arXiv subjects

John Karakla

Publications and source records attributed to John Karakla.

18 recordsLinked to original sources

arXiv2026

Detectors for CLASS-W2: The second 90 GHz telescope of the Cosmology Large Angular Scale Surveyor

The Cosmology Large Angular Scale Surveyor (CLASS) is measuring the Cosmic Microwave Background (CMB) polarization anisotropy on the largest angular scales (>1 degree) to probe the epochs of inflation and reionization. To enhance the CMB mapping speed, we have built, tested, and commissioned in August 2025 a second 90 GHz receiver (CLASS-W2) with a detector focal plane composed of feedhorn-coupled Transition Edge Sensor (TES) bolometers fabricated at NIST-Boulder. The focal plane consists of four modules, each containing 37 feedhorns coupling orthogonal polarizations onto two TES bolometers, for a total of 296 optically sensitive detectors. Laboratory tests show highly uniform TES properties with an array average critical temperature of 184+-3 mK, a thermal conductance of 460+-47 pW/K, and a normal resistance of 7.8+-0.3 mOhms. The detector array has an average band center frequency of 95.2 GHz with 28.3 GHz bandwidth, and achieves a detector yield of 94%. On-sky measurements indicate a mean detector optical load of 3.3 pW, corresponding to an antenna temperature of ~23 K. The array's average beam solid angle is 124 $\mu$sr, with a full width at half maximum of 0.592 degrees, and the end-to-end average optical efficiency is 0.37. We find that high-frequency 'blue-leak' radiation couples directly to the TES bolometer islands; adding a metal-mesh low-pass filter with cutoff frequency of 157 GHz in front of the focal plane suppresses the 'blue-leak' power by 0.9 pW. The four-module array achieves a noise-equivalent temperature of NET= 16 uKrtS. Adding this array has boosted the CLASS 90 GHz mapping speed by 41%.

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arXiv2025

A Measurement of the Largest-Scale CMB E-mode Polarization with CLASS

We present measurements of large-scale cosmic microwave background (CMB) E-mode polarization from the Cosmology Large Angular Scale Surveyor (CLASS) 90 GHz data. Using 115 det-yr of observations collected through 2024 with a variable-delay polarization modulator, we achieved a polarization sensitivity of $78\,\mathrm{\mu K\,arcmin}$, comparable to Planck at similar frequencies (100 and 143 GHz). The analysis demonstrates effective mitigation of systematic errors and addresses challenges to large-angular-scale power recovery posed by time-domain filtering in maximum-likelihood map-making. A novel implementation of the pixel-space transfer matrix is introduced, which enables efficient filtering simulations and bias correction in the power spectrum using the quadratic cross-spectrum estimator. Overall, we achieved an unbiased time-domain filtering correction to recover the largest angular scale polarization, with the only power deficit, arising from map-making non-linearity, being characterized as less than $3\%$. Through cross-correlation with Planck, we detected the cosmic reionization at $99.4\%$ significance and measured the reionization optical depth $\tau=0.053^{+0.018}_{-0.019}$, marking the first ground-based attempt at such a measurement. At intermediate angular scales ($\ell>30$), our results, both independently and in cross-correlation with Planck, remain fully consistent with Planck's measurements.

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arXiv2024

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

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 $50~\mathrm{\mu m}$, $175~\mathrm{\mu m}$ pitch) positioned $0.88~\mathrm{mm}$ from an aluminum mirror. In lab tests, it was confirmed that the wire resonance frequency ($f_\mathrm{res}$) profile is consistent with the target, $139~\mathrm{Hz}<f_\mathrm{res}<154~\mathrm{Hz}$ in the optically active region (diameter smaller than $150~\mathrm{mm}$), 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 $0.048^{+0.013}_{-0.014}~\mathrm{mm}$ in the optically active region, achieving a modulation efficiency $\epsilon=96.2_{+0.5}^{-0.4}\%$ 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 ($2.5~\mathrm{Hz}$). 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.

