and 39 GHz. The LF focal plane will primarily characterize Galactic synchrotron emission as a critical part of foreground subtraction from CMB data. We will discuss the design, optimization, and current testing status of these pixels.

astro-ph.IM↗

arXiv2024

The Simons Observatory: laboratory beam characterization for the first small aperture telescope

The Simons Observatory is a ground-based telescope array located at an elevation of 5200 meters, in the Atacama Desert in Chile, designed to measure the temperature and polarization of the cosmic microwave background. It comprises four telescopes: three 0.42-meter small aperture telescopes (SATs), focused on searching for primordial gravitational waves, and one 6-meter large aperture telescope, focused on studying small-scale perturbations. Each of the SATs will field over 12,000 TES bolometers, with two SATs sensitive to both 90 and 150GHz frequency bands (SAT-MF1, and SAT-MF2), while the third SAT is sensitive to 220 and 280GHz frequency bands. Prior to its deployment in 2023, the optical properties of SAT-MF1 were characterized in the laboratory. We report on measurements of near-field beam maps acquired using a thermal source along with measurements using a holographic method that enables characterization of the amplitude and phase of the beam response, yielding an estimate of the far-field radiation pattern received by SAT-MF1. We find that the near-field half-width-half-maximum (HWHM) requirements are met across the focal plane array for the 90GHz frequency band, and through most of the focal plane array for the 150GHz frequency band. The mean of the bandpass averaged HWHM of the edge-detector focal plane modules match the simulated HWHM to 10.4%, with the discrepancy caused by fringing in the simulation. The measured beam profiles match simulations to within 2dB from the beam center to at least the -10dB level. Holography estimates of the far-field 90GHz beams match the full-width-half-maximum from simulation within 1%, and the beam profiles deviate by less than 2dB inside the central lobe. The success of the holography and thermal beam map experiments confirmed the optical performance was sufficient to meet the science requirements. On-site observations are currently underway.

astro-ph.IM↗