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arXiv2024

CLASS Angular Power Spectra and Map-Component Analysis for 40 GHz Observations through 2022

Measurement of the largest angular scale ($\ell < 30$) features of the cosmic microwave background (CMB) polarization is a powerful way to constrain the optical depth to reionization and search for the signature of inflation through the detection of primordial $B$-modes. We present an analysis of maps covering 73.6\% of the sky made from the $40\,\mathrm{GHz}$ channel of the Cosmology Large Angular Scale Surveyor (CLASS) from 2016 August to 2022 May. Taking advantage of the measurement stability enabled by front-end polarization modulation and excellent conditions from the Atacama Desert, we show this channel achieves higher sensitivity than the analogous frequencies from satellite measurements in the range $10 < \ell < 100$. Simulations show the CLASS linear (circular) polarization maps have a white noise level of $125 \,(130)\,\mathrm{μK\, arcmin}$. We measure the Galaxy-masked $EE$ and $BB$ spectra of diffuse synchrotron radiation and compare to space-based measurements at similar frequencies. In combination with external data, we expand measurements of the spatial variations of the synchrotron spectral energy density (SED) to include new sky regions and measure the diffuse SED in the harmonic domain. We place a new upper limit on a background of circular polarization in the range $5 < \ell < 125$ with the first bin showing $D_\ell < 0.023$ $\mathrm{μK^2_{CMB}}$ at 95\% confidence. These results establish a new standard for recovery of the largest-scale CMB polarization from the ground and signal exciting possibilities when the higher sensitivity and higher-frequency CLASS channels are included in the analysis.

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arXiv2023

CLASS Data Pipeline and Maps for 40 GHz Observations through 2022

The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background over 75\% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220~GHz. This paper describes the CLASS data pipeline and maps for 40~GHz observations conducted from August 2016 to May 2022. We demonstrate how well the CLASS survey strategy, with rapid ($\sim10\,\mathrm{Hz}$) front-end modulation, recovers the large-scale Galactic polarization signal from the ground: the mapping transfer function recovers $\sim75$\% of $EE$, $BB$, and $VV$ power at $\ell=20$ and $\sim45$\% at $\ell=10$. We present linear and circular polarization maps over 75\% of the sky. Simulations based on the data imply the maps have a white noise level of $110\,\mathrm{μK\, arcmin}$ and correlated noise component rising at low-$\ell$ as $\ell^{-2.2}$. The transfer-function-corrected low-$\ell$ component is comparable to the white noise at the angular knee frequencies of $\ell\approx16$ (linear polarization) and $\ell\approx12$ (circular polarization). Finally, we present simulations of the level at which expected sources of systematic error bias the measurements, finding sub-percent bias for the $Λ\mathrm{CDM}$ $EE$ power spectra. Bias from $E$-to-$B$ leakage due to the data reduction pipeline and polarization angle uncertainty approaches the expected level for an $r=0.01$ $BB$ power spectrum. Improvements to the instrument calibration and the data pipeline will decrease this bias.

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arXiv2023

Cosmology Large Angular Scale Surveyor (CLASS): 90 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance

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 instrument 1D beam estimated at 90 GHz has an FWHM of 0.620+/-0.003 deg and a solid angle of 138.7+/-0.6(stats.)+/-1.1(sys.) usr integrated to a radius of 4 deg. The corresponding beam window function drops to b_ell^2 = 0.93, 0.71, 0.14 at ell = 30, 100, 300, respectively. 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. The instrumental temperature-to-polarization (T-to-P) leakage fraction, inferred from per-detector demodulated Jupiter scan data, has a monopole component at the level of 1.7 x 10^-3, a dipole component with an amplitude of 4.3 x 10^-3, with no evidence of quadrupolar leakage.

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arXiv2022

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

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 $μ$m} diameter wires with \mbox{175 $μ$m} 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.

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arXiv2022

Four-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: On-sky Receiver Performance at 40, 90, 150, and 220 GHz Frequency Bands

The Cosmology Large Angular Scale Surveyor (CLASS) observes the polarized cosmic microwave background (CMB) over the angular scales of 1$^\circ \lesssim θ\leq$ 90$^\circ$ 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, 23, and 71 $\mathrm{μK}_\mathrm{cmb}\sqrt{\mathrm{s}}$ 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.

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arXiv2021

Venus Observations at 40 and 90 GHz with CLASS

Using the Cosmology Large Angular Scale Surveyor, we measure the disk-averaged absolute Venus brightness temperature to be 432.3 $\pm$ 2.8 K and 355.6 $\pm$ 1.3 K in the Q and W frequency bands centered at 38.8 and 93.7 GHz, respectively. At both frequency bands, these are the most precise measurements to date. Furthermore, we observe no phase dependence of the measured temperature in either band. Our measurements are consistent with a CO$_2$-dominant atmospheric model that includes trace amounts of additional absorbers like SO$_2$ and H$_2$SO$_4$.

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arXiv2020

Two Year Cosmology Large Angular Scale Surveyor (CLASS) Observations: Long Timescale Stability Achieved with a Front-End Variable-delay Polarization Modulator at 40 GHz

The Cosmology Large Angular Scale Surveyor (CLASS) is a four-telescope array observing the largest angular scales ($2 \lesssim \ell \lesssim 200$) 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 ($T\rightarrow P$) 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 $T\rightarrow P$ leakage of $<3.8\times10^{-4}$ (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 $203 \pm 12 \mathrm{μK_{RJ}\sqrt{s}}$ for 1 mm of PWV when evaluated at 10 mHz; accounting for $\sim32\%$ of the $1/f$ noise in the central pixels of the focal plane. The low level of $T\rightarrow P$ 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.

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arXiv2020

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance

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 ($1^\circ\lesssimθ\leqslant 90^\circ$) CMB polarization to constrain the tensor-to-scalar ratio at the $r\sim0.01$ 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 2016 September to 2018 February. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is $<0.023^\circ$ ($<1.6$% 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 an FWHM of $1.579^\circ\pm.001^\circ$ and a solid angle of $838 \pm 6\ μ{\rm sr}$, consistent with physical optics simulations. The corresponding beam window function has a sub-percent error per multipole at $\ell < 200$. An extended $90^\circ$ 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.

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arXiv2019

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz

We report circular polarization measurements from the first two years of observation with the 40 GHz polarimeter of the Cosmology Large Angular Scale Surveyor (CLASS). CLASS is conducting a multi-frequency survey covering 75% of the sky from the Atacama Desert designed to measure the cosmic microwave background (CMB) linear E and B polarization on angular scales $1^\circ \lesssim θ\leq 90^\circ$, corresponding to a multipole range of $2 \leq \ell \lesssim 200$. The modulation technology enabling measurements of linear polarization at the largest angular scales from the ground, the Variable-delay Polarization Modulator, is uniquely designed to provide explicit sensitivity to circular polarization (Stokes $V$). We present a first detection of circularly polarized atmospheric emission at 40 GHz that is well described by a dipole with an amplitude of $124\pm4\,\mathrm{μK}$ when observed at an elevation of $45^\circ$, and discuss its potential impact as a foreground to CMB experiments. Filtering the atmospheric component, CLASS places a 95% C.L. upper limit of $0.4\,\mathrm{μK}^2$ to $13.5\,\mathrm{μK}^2$ on $\ell(\ell+1)C_\ell^{VV}/(2π)$ between $1 \leq \ell \leq 120$, representing a two-orders-of-magnitude improvement over previous limits.

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arXiv2019

On-sky performance of the CLASS Q-band telescope

The Cosmology Large Angular Scale Surveyor (CLASS) is mapping the polarization of the Cosmic Microwave Background (CMB) at large angular scales ($2<\ell\lesssim200$) in search of a primordial gravitational wave B-mode signal down to a tensor-to-scalar ratio of $r \approx 0.01$. The same data set 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 $32~μ\mbox{K}_{cmb}\sqrt{\mbox{s}}$. We demonstrate that the detector optical loading ($1.6~\mbox{pW}$) and noise-equivalent power ($19~\mbox{aW}\sqrt{\mbox{s}}$) match the expected noise model dominated by photon bunching noise. We derive a $13.1\pm0.3~\mbox{K/pW}$ calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of $0.48\pm0.04$ and a $27~\mbox{K}$ system noise temperature. Finally, we report a Tau A flux density of $308\pm11~\mbox{Jy}$ at $38.4\pm0.2~\mbox{GHz}$, consistent with the WMAP Tau A time-dependent spectral flux density model.

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arXiv2018

The Cosmology Large Angular Scale Surveyor Receiver Design

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.

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arXiv2018

Variable-delay Polarization Modulators for the CLASS Telescopes

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 $45^\circ$) 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.

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arXiv2016

The Cosmology Large Angular Scale Surveyor

The Cosmology Large Angular Scale Surveyor (CLASS) is a four telescope array designed to characterize relic primordial gravitational waves from inflation 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.).

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arXiv2014

The Cosmology Large Angular Scale Surveyor (CLASS): 38 GHz detector array of bolometric polarimeters

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.

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arXiv2014

CLASS: The Cosmology Large Angular Scale Surveyor

The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment to measure the signature of a gravita-tional-wave 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 $\ell$. 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, $τ$.

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